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Geophysical Research Letters

RESEARCH LETTER observations of large-scale compressions 10.1002/2017GL073132 of ’s dawnside Special Section: Daniel J. Gershman1,2 , Gina A. DiBraccio2,3 , John E. P. Connerney2,4 , George Hospodarsky5 , Early Results: Juno at Jupiter William S. Kurth5 , Robert W. Ebert6 , Jamey R. Szalay6 , Robert J. Wilson7 , Frederic Allegrini6,7 , Phil Valek6,7 , J. McComas6,8,9 , Fran Bagenal10 , Key Points: Steve Levin11 , and Scott J. Bolton6 • Jupiter’s dawnside is highly compressible and subject to 1Department of , University of Maryland, College Park, College Park, Maryland, USA, 2NASA Goddard Spaceflight strong Alfvén-magnetosonic mode Center, Greenbelt, Maryland, USA, 3Universities Association, Columbia, Maryland, USA, 4Space Research coupling 5 • Magnetospheric compressions may Corporation, Annapolis, Maryland, USA, Department of and Astronomy, University of Iowa, Iowa City, Iowa, USA, 6 7 enhance reconnection rates and Southwest Research Institute, San Antonio, Texas, USA, Department of Physics and Astronomy, University of Texas at San increase mass transport across the Antonio, San Antonio, Texas, USA, 8Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey, USA, magnetopause 9Office of the VP for the Princeton Physics Laboratory, Princeton University, Princeton, New Jersey, USA, • Total pressure increases inside the 10Laboratory for Atmospheric and , University of Colorado Boulder, Boulder, Colorado, USA, 11Jet Propulsion magnetopause with durations of hours are indicative of strong solar Laboratory, Pasadena, California, USA -magnetosphere energy transport Abstract We investigate the structure of Jupiter’s dawnside magnetopause using observations obtained by particle and fields instrumentation on the Juno spacecraft. Characterization of Jupiter’s magnetopause Supporting Information: • Supporting Information S1 is critical for the understanding of mass and energy transport between the and the magnetosphere. We find an extended magnetopause boundary layer (MPBL) during a magnetopause Correspondence to: crossing on 14 July 2016. This thick MPBL, in combination with a large magnetic field component normal D. J. Gershman, to the magnetopause boundary, suggests that strong magnetospheric compression enhances mass daniel.j.gershman@.gov transport across the magnetopause via . We further identify ~2 h increases in the total magnetospheric pressure adjacent to the magnetopause on 14 July 2016 and 1 August 2016. These Citation: large-scale structures provide evidence of focused energy transport into the magnetosphere via Gershman, D. J., et al. (2017), Juno observations of large-scale magnetohydrodynamic structures. compressions of Jupiter’s dawnside magnetopause, Geophys. Res. Lett., 44, doi:10.1002/2017GL073132. 1. Introduction ’ fi Received 17 FEB 2017 Jupiter s magnetosphere is the largest in the , due to its strong internal magnetic eld (magnetic Accepted 19 JUN 2017 moment ~20,000 times that of ). Centrifugal forces generated by Jupiter’s rotation result in the outward Accepted article online 27 JUN 2017 diffusion of thermal plasma, and this radial transport stretches Jupiter’s dipolar magnetic field out into a mag- netodisc configuration [Gledhill, 1967; Smith et al., 1974; Caudal, 1986; Caudal and Connerney, 1989; Connerney et al., 1981; Kivelson, 2014; Achilleos et al., 2015; Szego et al., 2015; Delamere et al., 2015a]. Outside of the mag- netodisc is a so-called “cushion region,” a region of strongly fluctuating magnetic field that forms preferentially on the dawnside of the magnetosphere from the transport of empty flux tubes back toward the dayside [Smith et al., 1976; Kivelson and Southwood, 2005; Delamere and Bagenal, 2010; Went et al., 2011; Delamere et al., 2015b]. Hot magnetodisc plasmas provide an additional source of pressure with which to stand off the solar wind [Huddleston et al., 1998]. This plasma thermal pressure inflates the size of Jupiter’s magnetospheric cav- ity, and its variability makes the magnetosphere a more compressible obstacle [Slavin et al., 1985; Kurth et al.,

2002]. Consequently, Jupiter’s subsolar magnetopause standoff distance RMP varies substantially between ~50 and 100 RJ [Joy et al., 2002], where RJ is a Jupiter radius (1 RJ = 71,492 km). This variation (ΔRMP), as shown in Figure 1, is the largest in the solar system both in absolute scale and relative to the size of the .

