1St Joint THEMIS-TWINS Science Workshop Abstracts

1st Joint THEMIS-TWINS Science Workshop March 21 - 25, 2010, UCLA Abstracts

TWINS ENA Primer P. W. Valek (1) and the TWINS science team Space Science and Engineering, Southwest Research Institute, San Antonio, Tx [email protected] The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission images the inner magnetosphere using Energetic Neutral Atoms (ENAs) in the energy range from 1 to 100 keV. The TWINS Mission Of Opportunity consists of Ly-alpha monitors to measure the geocoronal hydrogen density to aid in the ENA analysis, environmental sensors provide con- temporaneous measurements of the local charged particle environment, and ENA imagers to produce images with typical angular resolutions of 4 x 4 degree and time resolutions of 10 min (ENA images with 1 x 1 degree and 1 minute resolutions are possible with sufficient ENA flux). TWINS images ENAs with identical instruments aboard two widely spaced high-inclination spacecraft both in Molniya orbits. In this talk we will briefly review TWINS mission, how it performs these ENA measurements, and discuss the interpretation of the ENA images. The production of the images and instrumental effects that need to be considered will be discussed.

Comparison of ion distributions from TWINS ENA images and THEMIS measurements J.D. Perez (1) et al. Physics Department, Auburn University, AL 36849 [email protected] Ion distributions obtained from TWINS ENA images will be presented for geomagnetic storm periods, Oct. 11, 2008, July 22, 2009, May 29, 2010, and Aug. 3-4, 2010. Comparisons with THEMIS in-situ measurements will be made whenever they are available.

New science questions raised by event studies combining TWINS, THEMIS and other satellite and groundbased data sets J.U. Kozyra (1), M.W. Liemohn, (1), Co-authors from TWINS and other instrument teams (1) University of Michigan [email protected] We will present information on TWINS events, interesting signatures in TWiNS, THEMIS, and other related satellite and ground-based data sets, and the science questions that are raised by these signatures. For example, during the 22 July 2009 CIR event, TWINS and THEMIS observed the capture of magnetosheath plasma on the dayside during obliquely southward IMF and high dynamic pressure. The captured plasma appeared to subsequently move into the inner magnetosphere. This type of ring current source population has never before been reported. Another example, during the 3-5 August 2010 CME-driven magnetic storm, TWINS

1 observed unusual ring current signatures at a time when maps of total electron content (TEC) in the ionosphere show evidence of strong prompt penetration electric fields. These types of TEC signatures are usually seen only in major magnetic storms but this storm had a min-Dst of only -70 nT. These features are likely due to the exceptionally low ionospheric densities (and conductivities) this solar minimum, that are resulting in stronger electric fields from magnetospheric current closure. In addition, the August 2010 storm has two Dst minima. The second minimum is driven by filament material in the ICME. An interesting density structure, preceding the filament material, compresses and then expands the magnetosphere resulting in a peak in low-altitude ENA, and an adiabatic drop-out and recovery of the radiation belts. Description of other events will follow.

Global Ring Current ion and Pitch-Angle Distributions During Storm Main and Recovery Phases Obtained Trough Stereo Observations by TWINS and Compar- isons with THEMIS/ESA P. C. Brandt (1), K. Keika (2), E.C. Roelof (1), J. Redfern (3), P. Valek (3), V. Angelopoulos (4), S.-Y. Hsieh (1), D. Bazell (1), and the TWINS Team (1) The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723; (2) New Jersey Institute of Technology; (3) Southwest Research Institute, San Antonio, TX; (4) CESR, Toulouse, France [email protected] We compare ring current proton distributions retrieved from TWINS observations with measurements obtained by the Electro Static Analyzer (ESA) on board THEMIS during storm main phases and a storm recovery phases. We find that the spatial, pitch-angle distributions (PADs) and spectral distributions of the TWINS and THEMIS measurements are consistent. While isotropic PADs are retrieved during the main phase, TWINS clearly observes more pancake PADs during storm recovery phase, as expected. By using two imaging vantage points we also demonstrate possible spatially dependent pitch-angle distributions (PADs). To retrieve the proton distributions from TWINS images we use a forward modeling technique, in which Energetic Neutral Atom (ENA) images are simulated from a parameterized proton distribution through the point-spread function of the TWINS cameras. The simulated images are compared to the observed ENA images and the parameterized proton distribution is modified until the difference between the simulated and observed ENA image is minimized.

An introduction to the new GOES 13-15 magnetospheric electron and proton data J. V. Rodriguez (1), T. G. Onsager (2), J. C. Green (3) (1) University of Colorado, Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado 80309 (2) NOAA Space Weather Prediction Center, Boulder, Colorado 80305 (3) NOAA National Geophysical Data Center, Boulder, Colorado 80305 [email protected] The Space Environment Monitor (SEM) suite on the GOES 13-15 satellites includes two new sensors, the Magnetospheric Electron Detector (MAGED) and the Magnetospheric Proton Detector (MAGPD). Each provides diﬀerential ﬂux measurements at 9 pitch angles in 5 energy

2 channels (30-600 keV electrons, 80-800 keV protons). This tutorial will include an overview of the measurements and a description of data availability and access. Examples of events of interest to the THEMIS community will be shown.

Magnetic Flux Transfer in the April 5, 2010 Galaxy 15 Substorm: An Unprece- dented Observation M. Connors (1), C. T. Russell (2), V. Angelopoulos (2) (1) Athabasca University, Athabasca AB, T9S 3A3 Canada (2) IGPP, UCLA, Los Angeles CA, 90095 USA [email protected] At approximately 0825 UT on April 5, 2010, a CME-driven shock encountered Earth’s magnetosphere. The IMF, slightly southward since 0805 UT, turned more so, to an average value close to -15 nT, which was maintained for nearly an hour under high dynamic pressure conditions. Following a substorm growth phase, a localized dipolarization took place in the midnight sector at 0847 UT. Large dipolarizations at 0903 UT and 0908 UT were observed by GOES West (11) in the midnight sector, and by three THEMIS spacecraft near X=-11, Y=-2 RE. A large electric field detected at the THEMIS spacecraft indicates so much flux transfer to the inner magnetosphere that ”overdipolarization” took place at GOES 11. This transfer is consistent with the signature of the substorm current wedge including its ground magnetic perturbations. Significant particle injections were also observed. The ensemble of extreme geophysical signatures presented here has never been previously observed, but is consistent with the Near-Earth Neutral Line interpretation of substorms. Shortly after their occurrence, anomalies were reported on the Galaxy 15 geosynchronous communications satellite, stationed close to GOES 11. We interpret the sequence of activity in this event, with emphasis on the transfer of flux in the period 0902 to 0909 UT. The location of Galaxy 15 near midnight subjected it to extreme space weather conditions, suggesting that they were causative in disruption of its operation.

Multipoint Observations of the Large Substorm Associated with the Galaxy 15 Anomaly H. J. Singer (1), P.T.M. Loto’aniu (1,2), J.C. Green (1), J.V. Rodriguez (1,2), B.J. Anderson (3), J.J. Love (4), V. Angelopoulos (5), D.N. Baker (6), M.G. Connors (5,7), W.F. Denig (8), E.F. Donovan (9), O. LeContel (10), T.G. Onsager (1), X. Nagatsuma (11), A. Runov (5), E.L. Spanswick (9) (1) Space Weather Prediction Center, Boulder, CO, United States (2) Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, United States (3) Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, United States (4) USGS, Denver, CO, United States (5) Institute for Geophysics and Planetary Physics, University of California Los Angeles, Los Angeles, CA, United States (6) Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, United States (7) Center for Science, Athabasca University, Athabasca, AB, Canada

3 (8) National Geophysical Data Center, NOAA, Boulder, CO, United States (9) Dept. of Physics and Astronomy, University of Calgary, Calgary, AB, Canada (10) LPP-CNRS, Ecole Polytechnique, Vlizy, France (11) National Institute of Information and Communications Technology, Koganei, Japan [email protected] On April 5, 2010 around 09 UT, the NOAA Geosynchronous Operational Environmental Satellites (GOES) observed a large reconfiguration of the magnetospheric magnetic field in the midnight to dawn local time sector. Specifically, near midnight, the GOES-11 Hp (north-south) magnetic field increased by approximately 100 nT in 10 minutes. This is one of the largest dipolarizations of Earths field ever observed by GOES. At the same time, there were large increases in the flux of energetic electrons and protons and large ground magnetic perturbations. While the auroral electrojet index (AL) was extreme, being less than -2000 nT during this event, the preliminary, real-time Dst storm index was only on the order of -30 nT during the event (although it reached somewhat more intense values during the following day). The Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) monitored currents at low altitude during this event, and these observations can be used to place the localized measurements at geosynchronous orbit in a global context. Additional measurements were made by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft just tailward of the GOES satellites. Minutes after the initiation of this intense substorm, at 0948 UT, Galaxy 15, located at geosynchronous orbit, just a few degrees away from GOES-11, suffered a serious anomaly. At the time, it was reported that this anomaly was likely related to the solar generated event; however, the anomaly is still under investigation. The purpose of this presentation is not to evaluate the anomaly, but rather to describe the large response of the magnetosphere and ionosphere system to the solar and solar wind conditions that caused this extremely large substorm.

