sign in sign up
<<
Home , Explorer 1, Polar wind

Mechanisms of Energetic Ejection (MEME)

T. E. Moore1, L. Andersson2, C. R. Chappell3, G. I. Ganguli4, T. I. Gombosi5, G. V. Khazanov1, L. M. Kistler6, D. J. Knudsen7, M. R. Lessard6, M. W. Liemohn5, J. P. McFadden8, A. F. Nagy5, W. K. Peterson2, C. J. Pollock1, A. J. Ridley5, D. E. Rowland1, R. W. Schunk9, J. L. Semeter10, R. J. Strangeway11, J. P. Thayer12, R. Winglee13, A. Yau7 1. Executive Summary

MEME will achieve the overarching objective: “Origins of Near Earth Plasmas; to understand the transport of terrestrial gas and from its atmospheric source into the and downstream Solar ” [2009 Heliophysics Roadmap]. Plasma of atmospheric origin is widely recognized as a critical constituent of magnetospheric dynamics, providing the primary source of plasma for the and during dynamic conditions. The key observables are the signatures of the process, involving several mechanisms, by which gas and plasma interact and are transported and heated or accelerated such that they escape Earth’s gravity and are ejected into the Magnetosphere and downstream . Candidate mechanisms include convection into auroral regions, auroral kinetic and electromagnetic energy fluxes into the atmosphere, Joule dissipation through ion-neutral collisions, imposed convection and resulting ion pickup, plasma wave generation, and resultant plasma heating and acceleration. Plasma physical mechanisms are fundamental aspects of atmospheric energetic mass ejection from gravitational confinement, anywhere in the universe. MEME will provide comprehensive multi-point fields and plasma observations from the topside ionospheric ejection region, completing our knowledge of these mechanisms. MEME is essential to build predictive models of the energy transfer between drivers and loads, and the resultant mass ejection fluxes needed in support of predictive global circulation models of geospace weather. 2. Description

The MEME science problem comes down to understanding the varying degree to which terrestrial material mass loads the hot Solar Wind plasma and magnetospheric energetic particles [Szégo et al., 2000]. Known terrestrial plasma escape mechanisms include the wind [Barakat & Schunk, 2006]; the and its convection plumes, with their topside “footprints”, consisting mainly of light ion outflows; and auroral ionospheric heating mechanisms, which eject highly variable heavy ion outflows including O+ and molecular ions into the steady and pervasive polar wind light ion outflows [Moore & Horwitz, 2007; Lotko, 2007]. Reconnexion opens the Magnetosphere to exchange of solar and terrestrial plasmas and energy, to a degree that increases as the Solar Wind magnetic field turns southward and magnetospheric convection activity increases. Yet, paradoxically, the more open the Magnetosphere, the more it becomes mass loaded with terrestrial material, mainly the heavy O+ component [Kistler et al. 2006]. This basic fact argues strongly that reconnexion of the Solar Wind to the Magnetosphere and Ionosphere produces more ablative ejection of terrestrial plasma than entry of solar plasma. This escape process has been extensively observed and hypothesized, but its component mechanisms remain poorly understood because of the challenges of comprehensively observing low energy plasmas and fields, combined with the space-time ambiguities of single point observation.

Known auroral mass ejection mechanisms include ionization and heating by soft precipitation, with resultant enhancement of the ambipolar electric field; Alfvén waves propagating into the Ionosphere from turbulent magnetospheric boundary layers; current- driven instabilities leading to lower hybrid waves in the auroral acceleration region; and Mechanisms of Energetic Mass Ejection convection and shear driven instabilities along auroral flux tubes, including pick-up ions in the topside [Wilson, 1994; Moore & Khazanov, 2010; Paterson and Frank, 1989], and Joule- frictional heating in the collision dominated F region. Much of the energy is dissipated in the neutral gas, leading to observable atmospheric upwelling features above the , including the ejection of molecular ions. Auroral heavy ion outflows are observed to be dominated by superthermal (eV to 10's eV) ions that are transversely heated and whose flux rises in a power law relationship to the incident DC and-or AC electromagnetic energy fluxes and the auroral production of soft [Strangeway et al., 2005; Zheng et al., 2005; Zeng & Horwitz, 2007]. As summarized in Figure 1, these facts may be consistent with lifting of ions by the ambipolar electric field, enhanced by soft electron precipitation [Burchill et al., 2009], combined with heating and-or ponderomotive acceleration of the ions by broadband waves in the cyclotron frequency range. The ambipolar electric field can be derived from hot plasma precipitation rates, but the source of ion resonant waves is indeterminate, so neither their amplitudes nor heating rates are derivable from macroscopic disturbance conditions such as magnetohydrodynamic (MHD) flow, current, and thermal conditions.