Variability in ΔRMP is associated with large-amplitude transverse perturbations of the magnetopause bound- ary. At Jupiter, these perturbations can be larger than the radius of curvature (RC) of the stretched magneto- disc magnetic fields. High ΔRMP/RC ratios promote increased Alfvén-magnetosonic mode coupling [Wentzel, 1974; Southwood and Saunders, 1985] that can influence the propagation of magnetohydrodynamic (MHD) structures generated at the magnetopause. As shown in Figure 1, this ratio is highest at local times, where the magnetodisc is a factor of ~3 thinner than that either at dusk or on the dayside [Mauk and

©2017. American Geophysical Union. Krimigis, 1987; Khurana et al., 2004]. This enhanced mode coupling at Jupiter may result in dynamics not All Rights Reserved. readily observable in other planetary .

GERSHMAN ET AL. JUPITER’S DAWNSIDE MAGNETOPAUSE 1 Geophysical Research Letters 10.1002/2017GL073132

The transport of mass and energy across Jupiter’s magnetopause occurs due to a combination of magnetic reconnection, viscous processes, and MHD fluctua- tions [Delamere and Bagenal, 2010]. The high ratio of thermal to magnetic pressure (i.e., β) in the and strong flow shear along the flanks are thought to suppress magnetic reconnection at Jupiter [Swisdak et al., 2003; Desroche et al., 2012]. However, Ebert et al. [2017] recently observed Hall magnetic field structures and Alfvénic jets at Jupiter’s magnetopause near the dawn terminator, indicating that reconnection may yet play a role in mass transport. -Helmholtz vortices, which are observed at other planetary magnetospheres [Hasegawa et al., 2004; Masters et al., 2010; Figure 1. Radius of curvature, RC, in the equatorial magnetosphere as a Delamere et al., 2013; Sundberg et al., function of the range in subsolar magnetopause standoff distance, 2012; Gershman et al., 2015; Kavosi ΔR , at [Winslow et al., 2013], Earth [Fairfield, 1971], MP and Raeder, 2015; Ma et al., 2014a, [Achilleos et al., 2008], and Jupiter [Joy et al., 2002; Khurana et al., 2004]. The magnetic field at the terrestrial (i.e., Mercury and Earth) is more 2014b, 2015], are expected to be pre- dipolar than in the stretched magnetodisc configurations of the giant sent, but these structures at Jupiter > Δ planets (i.e., Saturn and Jupiter) such that RC RMP. At the more com- have not yet been reported. Finally, pressible magnetospheres of Saturn and Jupiter, the variation in magne- low-frequency MHD fluctuations with topause standoff distance can exceed the curvature such that ΔR > R . MP C periods up to ~100 s have been Because of Jupiter’s thin magnetodisc on the dawnside [Khurana et al., observed inside of Jupiter’s magneto- 2004], both ΔRMP and the ratio ΔRMP/RC are the largest in the solar system. sphere and in the magnetopause boundary layer (MPBL) [Tsurutani et al., 1993]. These structures can lead to diffusive transport of plasma across the magnetopause [Delamere and Bagenal, 2010]. The Juno spacecraft, which entered into a highly elliptical orbit around Jupiter on 5 July 2016, carries particle and fields instrumentation particularly well suited to the investigation of magnetopause dynamics at Jupiter [Bagenal et al., 2014; Bolton, 2010]. Here we combine magnetic field, electric field, , and data to map Juno’s trajectory through the (PS), plasma sheet boundary layer (PSBL), and the MPBL near local dawn. In addition, we identify distinct, long-duration (~2 h) increases in the total pressure adjacent to the magnetopause, indicative of strong solar-wind-magnetospheric coupling during magnetospheric compressions.

2. Data Analysis

This study used the Jupiter--Orbit (JSO) coordinate system, where the XJSO axis is defined as the unit vector from the center of Jupiter to the Sun, the YJSO axis is defined as the unit vector opposite that of Jupiter’s motion around the Sun, and the ZJSO axis completes the right-handed coordinate system [Bagenal et al., 2014]. The data presented here were collected by the Juno spacecraft at a magnetic local time ~0600 h, i.e., near the dawn terminator. Particle and field data were obtained from the (MAG) instru- ment [Connerney et al., 2017], the plasma investigation (Waves) [Kurth et al., 2017], and the Jovian Auroral Distributions Experiment (JADE) [McComas et al., 2013]. MAG consists of a set of two independent triaxial fluxgate (FGM) sensors each mounted on an ultrastable optical bench with two cameras that provide precise attitude knowledge (up to ~20 arcsec) at each magnetic sensor [Connerney et al., 2017]. FGM sensor data, after application of offsets and scale factors in each axis, were transformed from sensor coordinates into the spacecraft payload frame and