Comparison of Simulated and Observed Ring Current Magnetic Field and Ion Fluxes During 5 Aprils 2010 Storm M.W. Chen (1), C.L. Lemon (1), T.B. Guild (1), M. Schulz (2), A.T.Y. Lui (3), A. Keesee (4), and J. Goldstein (5) (1) The Aerospace Corporation, Los Angeles, CA; (2) Lockheed Martin Retiree; (3) John Hopkins University Applied Physics Laboratory; (4) West Virginia University; (5) Southwest Research Institute [email protected] In this study we compare simulated and observed stormtime magnetic intensities, proton flux spectra and/or ENA fluxes for the 5 April 2010 storm (minimum Dst < 73 nT) to test how well self-consistent simulations can simultaneously reproduce these quantities. We simulate the ring current and plasma sheet using the magnetically and electrostatically self-consistent Rice Convection Model-Equilibrium (RCM-E) [Lemon et al., JGR, 2004] with a time-varying magnetopause driven by upstream solar wind and interplanetary magnetic field (IMF) conditions. We use ion temperatures inferred from TWINS energetic neutral atom (ENA) images, THEMIS/ESA and SST ion data, and the empirical IMF-dependent model of Tsyganenko and

4 Mukai [JGR, 2003] to guide our specification of the plasma sheet ion pressure and density at 10 RE, our plasma boundary location in the RCM-E. We compare the simulated magnetic intensity with the magnetic intensity measured by magnetometers on the GOES satellites at geosynchronous altitude (6.6 RE) and on THEMIS satellites. The simulated and observed proton spectra (THEMIS) and (ENA) flux (TWINS) will be compared. We discuss the response of the ring current magnetic field and ion flux distribution to expansions and compressions of the magnetosphere associated with the dynamic solar wind pressure for the storm event.

Storm-time changes in the outer magnetosphere A.T.Y. Lui Applied Physics Laboratory, The Johns Hopkins University, 11100 Johns Hopkins Rd., Laurel, MD 20723-6099, USA [email protected] I plan to give a preliminary report on the investigation of storm-time changes of particle energy density and magnetic ﬁeld in the outer magnetosphere.

Flux Transport During Steady Magnetospheric Convection J. Kissinger (1), R.L. McPherron (1), V. Angelopoulos (1), T.-S. Hsu (1), X. Chu (1) (1) Earth and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095 [email protected] Steady magnetospheric convection (SMC) events are a mode of magnetospheric response thought to result from balanced rates of dayside and nightside reconnection, usually preceded by a substorm. How the magnetosphere reaches a steady-state configuration after substorm expansion remains an outstanding question. Flux must return from the tail x-line to the dayside reconnection region fast enough to maintain the reconnection rate balance for several hours. Previous case studies suggest that the large-scale magnetosphere configuration and plasma convection remain stable, but are less steady on short time scales. Using Geotail and THEMIS data, we present a statistical survey of plasma flows and flux transport in the magnetotail during a comprehensive list of over 3000 SMC events from 1997-2009. We find numerous earthward fast flows throughout the entire tail, which are diverted to the dawn and dusk flanks, away from the inner magnetosphere.

Properties of Stream Interfaces during the Prolonged Declining Phase of Solar Cycle 23 R.L. McPherron, T.-S. Hsu, J. Kissinger, X. Chu Inist. Geophys. Planet. Physics, Univ. Calif. Los Angeles [email protected] The Themis Mission was dedicated to resolution of the substorm controversy. Unfortunately, the two years during which these spacecraft passed through the tail were among the most magnetically quiet years ever observed in the space age. We illustrate the diﬀerence between substorm in these years and other times with a statistical investigation of the properties of the solar wind and geomagnetic indices. Most substorms during this interval occurred within high-speed streams following the interface between low and high-speed streams. Paradoxically,

5 the coupling of the solar wind electric ﬁeld to the indices during this time was more eﬃcient that at any other times in the solar cycle. We explain this observation as a consequence of the weak solar wind driver and the absence of saturation of the polar cap potential.

A Comparative Study of Propagated Solar Wind and Themis Solar Wind Obser- vations Tung-Shin Hsu, Robert McPherron, James Weygand, Xiangning Chu, Jennifer Kissinger Institute of Geophysics and Planetary Physics, University of California, Los Angeles, USA [email protected] A more precise solar time propagation method, which is based on minimum variance analysis, has been used wildly in the magnetospheric studies. However, a comparison between the near-earth solar wind and propagated solar wind observation has seldom been investigated. Particularly, how often a near-earth IMF structure can be missed by the propagated solar wind and vice versa is the primary interest. In this study, we will use the Themis solar wind observations from 2008 - 2009 to compare with the Weimer propagated solar wind data to examine how often the propagated solar wind is matched with the near-earth solar wind observations. The result may have signiﬁcant importance for the solar wind and magnetosphere coupling study.

Statistical study of the effect of ULF fluctuations in IMF on the cross polar cap potential drop H.-J. Kim (1), L. Lyons (1), A. Boudouridis (2) (1) Dept. of Atmospheric and Oceanic Sciences, UCLA, Los Angeles, CA 90095-1565, USA (2) Space Science Institute, Boulder, CO 80301, USA [email protected] Recent studies showed that, regardless of the orientation of IMF, high level of ULF wave activity in the solar wind can substantially enhance the convection strength in both the dayside and nightside high latitude ionosphere, suggesting that ULF fluctuations may also be an important contributor to the coupling of the solar wind to the magnetosphere-ionosphere system. We conduct a statistical study to understand the effect of interplanetary ULF fluctuations on the electric potential drop across the polar cap, primarily focusing on northward IMF conditions. We have analysed the Assimilative Mapping of Ionospheric Electrodynamics [AMIE] calcula- tions of the cross polar cap potential, hourly ULF index that is defined as the logarithm of Pc5 ULF wave power in IMF Bz, and solar wind parameters measured by the ACE spacecraft for a total of 249 days in year 2003. The averaged cross polar cap potential drop tends to increase linearly with the ULF index, when the effects of the solar wind speed and the solar wind dynamic pressure are suppressed. This result suggests that while the cross polar cap potential drop is predominantly controlled by the IMF and solar wind speed, enhanced ULF wave activity in IMF can also drive a substantial increase of trans-polar potential drop, which is consistent with the previous finding of the positive correlation between the interplanetary ULF wave power and the convection strength in the polar cap.

6 Extended Low Shear (or large guide field) Magnetic Reconnection in the Solar Wind M.L. Cartwright (1), T. Phan (1), V. Angelopoulos (2), J.P. McFadden (1), D. Larson (1), K.-H. Glassmeir (3) (1) Space Sciences Laboratory, UC Berkeley (2) UCLA (3) TU Braunschweig [email protected] Extended magnetic reconnection X-lines in the solar wind have recently been reported. All reported extended X-lines have high field shear angles (greater than 60 deg) [Phan et al., 2009]. This suggests that high shear magnetic reconnection in the solar wind results in a dominant X- line rather than a network of patchy and randomly distributed X-lines. However, reconnection exhausts in the solar wind occur most frequently with small field shear angles [Gosling et al., 2007]. It is unclear if low shear reconnection X-lines are extended similar to high shear X-lines. Small shear angle reconnection exhausts are typically quite narrow and convect past spacecraft in less than 100s [Gosling et al., 2007]. This has made previous identification of low shear reconnection exhausts difficult with data resolutions greater than 60s. The Wind and Themis spacecrafts have plasma data resolutions of 3s making them ideal for multi-spacecraft studies of low shear reconnection; and we have used them to survey reconnection exhausts in the solar wind. We present examples of extended low shear (less than 60 deg) reconnection exhausts in the solar wind observed by Wind and Themis. The exhausts are quite narrow and convect past the spacecraft on the order of tens of seconds. In these examples, the X-line extended between 20 and 100 Earth radii.

THEMIS observations of a foreshock induced magnetopause disturbance exhibiting distinct plasma flows and an intense density compression D.L. Turner (1), S. Eriksson (2), T.D. Phan (3), V. Angelopoulos (1,4), N. Omidi (5), J.P. Mc- Fadden (3), K.-H. Glassmeier (6,7) (1) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA, United States (2) Laboratory for Atmospheric and Space Physics, Univ. of Colorado, Boulder, CO, United States (3) Space Sciences Laboratory, Univ. of California, Berkeley, CA, United States (4) Dept. of Earth and Space Sciences, University of California, Los Angeles, CA, United States (5) Solana Scientific Inc., Solana Beach, CA, United States (6) Institut fu?r Geophysik and Extraterrestrische Physik, Technische Universitt Braunschweig, Braun- schweig, Germany (7) Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany [email protected] Large-scale magnetopause disturbances can result from several different types of events, including those resulting from phenomena in the foreshock region. In this observational report, we present multipoint THEMIS observations of a magnetopause disturbance along the dawn-side, equatorial flank that exhibits distinct flows in the magnetospheric plasma and an abnormally strong compression of the plasma density within it, which peaks at >7× the density of the near-Earth solar wind. We find that the fastest ion and electron flows are related to two differ-

7 ent processes: the ion flows resulting from plasma being displaced around the disturbance and field-aligned electron flows, possibly related to magnetic reconnection. Interestingly, the magnetospheric plasma flows around the disturbance are very similar to those previously reported around flux transfer events, but we conclude that the disturbance is most likely the result of regions of compressed and rarefied plasma density in the sheath resulting from a foreshock cavity. We present simple schematics of this foreshock cavity and its leading-edge compression region that explain many of the observed features and discuss possibilities for the intense density enhancement. Using the simultaneous THEMIS observations from the magnetosphere, magnetosheath, and solar wind, we propose that the abnormal density enhancement was the result of a combination of compression effects due to the magnetosheath and the cavity’s leading-edge compression region coupled with some complex interaction near the magnetopause along the event’s distinct boundary layer.