Mass Ejection Mechanisms HCentrifugal Horizon Charge exchange Charge

FElectroMag FKinetic DC AC e-, i+ Precip

Topside TAI f(v)

HExobase Photoionization

Ambipolar Lift, Ion Pick-up F i+ heating gas heating e- heating Collisions

Magnetized => HIsotropause E (Dense atmosphere) T E Moore, NASA Goddard 1 27-28 Sep 2009, LWS TRT Ion Outflow Workshop

Figure 1. Schematic diagram of mass ejection mechanisms after Moore and Khazanov, [2010 JGR] and Strangeway et al., [2005 JGR]

The Outstanding Question is then: "What are the mechanisms by which Solar Wind energy flux is dissipated in planetary gas and plasma to produce ejected mass flux?" To answer this question, observations are needed to provide a comprehensive picture of ionospheric mass ejection, including: i) detailed observations of the 3D energy-angle distribution of transversely accelerated ions and electrons down to zero energy (≤ 0.1 eV); ii) control of sensor potential at or near the plasma potential; iii) true measurements based on composition analysis; iv) observation of plasma wavelengths and frequencies for definitive mode identification; v) direct observations of auroral neutral gas upwelling to complement recent CHAMP and other

2 Mechanisms of Energetic Mass Ejection accelerometer observations. The successful mission design will deliver simultaneous conjugate diagnostics from the F region (200-500 km), from the topside exobase region (500-1500 km), and from the auroral acceleration region (1500-3000 km). Advanced incoherent scatter radar (AISR) provides needed diagnostics of the F region. As illustrated in Figure 2, two identical spacecraft, placed in high inclination 500 km x 3000 km , with oppositely oriented semi-major axes through the radar sampled zone, can be maintained out of phase so they pass over the auroral zone in conjunction. In this configuration, one would be near apogee and the other near perigee, with alternating placements in the northern and southern auroral zones, with fixed orientation in inertial space so that a scan of all local times is performed each year. Measurement Requirements

We seek the operational capability to predict ionospheric mass ejection fluxes from imposed Solar Wind conditions, via global simulation models capable of computing the energetic excitation of the Ionosphere. The key element now missing from our understanding of this process is the ability to compute ionospheric response to those excitations, including the generation of instabilities that turn a portion of the available energy into ionization, heating, and acceleration of ionospheric gas and plasmas. MEME will achieve this overarching goal, advancing Heliophysics toward improved operational capabilities, as follows:

1. Determine the excitation of the global topside Ionosphere by solar and magnetospheric energy inputs. a. Measure the DC and AC Poynting flux into the Ionosphere from high altitude energy sources, in the range 0.01-100 mW m-2 with a resolution of 0.1 sec (1 km). b. Measure the frequency and wave vector spectrum associated with significant AC Poynting flux to determine the wave modes, propagation direction, and identify source regions. c. Measure the kinetic momentum and heat flux into the Ionosphere from high altitude particle sources, from thermal to auroral energies (0 eV - 20 keV).

2. Determine the response of the global topside Ionosphere to solar and magnetospheric forcing. a. Measure the local plasma wave environment generated by solar and magnetospheric forcing and the relaxation of auroral electron beams and pick-up ion ring beams. b. Determine the capability of such waves to produce superthermal power law tails in relevant velocity distributions. c. Measure the plasma upflow and outflow in the magnetized collisional zone (125-500 km), the exobase transition zone (~500-1500 km), and the centrifugal forcing zone of the auroral acceleration region (~1500-3000 km). d. Measure thermospheric upwelling and fast atom emission in response to auroral energy inputs throughout, and determine its effect on collisionality and exobase height.

3. Use physical models of the Ionosphere to predict the responses from the excitations observed, and refine these models to the point where they are demonstrably successful.

3. Cost

MEME requires a multi-point in situ observation program allowing simultaneous observations in the magnetized collisional F region, in the topside exobase transition zone, and in the region of highly structured field-aligned currents and shear flows of the auroral acceleration region. As such its cost will be comparable to the inflation-adjusted cost of previous multi-spacecraft

3 Mechanisms of Energetic Mass Ejection missions such as the International Sun Earth Explorer (ISEE), Cluster, and Time History of Events and Microscale Interactions during Substorms (THEMIS), albeit with fewer spacecraft, and therefore lower costs. The closest cost comparison may be with the -1 and -2 mission (DE-1,-2), although MEME’s emphasis is on lower altitude phenomena associated with the Ionosphere, in contrast with DE-1’s focus on of the plasmasphere or polar wind. Achieving the synchronized opposing major axis orbits over the appropriate auroral radar facilities and scanning all local times may consume the apparent propulsion savings over higher altitude orbits, so this orbital strategy requires study to determine its feasibility and cost.