GERSHMAN ET AL. JUPITER’S DAWNSIDE MAGNETOPAUSE 2 Geophysical Research Letters 10.1002/2017GL073132

ultimately inertially referenced using star camera-derived attitudes. We used magnetic field vectors (B) data averaged to 1 s. The Waves investigation measured the magnetic field and electric field from 50 Hz to 20 kHz and 50 Hz to 40 MHz, respectively [Kurth et al., 2017]. In its survey mode of operation, used here, low-frequency (<21 kHz) power spectra were produced at a 2 s cadence. Inside the magnetosphere, continuum emission above the plasma frequency was frequently evidenced in the electric field measurements. This cutoff frequency could be used to estimate the total plasma (n)[Hospodarsky et al., 2017]. Furthermore, a sharp onset or cessation in continuum emission can be used to identify the presence of a magnetopause boundary crossing. Finally, JADE measured (JADE-E) and (JADE-I) between 0.1–100 keV and 0.05–50 keV, respec- tively [McComas et al., 2013]. JADE-I provides an all- of ions every 30 s using the spacecraft rotation to scan the environment. In addition, JADE-I provided ion composition measurements between 1 and 50 amu/e with mass per charge resolution of ~2.5. We used H+ data from JADE-I to study Jupiter’s magneto- pause boundary. The JADE-E observations presented here were obtained from a single sensor in low-rate science mode, where an electron distribution was acquired in 1 min. The field of view of JADE-E near Jupiter’s magnetopause was often sufficient to obtain a complete 0 to 180° distribution of fluxes ordered by magnetic pitch angle. JADE was not at this time operated continuously in orbit around Jupiter, and as such data were only available for one of the events discussed here.

3. Results 3.1. Magnetopause Crossing on 14 July 2016 Data collected from MAG, Waves, and JADE during a magnetopause crossing on 14 July 2016 are shown in Figure 2 with an illustration of Juno’s trajectory during this interval transformed into a frame of reference that

moves with the magnetopause boundary. At a radial distance of ~80 RJ near the dawn terminator, Juno was inside of the magnetosphere and south of the magnetodisc current sheet (from 0000 to 0400 UT, first light

blue bar in Figure 2) as evidenced by the stretched BX > 0, BY > 0 magnetic field and the presence of counter- À streaming electrons (n ~ 0.01 cm 3, B~6nT). At ~0400 UT, the field magnitude decreased and the plasma (estimated from the peak in the energy spectrogram) increased substantially, and the electrons became more isotropic. These signatures indicated the passage of the spacecraft into plasma sheet À (n ~ 0.002 cm 3). The PSBL (first dark blue bar in Figure 2) on the edge of the plasma sheet was identified as a region with both hot plasmas and counterstreaming electrons. At 10:52 UT, Juno entered a MPBL containing mixed magnetosheath (cold) and magnetospheric (hot) mate- À rial (n ~ 0.01 cm 3)(first purple bar in Figure 2). The presence of counterstreaming electrons in this region suggests that the observed MPBL was topologically connected to Jupiter on closed magnetic field lines. A partial magnetopause crossing was observed at 13:35 UT where the continuum emission above the plasma frequency ceased and lower energy particle fluxes increased without a large rotation of the magnetic field vector. Juno then remained in the MPBL until reentry into the plasma sheet at 16:00 UT, as a result of expan- sion of Jupiter’s magnetosphere. In the plasma sheet, from 15:36 to 17:45 UT instruments observed a strong enhancement of total pressure, as evidenced by increased plasma fluxes and a doubling of the magnetic field À strength (n ~ 0.005 cm 3, B~10 nT). This compression region appears shaded in Figure 2. Juno finally crossed the magnetopause (see Figure S1 in the supporting information for a close-up view) at

21:18 UT after passing through the PSBL and closed-field MPBL at ~80 RJ (i.e., a compressed magnetosphere [Joy et al., 2002]). The magnetopause boundary was identified by a sudden dropout in emission, increased fluxes of magnetosheath ions, and a large rotation in the direction of the magnetic field. However, beyond the magnetopause, magnetospheric electrons (E > 100 eV) are observed from 21:18 to 22:20 UT. These electrons were on magnetosheath field lines as evidenced by the unidirectional field-aligned beam associated with a magnetic connection to the Sun. The MPBL, therefore, is present both inside and outside of the magnetosphere. We assumed a pressure-balanced magnetopause and took the maximum

pressure to be equal to the magnetic pressure outside of the plasma sheet, where βMS ≪ 1. Using βMSH ≫ βMS, [see DiBraccio et al., 2013], we estimated a maximum Δβ ~ 3 across the magnetopause, which represents an upper bound due to increased βMS values in the plasma sheet.