Foreshock Cavities: Origins and Magnetospheric Impacts N. Omidi (1), D.G. Sibeck (2) (1) Solana Scientific Inc. (2) NASA/GSFC [email protected] Foreshock cavities correspond to events observed upstream of the bow shock where the solar wind magnetic field and density decrease by a significant ( 50%) amount bounded by regions of increase in the ambient field and density. Two possible interpretations of foreshock cavities have been suggested. In one scenario, a foreshock cavity corresponds to a finite region of space with magnetic field lines connected to the quasi-parallel bow shock and bounded by field lines not connected to the bow shock. In the second interpretation, foreshock cavities are due to spacecraft encountering the foreshock compressional boundary (FCB) as it moves back and forth pass the spacecraft. In order to understand the magnetospheric impacts of foreshock cavities it is important to distinguish between these two possibilities. In this presentation, we use the results of hybrid (kinetic ions, fluid electrons) simulations and spacecraft observations to examine the origins and magnetospheric impacts of foreshock cavities.

Remotely sensing the dayside magnetopause with TWINS in-situ observations T. B. Guild, J. H. Clemmons, M. W. Chen, J. F. Fennell, J. L. Roeder The Aerospace Corporation, 15049 Conference Center Drive, Chantilly, VA and 2310 E. El Segundo Blvd, El Segundo, CA [email protected] Plasma observations in or nearby the magnetospheric cusp provide a powerful vantage point with which to remotely sense a large portion of the solar wind-magnetosphere boundary. Distant reconnection regions inject particles along ﬁeld lines into the cusp with observable energy-time and energy-latitude signatures indicative of their origin. We here present preliminary plasma observations of high latitude ion dispersions near the earths geomagnetic cusp, and use these observable signatures to understand the nature of the injection at the magnetopause. The dispersions are observed with the Surface Charging Monitor (SCM), a top-hat electrostatic plasma analyzer on both vehicles of the Two Wide angle Imaging Neutral-atom Spectrometers (TWINS) mission of opportunity. The TWINS orbits are high-inclination elliptical orbits with

8 apogees near 7 RE, and during certain seasons the sensors dwell in the northern cusp at apogee, providing a unique platform to statistically characterize these ion dispersions. By performing simple time-of-flight analysis on the dispersions, we determine that many map to the low- latitude flank, and some are periodic. These results are the beginning of a broader study to understand the distribution of magnetopause signatures observed at mid-altitude with minimal, or at least different, orbital selection effects.

The Cluster Observations of a Magnetic Reconnection Event at the Dayside Mag- netopause S.H. Lee (1), H. Zhang (1), K.-H. Glassmeier (2), P.W. Daly (3), H. Reme (4) (1) Physics Department and Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA (2) Institute for Geophysics and Extraterrestrial Physics, Braunschweig, Germany (3) Max Planck Institute for Solar System Research, Lindau, Germany (4) Centre dEtude Spatiale des Rayonnements, Toulouse, France [email protected] We present preliminary results on a magnetic reconnection event observed by the Cluster spacecraft at the dayside magnetopause. Cluster 1 crossed the magnetopause at (x, y, z)=(7.6, 2, 5.3) RE GSM while traveling outbound in the northern hemisphere on March 5, 2007. High speed ﬂows (Vz ∼250 km/s, Vx∼-200 km/s) were observed from 18:52 UT to 19:03 UT. Meanwhile, energetic protons (>30 keV) were observed by the RAPID instrument. The spatial extension of the energetic particles are examined by four Cluster spacecraft. During this time interval, the IMF was southward with (Bx, By, Bz) = (-4, 0, -4) nT GSM and the solar wind speed was 400 km/s.

FTE-like magnetic reconnection signatures on the dayside magnetopause H. Zhang, M. G. Kivelson, K. K. Khurana, V. Angelopoulos Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA, USA [email protected] A bipolar magnetic perturbation in the normal direction to the dayside magnetopause associated with variations in the field strength is generally referred to as a flux transfer event (FTE). According to the widely-accepted interpretations, these signatures are induced by the passage of magnetic bubbles ejected from reconnection site or flux ropes. For some cases, however, the plasma and electric field data suggest that these FTE like signatures may not relate to those traditional interpretations. In these cases, the velocity tangential to the local magnetopause changes its sign right in the middle of the bipolar magnetic perturbation. The ion pitch angle distributions change its polarity (parallel or anti-parallel to the magnetic field line). These observations may indicate that the observing spacecraft traverses a reconnection structure on the magnetosphere side (from one jet flow region to another). The magnetic field component tangential to the magnetopause also shows bipolar variations, which is related to the Hall current system of a reconnection structure. The electric field waves burst, which is also a common feature near the reconnection separatrices.

9 Period-doubling in drift-mirror pulsations: THEMIS observations in the morning magnetosphere D.G. Sibeck (1), G.I. Korotova (2) (1) NASA/GSFC, Code 674, Greenbelt, MD 20771 (2) IPST/UMD, College Park, MD 20742 david.g.sibeck@nasa.gov Korotova et al. [2009] recently presented THEMIS observations of drift-mirror mode waves in the outer dawn magnetosphere during a geomagnetic quiet period from 0830-1230 UT on November 7, 2007, noting the presence of period doubling during a portion of the interval Here we examine the period-doubling during this interval in greater detail, showing (1) it became more prominent as THEMIS-A approached the geomagnetic equator, (2) it was part of a nearly purely second harmonic poloidal oscillation, (3) although there is some evidence for drift-bounce resonances and spatial gradients sweeping past the spacecraft, including a 180 shift in phase across a resonant energy near 25 keV, neither explanation satisfactorily accounts for the period doubling, (4) ions with energies greater than 10 keV exhibit butterﬂy pitch angle distributions when the magnetic ﬁeld strength weakens, and (5) the same ions frequently exhibit bounce phase bunching consistent with that expected for mirror-mode squeezing (but not a drift-bounce resonance).

Electromagnetic waves on ion gyro-radii scales across the magnetopause Y. Yao (1), C.C. Chaston (1), K.-H. Glassmeier (2), V. Angelopoulos (3) (1) Space Sciences Laboratory, University of California, Berkeley (2) Institut für Geophysik und Extraterrestrische Physik, Technische Universität Braunschweig, Braun- schweig, Germany (3) Institute for Geophysical and Planetary Physics, University of California, Los Angeles [email protected] This report presents the first study of the global distribution and properties of kinetic scale electromagnetic waves on the Earth’s magnetopause. From a statistical study using THEMIS fields observations over a frequency range from 0.01 − 2Hz, we find waves on scales of the order of ion gyro-radii at all magnetic local times surveyed (4 − 20 MLT). The spectral energy densities (ǫ) within these waves across the magnetopause are larger on the dawn (4 − 12 MLT) than duskside (12 − 20 MLT) irrespective of the direction of the magnetosheath magnetic fields BZ sheath component. Furthermore, these observations reveal that ǫ is magnetic shear dependent with large enhancements for shear angle > 60◦. The similarity between dawn-dusk asymmetry in ǫ we observe and reported distribution of magnetosheath ions in the magnetosphere suggest that gyro-radii scale magnetic field structures/waves may facilitate a significant fraction of cross magnetopause plasma transport.

Observations of the Poynting vector near a ﬁeld line resonance Michael Hartinger (1), Vassilis Angelopoulos (1), Mark B. Moldwin (1,2), Karl-Heinz Glass- meier (3), Yukitoshi Nishimura (4,5) (1)Earth and Space Sciences Department, UCLA (2)Atmospheric, Oceanic, and Space Sciences Department, University of Michigan

10 (3)Institute for Geophysics and Space Physics, Technical University of Braunschweig, Germany (4)Atmospheric and Oceanic Sciences Department, UCLA (5)Solar-Terrestrial Environment Laboratory, Nagoya University [email protected] The field line resonance (FLR) mechanism plays an important role in transferring energy from fast mode waves to shear Alfvén waves in the Earths magnetosphere. Multi-spacecraft observations, combined with ground observations, are needed to trace the energy transfer associated with FLRs from sources in the solar wind (or sheath) to localized shear Alfvén waves, and finally to the ionosphere. We use satellites from the THEMIS mission and IMAGE ground magnetometers to study energy transfer near an FLR. We determine that energy was being diverted from the magnetic equatorial plane to the ionosphere, and the net energy flux in the field aligned direction, mapped to the ionosphere, was 0.70 mW/m2. This was comparable to expected Joule dissipation rates. Ground observations demonstrate that the time evolution of the FLR and associated energy transfer were strongly linked to the dynamics of the magnetopause undulations.