Figure 2. Schematic diagram of an effective MEME orbital strategy that places both spacecraft over auroral forms in approximate conjunction.

New instrument capabilities not seen on DE-1 and -2 are required and available for MEME. These will enhance the spatiotemporal resolution of plasma measurements in support of diagnostics of thin structures (km) sampled at orbital velocity (10 km/s). It is a requirement to observe the lowest energy ionospheric plasmas without assuming that their velocity distributions are in local thermodynamic equilibrium. This requires fast 3D plasma instruments that multiple dimensions simultaneously, with the sensors maintained at or near the plasma potential to prevent exclusion of the core of ionospheric ions and electrons, which again, may not be “cold”. Such instruments require boom deployment to assure that they view normal to the residual spacecraft sheath. Wave measurements must be interferometric to permit wave vector measurement and definitive mode identification. Finally, there must at minimum be background gas composition measurements comparable to laboratory Residual Gas Analyzers (RGA), with gas transport analyzers preferred for completeness.

A trade must be made between onboard or ground based auroral imaging. Onboard imaging would give the tightest integration with the in situ observations. But the ground elements of MEME may include a network of all sky cameras and scanning photometers in addition to the required Advanced Incoherent Scatter Radar (AISR) diagnostics. Together these may provide

4 Mechanisms of Energetic Mass Ejection adequate diagnostics of auroral ionospheric context in support of interpreting the spacecraft data, providing definitive discrimination between ionospheric upwelling in response to electron heating and upwelling in response to ion heating.

Significant cost elements for the MEME mission include: 1. Collisional and Magnetized Zone Probe (125-500 km) a. Advanced Incoherent Scatter Radar (AISR) plasma diagnostics b. Ground optical observations and auroral imaging (baseline) 2. Exobase-Auroral Transition Zone Probe 1 (500-3000 km) a. Superthermal plasma (0 - 20 eV ions and electrons, resolving 0.1 eV, with aperture bias) b. Precipitating plasma (20 eV - 20k eV ions and electrons) c. E field and wave interferometer (DC - fcO2+ - fpe) d. Magnetometer (DC - fcO+) e. Neutral gas composition analyzer (RGA) 3. Exobase-Auroral Transition Zone probe 2 (500-3000 km) a. Same subelements as for Probe 1 4. Mission & Science Operations Center a. Maintains spacecraft systems and orbits b. Operates instruments c. Merges ground and space data sets into mission database d. Supports community research collaboration

MEME is comparable in scope to the NASA Dynamics Explorer program, which also consisted of two spacecraft, one in circular ~1000 km , the other in a 1000 km x 4.5 RE (geocentric) orbit. The reported cost of DE1-2, including two unique observatories and launch vehicle, was about $65M (FY81$), which inflates to approximately $160M (FY10$). However, DE was undertaken before full cost accounting of NASA missions. The value of additional civil service support to the DE mission is roughly estimated to be FY10$100M (~400 wk-yr). To the above must be added the cost of dedicated supporting ground observations. Thus the full cost of MEME is thought to be in the range $275-300M (FY10$) range. This is a generous figure, since the two MEME spacecraft are identical rather than unique. Ideally, the supporting ground observations would be provided by an agency that is responsible for the relevant facilities, and as such, could be contributed from outside the NASA budget, sharing the overall program cost. 4. Relevance

MEME is designed in response to the STP#5 (ONEP) Science Target based on the fundamental process question: “How are planetary parent gases and plasmas heated, accelerated and transported?” It also aims to answer the societal relevance investigation: “How do the Solar Wind and the Magnetosphere-Ionosphere-Thermosphere systems interact with each other?” Understanding planetary plasma production, energization and transport mechanisms is fundamental in determining how planetary atmospheres lose material and contribute to magnetospheric plasma populations and their dynamics. Some of the matter ultimately escapes into the Solar Wind. Loss of ionospheric plasma is effectively atmospheric loss, since the plasma is created through ionization of the neutral atmosphere. Moreover, upwelling gas and plasma often consists of heavier species, such as atomic oxygen, and during intense activity, molecular atoms and ions. Increased mass density changes the atmospheric drag on low orbiting spacecraft, and also the characteristics of the magnetospheric plasma, by loading down flux tubes with enhanced density, thereby slowing wave propagation and rate of reconnexion.