Monitoring magnetospheric density by ground magnetometers through normal- mode magnetoseismology P.J. Chi University of California, Los Angeles [email protected] In this presentation I report the latest results using ground magnetometer stations in North America to monitor the spatiotemporal evolution of magnetospheric plasma using normal-mode magnetoseismology (ﬁeld line resonance sounding). The data include those collected by the ground magnetometers in multiple arrays, including the THEMIS GBO/EPO stations.

High-resolution empirical geomagnetic field model TS07D: Scientific results, community service and perspectives M. I. Sitnov (1), G. K. Stephens (1), A. Ya. Ukhorskiy (1), J. D. Vandegriff (1), N. A. Tsyga- nenko (2) (1) Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA (2) St. Petersburg University, Russia [email protected] The increased volume of the geomagnetic field data required a qualitative transition in the empirical modeling tools. Classical empirical models, such as T96 and T02, used few custom- tailored modules to represent major magnetospheric current systems and simple data binning or loading-unloading inputs for their fitting with data and the subsequent applications. In the new model TS07D they have been replaced by systematic expansions of the equatorial current contributions and data-mining algorithms searching for events with the global activity parameters similar to those at the time of interest. The necessity to mine and fit data dynami- cally, with the individual subset of the database being used to reproduce the geomagnetic field pattern at every new moment in time, requires the corresponding transition in the use of the new empirical geomagnetic field models. It becomes more similar to runs-on-request offered

11 by the CCMC for ﬁrst-principles models. To provide this mode of operation a new web-based modeling tool has been created and tested at the JHU/APL

(http://geomag_field.jhuapl.edu/model/) and we discuss the first results of its performance testing and validation, including in-sample and out-of-sample modeling of a number of CME- and CIR-driven magnetic storms. We also report on the first tests of the forecasting version of the TS07D model, where the magnetospheric part of the macro-parameters involved in the data-binning process (Sym-H index and its trend parameter) are replaced by their solar wind-based analogs obtained using the Burton- McPherron-Russell approach. Perspectives of the model, including the reconstruction of the plasma pressure and the elaboration of a flexible field-aligned current module, are discussed.

OpenGGCM Simulation of a Substorm: Axial Tail Instability and Ballooning Mode Preceding Substorm Onset J. Raeder (1), P. Zhu (2), Y. Ge (1) (1) Space Science Center, University of New Hampshire, 8 College Rd, Durham, NH 03824, USA (2) Department of Physics, University of Wisconsin - Madison, 1500 Engineering Dr., Madison, WI 53706, USA [email protected] It is generally accepted that magnetic reconnection is the main mechanism that dissipates power during a substorm. It is less clear, however, whether the beginning of magnetic reconnection in the magnetotail also signifies the onset of the substorm expansion phase itself, i.e., whether the ”outside-in” scenario applies, or if a different process happens first closer to Earth that triggers the reconnection onset in the magnetotail, i.e., the ”inside-out” scenario. Global MHD simulations have generally supported the ”outside-in” scenario. However, ideal MHD instabilities that could possibly trigger tail reconnection may have been missed due to coarse numerical resolution or due to other numerical effects. Here, we present results from an OpenGGCM simulation of the March 23, 2007 substorm that clearly shows growth of the ballooning mode as suggested by our earlier analysis (Zhu et al., 2009), as well as growth of an ideal-like instability that is purely axial and was previously reported by Siscoe et al. (2009). Both instabilities occur simultaneously and are immediately followed by reconnection onset. The simulations results are in accordance with recent Geotail observations of ballooning with a wavelength of approximately 0.5 RE, and the time scales agree with that of the explosive growth phase. The exact relation between the three instabilities, i.e., ballooning, the axial mode, and tearing, is not entirely clear yet; however, having demonstrated that the OpenGGCM repro- duces all of them, they can now be analyzed in more detail. In particular, we present the expected auroral signature of these processes, which will allow for a more detailed comparison of the simulation results with ground based imagers.

Result update of the neutral, geocoronal Hydrogen 3D structure J.H. Zoennchen, U. Nass, H.J. Fahr Argelander Institut f¨ur Astronomie, University of Bonn [email protected]

12 We present revised H-density profiles and an updated structure of the neutral geocoronal H distribution using TWINS1-LAD data for quiet sun conditions. In the upsun direction these results are compared with IBEX/Cluster measurements and CASSINI/HDAC data. Based on that data an upsun H density profile valid up to 15Re is derived. By using both TWINS1/2-data a significant H-density profile difference between the upsun- and the geotailwards-direction is analysed and verified. Finally some LAD-calibration statements of TWINS1/2 are made.

The inﬂuence of the geocoronal H distribution on the modeling of ring current decay R. Ilie and R. Skoug Los Alamos National Laboratory [email protected] During times of moderate convective drift, the dominant mechanism for the decay of the ring current is the charge exchange of the ring current ions with the geocorona. The geocorona is a halo-like extension of the exosphere out to several Earth radii, consisting of relatively cold (<1000 K), very tenuous neutral hydrogen atoms. Because the geocoronal hydrogen density decreases exponentially with radial distance, at large altitudes down the magnetotail, the collisions with the neutral hydrogen become negligible. However, in the ring current region, these collisions become increasingly important and they account for signiﬁcant loss of the ring current particles, since magnetospheric H+ can be easily removed by charge exchange with the neutral exospheric hydrogen. Several models describing the neutral hydrogen distribution have been developed from var- ious data sets. Until recently, the neutral hydrogen distribution in the Hot Electron Ion Drift Integrator (HEIDI) ring current model was as described by Rairden et al. (1986), with the neutral hydrogen density showing only radial dependence. However, additional description of geocorona, such as the Hodges (1994) and Ostgaard et al. (2003) models have been implemented into HEIDI. The changes in the ring current development and decay are investigated.

CRCM simulation of “ring current index” N. Buzulukova (1), M.-C. Fok (1), P. Valek (2), J. Redfern (2), J. Goldstein (2), D. McComas (2) (1) NASA Goddard Space Flight Center, Greenbelt, MD (2) SWRI, San Antonio, TX [email protected] We present analysis of synthetic TWINS ENA images calculated with CRCM (Comprehen- sive Ring Current Model). We calculate optically thin ring current emissions for several storms during 2008-2010 using TWINS 1/2 geometry and H+ ﬂux from CRCM. We calculate so called ’ring current index’ (see Valek et al., 2010 for details) and compare with TWINS data. We try to infer ring current dynamics from CRCM results and associate it with ’ring current index’ from ENA.

RCM-E simulation of a moderate geomagnetic storm Jian Yang and Frank R. Toﬀoletto Department of Physics and Astronomy, Rice University, Houston, Texas, 77005 [email protected]

13 We present preliminary simulation results of a moderate geomagnetic storm using the Rice Convection Model coupled with an equilibrium magnetic field solver(RCM-E). The RCM-E combines the drift physics and magnetosphere-ionosphere coupling computational machinery in the RCM with a model of static force-balanced magnetic field from the magneto-friction code. The simulated storm occurred from May 28 to June 03, 2010 with a minimum Dst of about -80 nT and we used THEMIS data to provide the tailward plasma boundary conditions. The calculated magnetic field at geosynchronous orbit matches well with GOES observations. A synthetic magnetogram code is used to calculate virtual Sym-H, which also shows good agreement with observations. Contributions of different current systems to Sym-H will be analyzed and potential comparison against TWINS data will also be discussed.

Inner magnetosphere electrodynamics during 03-04 August 2010: results from CRCM N. Buzulukova (1), M.-C. Fok (1), j. Kozyra (2), P. Valek (3), J. Redfern (3), J. Goldstein (3), D. McComas (3) (1) NASA Goddard Space Flight Center, Greenbelt, MD (2) University of Michigan, Ann Arbor, MI (3) SWRI, San Antonio, TX [email protected] 03-04 August 2010 storm demonstrated a lot of interesting features seen in TWINS, DMSP, GPS TEC, Iridium (J. Kozyra, private communication). We show CRCM (Comprehensive Ring Current Model) results for the event. We calculate field-aligned currents, ring current fluxes, ring current pressure, electric field in the ionosphere and in the equatorial plane. We try to understand formation of prompt penetration electric field, SAPS and FAC structures during the event. A comparison with TWINS is also presented.

Effect of self-consistent magnetic field on plasma sheet penetration to the inner magnetosphere: force balanced RCM simulations and data comparisons Matina Gkioulidou (1), C.-P. Wang (1), L.R. Lyons (1), R.A. Wolf (2) (1) Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Califor- nia (2) Department of Physics and Astronomy, Rice University, Houston, Texas [email protected] Transport of plasma sheet particles into the inner magnetosphere is crucial to the development of the Region-2 field aligned current system, and as a result to the shielding of the penetration electric field and the formation of ionospheric features such as SAPS and the Ha- rang reversal. This transport is strongly affected by the magnetic field, which, in the inner plasma sheet is distinctly different from dipole. We have integrated the Rice Convection Model with a modified Dungey magnetic field solver to obtain the required force balance in the equatorial plane. Our outer plasma boundary conditions at r∼20 RE are a function of MLT, AE activity, and cross polar cap potential drop (CPCP) and are based on 11 years of Geotail data. We have run the self-consistent simulations for quiet time and enhanced convection. Comparing these results with those of a run using T96 magnetic field model we find that self- consistency gives weaker pressure gradients and thus weaker Region-2 field aligned currents

14 in the near Earth region, and the Harang reversal is located at higher latitudes and extends more dawnward. Also, the self-consistent magnetic field has a dawn-dusk asymmetry with field lines being more stretched in the pre-midnight sector, due to relatively higher plasma pressure there. This asymmetry affects in turn the radial distance and MLT variation of plasma sheet penetration into the inner magnetosphere. We compare plasma sheet properties of our results with statistical data obtained from Geotail and THEMIS.

Ion and electron pressure distributions from the tail plasma sheet to the inner magnetosphere: THEMIS-Geotail observations and comparisons with the RCM simulations C.-P. Wang (1), M. Gkioulidou1 (1) L.R. Lyons (1), V. Angelopoulos (2), R.A. Wolf (3), T. Nagai (4), J. Weygand (2), A.T.Y. Lui (5) (1)Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California (2)Department of Earth and Space Physics, University of California, Los Angeles, California (3)Department of Physics and Astronomy, Rice University, Houston, Texas (4)Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan (5)Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland [email protected] Spatial structures of plasma sheet ions and electrons and the associated field-aligned currents and magnetosphere-ionosphere coupling are crucial to development of storms and substorms. To determine global plasma sheet structures, we have statistically analyzed Geotail and THEMIS data from r = 4 to 30 Re under different convection strengths. To understand the dominant transport processes that result in the observed structures, we have compared the observed ion and electron distributions with simulation results of the Rice Convection Model (RCM). In our RCM runs, the electric and magnetic drift transport is simulated under self-consistent electric and magnetic fields, and the MLT-dependent boundary particle sources are established from Geotail. We found that for particles near and above thermal energy, their spatial distributions can be quantitatively accounted for by adiabatic electric and magnetic drift transport from their tail sources.

Spatial distribution of electron and proton aurora identified by NORSTAR MSP Y. Zou (1), Y. Nishimura (1, 2), L.R. Lyons (1), E.F. Donovan (3), V. Angelopoulos (4) 1 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, 90095 Los Angeles, USA 2 Solar-Terrestrial Environment Laboratory, Nagoya University, 464-8601 Nagoya, Japan 3 Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada 4 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, USA [email protected] Distribution and evolution of electron and proton auroras during steady magnetic conditions is studied using the coordinated observation of NORSTAR MSP and THEMIS ASI at Gillam. We firstly selected events where enhanced proton aurora in 486.0 nm extended equatorward of 557.7 nm electron aurora on the pre-midnight sector, and compared to simultaneous THEMIS white light ASI data. We found that the latitudinal profile of 486.0 nm intensity in

15 such proton-dominated latitudes is highly-correlated with that of diﬀuse aurora near the equatorward boundary detected by the THEMIS ASI, suggesting that proton-dominated aurora can be measured by white light ASIs. Secondly, we determined equatorward boundaries of proton and electron auroras using the 486.0 and 630.0 nm channels and investigated the local time distributions and magnetic activity dependence. Our preliminary case studies and statistics show that the duskside proton aurora extends equatorward of the electron aurora during magnetic quiet time, and that it is reversed in the dawnside. The equatorward boundaries of proton and electron aurora overlap near midnight. The overlapping region shifts duskward as magnetic activity increases. To the dusk of the overlapping region, the equatorward boundary of proton aurora is located equatorward of electron aurora, while in the dawnside, it is located poleward of electron aurora. Contrary to previous ﬁndings [e.g., Fukunishi, 1975], electron precipitation extends further equatorward of proton precipitation on the premidnight sector during enhanced magnetic activity.

A Three Year Study of Magnetic Field Fluctuations in the Magnetospheric Cusp Christina Chu et al., University of Alaska Fairbanks [email protected] The high-altitude magnetospheric cusps are regions of significant magnetic field turbulence. There are also magnetic field fluctuations that cannot be called turbulence. Some of the low frequency fluctuations observed in the cusp have been shown to be motion of the cusp structure by the spacecraft (back and forth motion of the boundaries surrounding the cusp); others are transient reconnection signatures or flux transfer events (FTEs). Turbulence is important to understand because it has been proposed as a mechanism for particle energization in the cusps to MeV levels. High resolution magnetic field data from the Fluxgate Magnetometer instruments on the Cluster satellites were used to statistically map the power of the magnetic field fluctuations in the frequency ranges of 0.017-10 Hz, 0.017-0.1 Hz, and 0.1-10 Hz for the high-altitude cusp and surrounding boundaries for northward, southward, and all interplanetary magnetic field clock angle orientations by using three years of Cluster data, 118 northern hemisphere cusp crossings, from 2001-2003. The mean (compressive), total, and perpendicular (transverse) power were calculated. Analysis was done to check whether the locations of enhanced power were statistically consistent with the locations of crossing of cusp boundaries and diamagnetic cavities. Sources of power in all frequency ranges studied, 0.017-10 Hz, include reconnection, a gradient in the magnetic field strength when moving from the magnetosphere to the magnetosheath, and possibly the earthward propagation of magnetotail reconnection fluctuations. Sources of wave power from 0.017-0.1 Hz include mirror mode, ion-cyclotron, and Alfvén waves. Sources of wave power from 0.1-10 Hz include ion-cyclotron and Alfvén waves.

Typical Characteristics of Rising and Falling Tones of Chorus Waves Observed on THEMIS W. Li (1), R. M. Thorne (1), J. Bortnik (1), Y. Nishimura (1, 2), V. Angelopoulos (3) (1) Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095-1565, USA; (2) Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan;

16 (3) Institute of Geophysics and Planetary Physics / Department of Earth and Space Sciences, Uni- versity of California, Los Angeles, Los Angeles, CA 90095-1567, USA [email protected] Chorus waves, which have received intense attention recently due to their significant role in radiation belt electron dynamics, frequently consist of discrete elements including rising and falling tones. We analyzed THEMIS wave burst data for rising and falling tones of lower-band chorus separately, in order to investigate their typical characteristics. Our results show that rising tones are more likely to be quasi field-aligned with higher ellipticity, whereas falling tones are very oblique with lower ellipticity. We also demonstrate that magnetic wave amplitude of rising tones is much larger than that of falling tones. Statistical results on the global distribution of rising and falling tones show that falling tones are observed from the premidnight to the noon sector, whereas rising tones extend further to the afternoon sector. Furthermore, on the dayside falling tones are more commonly observed at high latitudes, while rising tones do not show clear latitudinal dependence. These new findings provide an important implication on the chorus generation mechanism.

Explaining flux dropouts during the main phase of geomagnetic storms Drew L. Turner, M. Hartinger, Y. Shprits, and V. Angelopoulos Institute of Geophysics and Planetary Physics, University of California, Los Angeles [email protected] During the main phase of geomagnetic storms, relativistic electrons throughout Earths outer radiation belt experience rapid and drastic flux dropouts of up to several orders of magnitude. Previous studies have argued for different mechanisms to explain these dropouts, including fully adiabatic transport and non-adiabatic losses to Earths atmosphere and/or magnetopause. Here, we present observational evidence compiled from more than 10 different spacecraft, which are ideally located throughout different regions of the outer radiation belt during a recent storm driven by a co-rotating interaction region (CIR) in the solar wind. The particle fluxes and waves data demonstrate that the storm-time dropout of electrons is energy dependent and non-adiabatic, and furthermore, they clearly show that the loss is almost entirely to the magnetopause. This indicates that electron loss during CIR-driven storms is a result of magnetopause shadowing and subsequent outward radial diffusion.

Resolving the ring current and near-Earth tail current systems during storm times N.Yu. Ganushkina (1, 2), S. Dubyagin (2), M.V. Kubyshkina (3), M. Liemohn (1) (1) University of Michigan, Ann Arbor, MI, USA (2) Finnish Meteorological Institute, Helsinki, Finland (3) University of St.-Petersburg, St.-Petersburg, Russia [email protected] Understanding the time-varying electromagnetic field configuration and the consequent charged particle dynamics in the Earth’s space environment is fundamentally important both for scientific and space weather purposes. When discussing the relative contributions from the ring and near-Earth tail currents, the key question is how to separate partial ring current and tail current at the inner edge of the plasma sheet. There are two possible ways to give definitions: Using magnetic field observations and to measure the characteristic energy of the

17 current-carrying population. In reality, the ring and tail currents cannot be unambiguously separated in this transition region between dipole and tail-like field. We model the evolution of ring and near-Earth tail currents by using two models, namely, the time-evolving event- oriented model for the inner magnetosphere magnetic field and Inner Magnetosphere Particle Transport and Acceleration Model (IMPTAM). The event-oriented model based on available in-situ observations of the magnetospheric magnetic field gives a global representation of the magnetic field evolution during specified time periods. In IMPTAM model we trace particles with arbitrary pitch angles numerically in the drift approximation. A time-dependent model boundary outside of 6.6 RE gives a possibility to take into account the particles in the transition region (between dipole and stretched field lines) forming partial ring current and near-Earth tail current in that region. We compare the inner magnetosphere responses during two storms, two CIR-driven storms, which occurred at similar phases of solar cycles in 1996 and 2009. We focus mainly on the Dst index calculated by both models and contributions from the tail and ring current systems to it.