Assessment of ionospheric plasma distribution throughout the Magnetosphere requires knowledge of how the ions are energized and ejected from Earth’s gravitational potential well. For example, oxygen ions must be heated or accelerated to energies higher than typical

5 Mechanisms of Energetic Mass Ejection ionospheric temperatures to escape; however, can escape relatively easily at ionospheric temperatures. At the same time, since the heavier ions tend to have lower , they are more likely to be affected by convection. Distinctly non-thermal ion velocity distributions are known to be produced in the magnetized but still collisional F region. Prediction of the ejection of these ions requires detailed knowledge of their energy distribution as they leave the collisional Ionosphere.

The fundamental process of atmospheric mass ejection has especially important implications for life on Earth and other planets, not only in the impact of atmospheric loss but also in the understanding of geomagnetic storms, their magnitude, and duration. We must better understand the nonlinear interactions and feedbacks that enhance or moderate electrical currents in the Magnetosphere to successfully predict storm strength and subsequent societal effects. Ionospheric plasma is a major contributor to the plasma pressure and currents throughout the Magnetosphere during these storms. Because the plasma ejection derives from storm strength but also contributes to storm strength, the exchange of mass and energy between the Ionosphere and Magnetosphere constitutes the largest nonlinear feedback system among the terrestrial planets.

MEME satisfies the relevance criteria cited by the Decadal Survey panels, as follows:

a. It is a high priority science target of the 2009 NASA Heliophysics Roadmap/Science Plan.

b. It cuts across all NRC panel themes. Relevance to Solar and Heliospheric Physics lies in the physics of mass loaded plasmas, a common thread applying to the Solar Wind interaction with the solar photospheric gas, dust in the solar system, interstellar gas, planetary atmospheres, and and bolide outgassing.

c. It answers top scientific questions facing solar and space physics today, as identified in the 2003 Decadal survey and 2009 Heliophysics Roadmap, notably: i. What are the sources of energy that drive ionospheric outflows? ii. How are these energy sources partitioned as a function of altitude and location (e.g., the auroral oval versus the dayside cusp)? iii.Where do the different energy sources deposit their energy? iv.How does the neutral atmosphere respond to the energy deposition? v. Does neutral atmosphere variability affect ionospheric response to energy inputs? vi.To what degree do feedback and saturation processes control the outflow of plasma? vii. What feedback is there on the magnetospheric drivers of the outflows?

d. It contributes to applications and-or policy making because prediction cannot be accurate without a more realistic model of Atmosphere-Ionosphere- Magnetosphere-Solar Wind interactions. Contemporary models of the storm time ring current cannot produce a realistic ring current without being “spiked” with additional plasma at the boundary between the outer Magnetosphere and the inner Magnetosphere. This additional plasma is believed to originate in the Ionosphere but is not provided for by existing models, owing to the lack of an physical model of the outflows.

e. It complements and indeed is dependent upon ground observational systems including optical spectral imaging and IS radar observations of the auroral Ionosphere.

f. The cost of MEME to resolve these problems is in the mainstream of typical missions.

g. It is at a readiness level (technical, resources, staffing) that merits implementation.

6 Mechanisms of Energetic Mass Ejection

h. MEME is synergistic with other national and international plans and activities, such as the National Space Weather Program, and other missions such as Cluster, STEREO, THEMIS, SDO, Heliospheric Sentinels, Magnetospheric Multiscale, and RBSP. 5. Discussion

MEME will solve long-standing problems of the origin and loss of terrestrial plasma, from its source in the Atmosphere to the Magnetosphere and Solar Wind. It will use multipoint observations to relate simultaneous conjugate phenomena across key altitude ranges. It will evaluate the importance of electron precipitation, via the creation of superthermal electron distributions or heating of the thermal electrons. And it will determine the source and generation mechanism of the waves that are observed and known to heat ionospheric ions. When these mechanisms are fully understood, it will become possible to derive the heating rates, upward mass fluxes, and composition of mass loss produced by imposed Solar Wind conditions acting upon any upper atmosphere system. This in turn will make possible more accurate and assumption-free predictions of space weather in geospace, as driven by Solar Wind energy inputs and atmospheric ionization, ablation and circulation. MEME is a carefully considered response to the challenge of organizing an interdisciplinary study of the coupled Solar Wind-Magnetosphere-Ionosphere-Atmosphere system. There is an immediate need to advance the predictive capabilities of our increasingly competent models of solar variability and its interactions with planetary bodies, including magneto-plasma coupling leading to terrestrial space weather disturbances. But there is also a long term need to advance our basic understanding of the Heliophysics of planetary system evolution, including the important atmospheric effects of planetary magnetization. 6. Conclusion