Magnetospheric cross-field currents in the near-Earth tail Michael W. Liemohn (1) Darren L. De Zeeuw (1), Raluca Ilie (2), Natalia Yu. Ganushkina (1,3), and Janet U. Kozyra (1) (1) Atmospheric, Oceanic, and Space Sciences Department, University of Michigan, Ann Arbor, MI (2) Los Alamos National Laboratory, Los Alamos, NM (3) Also at the Finnish Meteorological Institute, Helsinki, Finland [email protected] Magnetospheric cross-field currents in the near-Earth tail are highly variable and continu- ously changing during magnetic activity. A single near-tail magnetic field line can be part of a variety of cross-field current systems. For example, in simulation results from the 22 October 1999 storm, a field line crossing downtail at L=8 during the main phase was found to contain partial ring current, symmetric ring current, and tail current simultaneously. Such field lines with multiple currents are common in the near-Earth tail. Another time from the same event found two closely-spaced field lines (L=6.0 and 6.5) with completely different current systems on them (one entirely symmetric ring current and the other entirely tail current). The inner- and outermost locations of these current systems are extracted from the simulation results and discussed in terms of how to define their spatial extents as a function of storm phase and driving conditions. Collaboration is asked of the THEMIS and TWINS teams to corroborate or refute these numerical findings.

Consequences of violation of frozen-in-flux - evidence from OpenGGCM simulations Bei Hu (1), Richard Wolf (1), Frank Toffoletto (1), Jian Yang (1), Joachim Raeder (2) (1) Department of Physics and Astronomy, Rice University, Houston, TX, 77005, USA 2Space Science Center, University of New Hampshire, Durham, NH, 03824, USA [email protected] It is widely believed that, during a substorm, plasma instabilities occur before the onset of magnetic reconnection, signaling the end of the growth phase. Despite many years of effort, however, the details of the instability and its impact on the plasmasheet before the onset of

18 reconnection are not well understood. In this paper, we study a simulation of a substorm that occurred on March 23, 2007, based on the Open Geospace General Circulation Model (OpenGGCM). Our analysis emphasizes the time development of the distribution of the entropy parameter and its convective time derivative, which should be zero in ideal MHD. In the late growth phase, the simulation exhibits, over a range of local times, a systematic violation of conservation of entropy that corresponds to what is called ”antidiﬀusion”. Out of this back- ground, a more localized disturbance develops in a region of high magnetic stretching, resulting in formation of a strong reduction of entropy (bubble) earthward of a local enhancement (blob). The process is accelerated when the current density exceeds a threshold for triggering an ex- plicit resistivity in the code. The bubble moves earthward and the blob tailward, which leads to a reduction of the normal magnetic ﬁeld and a thinning of the current sheet between them. This positive feedback gives rise to increased violation of the perfect-conductivity relation and eventually reconnection.

Dipolarization initiated by Cowling electrojet current loop substantiated by THEMIS observations of a substorm on March 1, 2008 J. R. Kan (1,2), H. Li (1), C. Wang (1), M. Kubyshkina (3), A. Runov (4), C. S. Deehr (2), C. J. Xiao (5), S. Y. Fu (6), W. Sun (2) (1) Center for Space Science and Applied Research, Chinese Academy of Science, Beijing, China (2) Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, USA (3) St Petersburg State University, St Petersburg, Russia (4) Institute of Geophysics and Planetary Physics, UCLA, Los Angelos, California, USA (5) School of Physics, Peking University, Beijing, China (6) School of Earth and Space Science, Peking University, Beijing, China [email protected] We present THEMIS observations of a substorm on March 1, 2008 to substantiate the M-I coupling substorm model that initiates dipolarization onset by the Cowling electrojet current loop (CECL). The Cowling electrojet is intensified by the induced equatorward E-field (Mozer, 1971) in the mid-night sector of the auroral oval driven by the earthward convection enhanced by the NEXL observed at X ∼ -20 ± 5 RE. Divergence of the Cowling electrojet current launches field-aligned currents propagated by the Alfvn wave to form the time-dependent CECL, which then interact with the plasma sheet to disrupt the cross-tail current in the dipolarization region as proposed by Kan (2007). The CECL model predicts: (I) Brightening of the substorm onset arcs preceded the dipolarization onset by ∼77 to ∼80 sec observed at TH-D. This is consistent with the Alfvén travel time from the footprint of TH-D to the neutral sheet estimated at ∼80.3 sec based on the AM02 field model (Kubyshkina et al., 2009). (II) The substorm onset arcs and the footprints of TH-D and TH-E are predicted to be imbedded in the Cowling electrojet channel by the CECL model. This is consistent with the observations that the substorm onset arcs and the footprints of TH-D and TH-E are aligned in the Cowling electrojet channel, which peaked around ∼67◦ to ∼68◦ GML and extended from ∼20 to ∼02 MLT.

Statistical Study of Magnetic Fluctuation Features Associated with Near-tail Dipo- larizations Observed by the THEMIS Spacecraft M.-Y. Park (1), D.-Y. Lee (1), S.-I. Ohtani (2), K.-C. Kim (3)

19 (1) Department of Astronomy and Space Science, University of Chungbuk National, Cheongju, South Korea; (2) Space Department, Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA; (3) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095- 1567, USA [email protected] In this work we examined a number of magnetic dipolarizations that occurred at X >∼-11 RE in the near-Earth plasma sheet. These dipolarization events were observed by the three near-tail THEMIS spacecraft. By employing the continuous wavelet transform technique, we found several interesting features. First, for the low frequency regime that we conveniently defined here, 0.005Hz to 0.03Hz (typically well below local proton gyro-frequency), we found that a significant percentage of dipolarizations indicate that the fluctuations are predominantly localized at one or more discrete frequencies. Many of them grow with growth time less than 2 min across onset time. Statistically, f ∼ 0.012-0.014 Hz is the dominant frequency, which is similar to frequency that has been referred to as ballooning mode frequency (e.g., Cheng and Lui, 1998; Horton et al., 1999). So, many of the dipolarizations can be viewed as manifestation of low frequency instability phenomenon. Further, we analyzed the polarization features (instability mode structure) of the dipolarization fluctuations in the low frequency domain, and found that the compressional component of magnetic fluctuations is significant for most of the studied events. It implies that the instability inevitably involves compression effect to a large extent. But it was also found that the relative significance of the perpendicular component of the fluctuations (so, the field line bending effect) increases with plasma beta which is thought to be proportional to the degree of field line stretching. When a perpendicular component is non-negligible, many events indicate that the magnetic oscillation on the perpendicular plane is mostly linearly polarized, and it is mostly in pshi direction, which is perpendicular to the azimuthal direction. This mode structure seems to be consistent with the conventional ballooning limit. Lastly, we have also studied the fluctuation features in the high frequency regime which we defined here is from 0.03 Hz to 0.1 Hz. Most importantly we found the tendency that the magnetic fluctuation intensity in the high frequency regime is localized near the neutral sheet, decreasing away from the neutral sheet. We have also estimated the relative significance between the low and high frequency fluctuations, the details of which will be presented in this paper.

On the nature of precursor flows upstream of advancing dipolarization fronts X.-Z. Zhou (1), V. Angelopoulos (1), V. A. Sergeev (2), A. Runov (1) (1) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90034, USA; (2) Institute of Physics, St. Petersburg State University, St. Petersburg, Russia [email protected] Earthward-propagating dipolarization fronts, interpreted as thin, vertical current sheets that separate plasmas of different origins in the Earth’s magnetotail, are embedded within flow bursts, often near the leading edge of bursty bulk flows. Observations have also shown that bursty bulk flow onset typically precedes dipolarization front arrival by ∼ 1 min. Ion distribution functions reveal that earthward flows in advance of front arrival are often caused

20 by the appearance of a new ion population atop a pre-existing plasma sheet component. Particle simulations suggest that this second population, which contributes most to the plasma velocity, is composed of ions that have been reflected at and accelerated by the approaching front. We propose that in the presence of a finite upstream Bz field, the reflected ions would be confined in a region with a size comparable to the ion thermal gyroradius over the upstream Bz. THEMIS observations confirm that the measured time difference δt between the appearance of earthward plasma flows and the dipolarization front arrival is consistent with the predicted size of the ion accessibility region.

TCR observations during ARTEMIS first tail crossings S.A. Kiehas (1,2), A. Runov (1), V. Angelopouplos (1) (1) Institute of Geophysics and Planetary Physics, UCLA, Los Angeles (2) Institute of Space Research, Austrian Academy of Sciences, Graz [email protected] We report about the first observations of TCRs and associated plasma flow structures in the Earth‘s magnetotail during ARTEMIS tail crossings. A series of TCRs observed on P1 associated with a series of flow events seen on P2 on October, 21, 2010 between 1700 and 1800 UT is discussed.

Low-Altitude Emission of Energetic Neutral Atoms: A New Diagnostic of Energetic Magnetospheric Ion Precipitation Patterns E.C. Roelof and H. Nair Johns Hopkins University/Applied Physics Laboratory, Laurel, MD 20723-6099, USA [email protected] We describe a new theoretical understanding of the emission of energetic neutral atoms (ENAs) generated by the precipitation of energetic magnetospheric ions into the Earths monatomic oxygen (O) exosphere (200-800 km). This low altitude emission (LAE) is the brightest ENA source in images obtained from Astrid-1/PIPPI, IMAGE/MENA/HENA, and TWINS1/2. The upward ENA albedo from the precipitating protons in the energy range 1-100 keV, can approach 50 percent of the incident proton intensity. Unlike FUV imaging, ENA imaging of the LAE allows us to extract the detailed (not integrated) energy spectrum of the precipitating protons. We have veriﬁed this claim by comparing ENA images from TWINS 1/2 with in situ ion spectra measured by DMSP spacecraft (825 km altitude) ﬂying simultaneously under the ENA LAE regions [Bazell et al., JGR, 115, A10204, 2010]. Quantitative extraction of proton spectra from the ENA images requires a thick-target theory that treats properly the multiple atomic collisions (charge exchange of protons, stripping ENA H-atoms) and associated energy losses (including ionization and excitation). Analytic solutions to the coupled proton/H-atom transport equations have been obtained, and they provide quantitative insight into the strong dependence of the ENA LAE upon the pitch angle and the energy of the precipitating protons. Simulated ENA images of LAE are compared with high-resolution all-sky ENA images by IMAGE/HENA from 2000 km altitude with exposure times of two minutes during perigee passes, demonstrating how the distribution of LAE in MLAT and MLT can reveal that of the ion precipitation process.

21 Comparison of TWINS images of low-altitude emission of energetic neutral atoms with DMSP precipitating ion ﬂuxes: 2010 events D. Bazell, T. S. Sotirelis, E. C. Roelof, H. Nair, and P. C. Brandt Johns Hopkins University/Applied Physics Laboratory, Laurel, MD 20723-6099, USA [email protected] Low altitude emissions (LAE) observed in TWINS1/2 stero energetic neutral atom (ENA) images (2-64 keV) are used to infer precipitating ion spectra using the thick target approximation for multiple atomic collisions between protons incident on the Earths monatomic oxygen exosphere (200-800 km). The technique has been described and validated for a particular TWINS substorm observation (11 October 2008) by Bazell et al. [JGR, 115, A10204, 2010]. We have been constructing a catalog of near-simultaneous DMSP observations (1-30 keV) of passes above the ENA LAE features in which we can compare the in situ DMSP precipitating ion spectra with the ion spectra inferred from the ENA spectra in the corresponding TWINS pixels. In order to make this comparison between the LAE (imaged by TWINS in 4degx4deg pixels) to DMSP observations at 825 km of ion precipitation (in 1 x7 km pixels), we time- average the DMSP observations corresponding to the largest scale auroral precipitation features (which will dominate the TWINS images). Only the spectral shapes are compared, because the precipitation pattern cannot be resolved by the ﬁnite-sized TWINS pixels. We show comparisons for several events 2010 and obtain reasonable agreement with the spectral shape between TWINS and DMSP, demonstrating the applicability of the thick target approximation for ENA imaging to this problem.

Triggering of substorms identified using modern optical versus geosynchronous particle data B. Gallardo-Lacourt (1), L. R. Lyons (1), Y. Nishimura (1, 2), V. Angelopoulos (3), E. F. Dono- van (4) (1) Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095-1565, USA (2) Solar-Terrestrial Environment Laboratory, Nagoya University, 464-8601 Nagoya, Japan (3) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095- 1565, USA (4) Physics and Astronomy, University of Calgary, Calgary, AB, Canada [email protected] Knowledge of the period before auroral onset of a substorm is crucial for understanding the sequence of events leading to substorm onset. Previous works [Lyons et al., 1997; Hsu et al., 2003] have been showed that more than 50% of the substorms can be triggered by the northward turning of the IMF Bz. However, Newell et al. [2010] recently found that just 24% of the substorms can be triggered by the IMF Bz. Now we have the capability to study substorm onsets with the high-resolution all-sky imager (ASI) array of the THEMIS program, obtaining onsets unambiguously, including for very weak substorms, which are difficult to identify with the AL index. Using the THEMIS ASIs, we found that only 18% of the substorms are associated with northward turning of the IMF Bz. We also considered whether or not high speed streams can be related to IMF triggering during different phases of the solar cycle. Finally, we compare the behavior of the IMF Bz in substorm onsets identified by

22 the THEMIS ASIs and events with injection of particles at geosynchronous orbit, obtaining a higher percentage, around 50%, of triggering associated to the injection. This result suggests that large substorms may be preferentially triggered by IMF northward turning, while small substorms have internal magnetospheric triggering.

Statistical relation of substorm onset to mid-latitude positive bay, geosynchronous dipolarization and near-earth dipolarization X. Chu, R.L. Mcpherron, T. Hsu, J. Kissinger (1) IGPP, UCLA, Los Angeles, CA, USA [email protected] Magnetospheric substorm plays a key role in the coupling of the solar wind and magnetosphere. A sudden intensiﬁcation of aurora which moves poleward and westward is usually identiﬁed as the indicator of substorm onset. Geomagnetic perturbation is another fundamen- tal observation signature of substorm. Rapid geomagnetic disturbances usually accompany the brightening of aurora. Geomagnetic disturbance exhibits an abrupt negative change at higher latitude, which is usually call negative bay onset. At mid-latitude, the change of geomagnetic disturbance is usually positive which is called mid-latitude positive bay (MPB) onset. Furthermore, an increase of magnetic inclination angle near mid-night sector, which is called dipolarization, is usually observed during a substorm. It is believed that these phenomena are highly correlated and they are manifestation of the formation of substorm current wedge (SCW) at substorm onsets. A large statistical study of the correlation among all these phenomena has not been previously reported. In this study, database of substorm onsets determined by AL indices and mid-latitude positive bay observed by INTERMAGNET chain are built. In addition, auroral substorm onset list determined by auroral observations from IMAGE will be established. Furthermore, onset lists of geosynchronous dipolarization from GOES satellites and near-earth dipolarization from THEMIS are created. The time delay and occurrence frequency among these onset lists are examined to see the relation among these onsets. A preliminary result shows that these phenomena are not always correlated in a one-to-one manner and a further investigation is necessary to clarify why this is the case.

Growth-phase auroral arcs and substorm breakups in global MHD simulations during THEMIS February 27 2009 substorm Y.S. Ge (1), J. Raeder (1), V. Angelopoulos (2), M.L. Gilson (1), A. Runov (2) (1) Space Science Center, University of New Hampshire (2) Institute of Geophysics and Planetary Physics, University of California Los Angeles [email protected] We investigated the auroral activity during February 27 2009 substorm observed by the THEMIS All-sky imagers (ASI) and well-conjugated THEMIS spacecraft, and performed a global MHD simulation to understand the relationship between the observed auroral activity and magnetospheric processes. One hour prior to the substorm major breakups, there is a pseudo-breakup and during the growth phase a relatively stable and longitudinally elongated auroral arc appears across the ﬁeld of view of Fort Smith, Gillam and Sanikiluaq. It was also recorded by THEMIS ASI that near the local midnight sector there was a North-South (N- S) arc formed 20 minutes before the major breakups. Recently, Nishimura et al. proposed

23 that the substorm auroral sequence was initiated by a poleward boundary intensification (PBI) and the following N-S arcs moving equatorward, and also suggested that a growth-phase arc was necessary for the N-S arc to trigger substorm. Thus the auroral sequence in February 27 substorm seems to follow the proposed paradigm. We used global MHD simulations to investigate the possible causes of these auroral activities in the magnetotail and to examine their relationship. In this study, our global simulation is able to reproduce the main auroral features with a small time delay to the observations, including the growth-phase arc, a faint N-S arc and the major breakups. By mapping these auroral features into the magnetotail, we found that the growth-phase arc in the simulation was related to the enhanced pressure gradient in the near-Earth tail region, which was probably caused by the enhanced earthward plasma-sheet convection during the southward IMF interval. However, the major breakup of the simulated substorm appears to be caused by a strong BBF at a different local time that is generated from the mid-tail X-line. While propagating earthward, the BBF generates strong flow shear at the edge of the flow channel and forms a large-scale flow vortex, which produces the field-aligned current (FAC) for the auroral breakup. This strong BBF also produces the observed earthward-propagating Dipolarization Front (DF) signatures. This simulation result suggests that during the February 27 substorm the growth-phase arc and the major breakups can be caused by different mechanisms and correspond to different dynamics. But by changing the pressure gradient in the near-Earth plasma sheet, the growth-phase arc may indicate the preconditioning of the magnetotail for the following onset, which should be further investigated in our future studies.