Solar variability and planetary magnetization control the flows of matter and energy in our upper atmosphere system, but we cannot begin to predict their nonlinear consequences without a more general understanding of the mechanisms through which stellar winds interact with planetary and atmospheres. Our own home planet offers the best possible opportunity to directly observe magnetized plasma media interacting. 7. Affiliations

1. Heliophysics Sci. Div. Code 670, NASA’s Goddard SFC, Greenbelt, MD 20771 USA 2. Laboratory for Atmos. & Space Phys., Univ. Of Colorado, Boulder, CO 80309 US 3. Dept. Of Physics and Astronomy, Vanderbilt Univ., Nashville, TN, 37235 USA 4. Plasma Physics Div., Naval Research Laboratory, Washington DC, 20007 USA 5. Dept. Atmos. Oceanic, Space Sci., Univ. Of Michigan, Ann Arbor, MI 48109 USA 6. University of NH, Space Science Center, Morse Hall, Durham NH 03824 ISA 7. Dept. Of Physics and Astronomy, Univ. Of Calgary, Calgary, AB T2N1N4 Canada 8. Space Sciences Laboratory, Univ. of Calif. Berkeley, Berkeley, CA 94720 USA 9. Center for Atmos. & Space Sci., Utah State University, Logan UT 84322 USA 10. Dept. Electrical and Computer Engineering, Boston Univ., Boston MA 02215 USA 11. Inst. Geophys. Planet. Phys., Univ. of Calif. in LA, Los Angeles, CA 90024 USA 12. Dept., Univ. Of Colorado, Boulder, CO 80309 US 13. Geophysics Program, Univ. Of Washington, Seattle, WA 98195 USA

7 Mechanisms of Energetic Mass Ejection

References

Barakat, A. R., and R. W. Schunk (2006), A three-dimensional model of the generalized polar wind, J. Geophys. Res., 111, A12314, doi:10.1029/2006JA011662. Burchill, J. K., D. J. Knudsen, et al. (2010), Thermal ion upflow in the cusp ionosphere and its dependence on soft electron energy flux, J. Geophys. Res., 115, A05206, doi: 10.1029/2009JA015006. Heliophysics Roadmap Team Report to the NASA Advisory Council, Heliophysics Subcommittee, (2009), THE SOLAR AND SPACE PHYSICS OF A NEW ERA, Recommended Roadmap for Science and Technology 2009–2030, May 2009, NP-2009-08-76-MSFC, 8-402847. Kistler, L. M., et al. (2006), Ion composition and pressure changes in storm time and nonstorm substorms in the vicinity of the near-Earth neutral line, J. Geophys. Res., 111, A11222, doi: 10.1029/2006JA011939. Lotko, W., (2007), The magnetosphere–ionosphere system from the perspective of plasma circulation: A tutorial, J. Atmos. Solar-Terrest. Phys. 69, p. 191–211. Moore, T. E., and J. L. Horwitz, (2007), Stellar ablation of planetary atmospheres, Rev. Geophys., 45, RG3002, doi:10.1029/ 2005RG000194. Moore, T. E., and G. V. Khazanov, (2010), Mechanisms of Ionospheric Mass Escape, 2010, J. Geophys. Res., DOI: 10.1029/JA014905. Paterson, W. R., and L. A. Frank (1989), Hot ion plasmas from the cloud of neutral gases surrounding the , 94(A4), p. 3721. Strangeway, R. J., R. E. Ergun, Y.-J. Su, C. W. Carlson, and R. C. Elphic (2005), Factors controlling ionospheric outflows as observed at intermediate altitudes, J. Geophys. Res., 110, A03221, doi:10.1029/2004JA010829. Szegö, K. et al. (2000), Physics of mass loaded plasmas, Space Science Reviews 94: 429–671, 2000. Wilson, G. R. (1994), Kinetic modeling of O+ upflows resulting from ExB convection heating in the high-altitude F region ionosphere, J. Geophys. Res., 99, A9, pp. 17453. Zeng, W, and J. L. Horwitz (2007), Formula representation of auroral ionospheric O+ outflows..., Geophys. Res. Lett. 34, L06103, doi:10.1029/2006GL028632. Zheng Y., et al. (2005), Polar study of ionospheric ion outflow versus energy input, J. Geophys. Res., 110, A07210, doi:10.129/2004JA010995.

8