Pre-onset time sequence of auroral substorms Y. Nishimura (1, 2), L.R. Lyons (1), V. Angelopoulos (2), S. Zou (3), and S.B. Mende (4) (1) Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, 90095 Los Angeles, USA (2) Solar-Terrestrial Environment Laboratory, Nagoya University, 464-8601 Nagoya, Japan (3) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 90095-1565 Los Angeles, USA (4) Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, 48109 Ann Arbor, USA (5) Space Science Laboratory, University of California, Berkeley, 94720-7450 Berkeley, USA [email protected] A critical, long-standing problem in substorm research is identification of the sequence of events leading to substorm auroral onset. Based on event and statistical analysis of THEMIS all-sky imager data, we have shown that there is a distinct and repeatable sequence of events that is initiated by a poleward boundary intensification (PBI) followed by a north-south (N-S) arc moving equatorward towards the onset latitude leading to substorm auroral onset. This sequence is common for both isolated onsets and onsets that are close together, indicating that the onset processes for both are similar. We have now investigated differences between N-S arc sequences that do and do not lead to substorm expansion onset. We find that the two types of N-S arcs have similar characteristics. There is, however, one difference between the sequences of N-S arc evolution. Each N-S arc leads to small intensification of the growth phase arc, and when the onset-related N-S arc reaches the equatorward portion of the auroral oval, the pre-existing

24 growth phase arc is much brighter than at the times of non-onset related N-S arcs. The onset arc is typically a thin arc near the poleward boundary of a diffuse growth-phase arc. Assuming that the growth-phase arc is related to enhanced pressure gradient, this difference indicates that the near-Earth plasma pressure distribution at the time of plasma sheet fast flows is crucial in substorm triggering. These observations suggest that substorm onset instability is possible only when the pre-existing inner plasma sheet pressure and pressure gradient is sufficiently large.

Nightside Ionospheric Flow Shear Associated with Poleward Boundary Intensifica- tions (PBIs) Yong Shi (1), E. Zesta (2), L. R. Lyons (1), A. Boudouridis (1, 3), H.-J. Kim (1), H. Frey (4), S. B. Mende (4), and J. M. Ruohoniemi (5) (1) Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Califor- nia (2) Air Force Research Laboratory/RVBXP, Hanscom AFB, Massachusetts (3) Space Science Institute, Boulder, Colorado (4) Space Science Laboratory, University of California, Berkeley, California (5) Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia [email protected] The association of PBIs with ionospheric convection flow is investigated by overlaying flow observations from the Super Dual Auroral Radar Network (SuperDARN) and concurrent auroral images from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE). For the studied events we find a clear flow shear around the PBI auroral forms occurring simultaneously with the PBI. There are southeast flows poleward of the PBI and nearly westward flows equatorward of the PBI with flow direction generally along the PBI orientation. The PBI is found to be slightly equatorward of the center of the shear. The flow magnitude typically enhances on one or both sides of a PBI. The flow enhancements are longitudinally localized and there can be multiple such longitudinally localized enhanced flow regions at the same time, associated with PBIs. Corresponding Defense Meteorological Satellite Program (DMSP) observations indicate there is upward field-aligned current (FAC) in the region of the PBI and downward FAC poleward of the PBI. PBI occurs near the open-closed field line boundary but on closed field lines, while the flow enhancement on the poleward side of the center of the flow shear extends poleward of that boundary into the polar cap region, i.e. on open field lines. This implies that there is separatrix crossing of new plasma brought into the plasma sheet from the polar cap by the enhanced flows. In situ observations from Geotail for a fortuitous conjunction confirm the connection of PBIs and the simultaneous ionospheric flow shear with midtail fast flows.

Possible Connection of Polar Cap Flow Channels to PBIs and Streamers, Including Those that lead to Substorms L. R. Lyons (1), Y. Nishimura (1.2), H.-J. Kim (1), E. Donovan (3), V. Angelopoulos (4), G. Sofko (5), M. Nicolls (6), C. Heinselman (6), J. M. Ruohoniemi (7) (1) Department of Atmospheric and Ocean Sciences, University of California, Los Angeles, USA (2) Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan

25 (3) Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada (4) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, USA (5) Department of Physics and Engineering, University of Saskatchewan, Saskatoon, Canada (6) Center for Geospace Studies, SRI International, Menlo Park, USA (7) Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA [email protected] Using Rankin Inlet SuperDARN and RISR observations in conjunction with the THEMIS auroral imagers observations, as well as Sondrestrom radar observations, we show that longitudinally narrow channels of enhanced flow within the nightside polar cap precede PBIs and appear to trigger PBIs that are followed by equatorward moving auroral streamers, including those leading to substorm onset instability within the near-Earth plasma sheet. The association between the ionospheric flows and PBIs indicates that enhanced flows on open field lines may contribute to enhancement of magnetotail reconnection and to the earthward flows leading to streamers.

Near-Earth Plasma Sheet Azimuthal Pressure Gradient and Associated Auroral Development Soon Before Substorm Onset X. Xing (1), L. R. Lyons (1), Y. Nishimura (1,2), V. Angelopoulos (3), E. Donovan (4), E. Span- swick (4), J. Liang (4), D. Larson (5), C. Carlson (5), and U. Auster (6) (1) Department of Atmospheric and Oceanic Science, UCLA, Los Angeles, CA 90095-1567, U.S.A (2) Solar-Terrestrial Environment Laboratory, Nagoya University, Furocho, Chikusa, Nagaya 464- 8601, Japan (3) IGPP/ESS UCLA, Los Angeles, CA 90095-1567, U.S.A (4) Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada (5) Space Sciences Laboratory, UCB, Berkeley, CA 94720-7450, U.S.A (6) Institut fr Geophysik und Extraterrestrische Physik der TUBM 3, 38106 Braunschweig, Germany [email protected] The azimuthal plasma pressure gradient in the near-Earth plasma sheet makes crucial contributions to field-aligned current (FAC) formation. Numerical simulations and statistical observations have shown that a plasma pressure peak tends to build up in the pre-midnight region of the near-earth plasma sheet during the substorm growth phase due to enhanced magnetic drift. This leads to azimuthal pressure gradients in this region. The temporal variation of the azimuthal pressure gradient may provide an indication for the FAC variations associated with the substorm growth phase and may setup a plasma sheet pre-condition for the substorm onset being triggered near this region. We take advantage of two of the THEMIS spacecraft separated azimuthally near the orbit apogee and investigate the azimuthal plasma pressure gradient before substorm onset in the R∼10-12 RE region. Equatorial plasma pressure is estimated by removing the curvature force effect. Five events with the spacecraft footprints mapped very close to the aurora onset region were selected. These events show substantial duskward pressure gradient enhancement 1-5 min before onset. The onset arc, which results from enhanced energetic electron precipitation, was found to intensify simultaneously with the pressure gradient enhancement before onset breakup occurs. Since the energy and energy flux of precipitating electrons reflect the upward FAC strength, these results indicate that the

26 duskward azimuthal pressure gradient enhancement is associated with enhanced upward FAC during the late-growth-phase and leads to the intensiﬁcation of the onset auroral arc soon before it breaks up. It is possible that this pressure gradient enhancement may lead to ballooning mode instability and thus substorm onset along the intensifying arc.

Field-aligned current distribution associated with substorm onset arcs Y. Nishimura (1, 2), L. R. Lyons (1), T. Kikuchi (2), V. Angelopoulos (3), E. F. Donovan (4), and H. Luhr (5) (1) Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, USA (2) Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan (3) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, USA (4) Department of Physics and Astronomy at the University of Calgary, Calgary, Canada (5) Helmholtz Centre Potsdam, Potsdam, Germany [email protected] The substorm onset arc is associated with an enhanced upward ﬁeld-aligned current (FAC). To understand the current system just before and after substorm onset, it is essential to determine the location of the upward FAC relative to well-known region 1 and 2 FACs. We surveyed coordinated observations of the THEMIS all-sky imagers and the CHAMP magnetometer during the 2007-2009 winter seasons and selected events where CHAMP passed over onset arcs within 5 min before and after substorm expansion onset. We found that the upward FAC on the expansion arc extends over a wide local-time range ( 21 to 3 MLT) and is adjacent to the ordinary region 2 FAC in latitude: the upward FAC is located just poleward of the downward region 2 FAC on the premidnight sector, and is connected to the poleward boundary of the postmidnight upward region 2 FAC. In contrast, the upward FAC of the expansion arc is located further equatorward than the nightside region 1 FAC. The latitudinal current distribution is essentially unchanged from the late growth phase except for the current intensity. These results suggest that the current system enhanced at substorm onset is related to the region-2 system, which is connected to an enhanced pressure gradient in the near-Earth plasma sheet, rather than to the nightside region 1 system. We suggest that the onset upward FAC is a latitudi- nally narrow part of the postmidnight upward region 2 that extends poleward of the downward premidnight region 2 FAC.