GERSHMAN ET AL. JUPITER’S DAWNSIDE MAGNETOPAUSE 3 Geophysical Research Letters 10.1002/2017GL073132

Figure 2. Juno’s passage through Jupiter’s magnetopause near the dawn terminator on 14 July 2016 from 78.25 to 82.53 RJ. (a) Electron energy-time count spectro- gram averaged over all look directions from JADE-E. (b) Magnetic pitch angle spectrogram of ~500 eV electrons from JADE-E. For a given spectra, each pitch angle bin is normalized by the average number of counts observed in all pitch angles to enhance relative structure in the distribution independent of incident particle flux. (c) H+ energy-time counts spectrogram averaged over all look directions from JADE-I. (d–g) Magnetic field vector B in JSO coordinates and field magnitude. (h) Frequency-time power spectrogram of electric field fluctuations between 50 Hz and 20 kHz from Waves. (i) Schematic of Juno trajectory in a frame of reference that moved with the magnetopause. For illustration purposes, the magnetopause is marked as a specific boundary, whereas for this event it is “open” due to reconnection between the magnetosheath and magnetospheric fields. The plasma sheet (PS), plasma sheet boundary layer (PSBL), high-latitude magnetosphere (HLMS), magnetopause boundary layer (MPBL), and magnetosheath (MSH) are indicated with different colors in both the illustration and as solid horizontal bars above Figures 2a, 2e, and 2h. The magnetopause is indicated with a vertical dashed line. A ~2 h pressure structure in the magnetic field magnitude is highlighted with a shaded gray region.

Minimum variance analysis [Sonnerup and Cahill, 1967] performed on the magnetopause boundary from

21:17:50 to 21:20:40 UT resulted in a boundary normal vector of [0.82,À0.57,À0.10]JSO. This normal direction with |YJSO| < |XJSO| was distorted from the average |YJSO| > |XJSO| direction expected near the dawn termina- tor and instead was tilted to what has been typically observed at the subsolar magnetopause. The normal component of the field (1.2 nT) was ~20% of the magnetospheric field strength (6.7 nT), indicating that the magnetopause was a rotational discontinuity, i.e., “open.” For this crossing, the local magnetic shear angle between the magnetosheath and magnetosphere fields was ~90o. The complete set of eigenvectors and

corresponding eigenvalues for this crossing were ([0.82, À0.57, À0.10]JSO, 0.06), ([0.46,0.54,0.71]JSO, 0.50), and ([À0.35, À0.62,0.70]JSO, 4.55). 3.2. Magnetopause Crossing on 1 August 2016

A magnetopause crossing observed by Juno at a distance of ~110 RJ (i.e., an expanded magneto- sphere [Joy et al., 2002]) on 1 August 2016 is shown in Figure 3. Although JADE was not operating during this time period, Waves and MAG data were. Continuum wave emissions above the plasma frequency indicated À that Juno was in the magnetosphere during the start of the interval (n ~ 0.002 cm 3, B~4 nT). The absence of

a ~10 h periodicity corresponding to the rotation of the magnetodisc around Jupiter, the fluctuations of BX

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Figure 3. Juno’s passage through Jupiter’s magnetopause near the dawn terminator on 1 August 2016 near ~113 RJ.(a–c) Magnetic field vector B in JSO coordinates, À1 À1 (d) azimuthal angle, i.e., tan (By, Bx) of the magnetic field, (e) angle, i.e., cos (Bz/B), of the magnetic field, (f) magnetic field magnitude, and (g) frequency-time power spectrogram of electric field fluctuations between 50 Hz and 20 kHz from Waves. The magnetosphere (MS) and magnetosheath (MSH) are indicated by blue and red horizontal bars above Figures 3a, 3d, and 3g, respectively. Multiple magnetopause crossings are indicated with vertical dashed lines. A ~2 h pressure structure in the magnetic field near the end of 31 July 2016 and beginning of 1 August 2016 is indicated with the shaded gray region.

and BY, and the low density may have indicated that Juno was in the cushion region. The magnetic field strength doubled from 23:00:00 UT on 31 July to 00:30:00 UT on 1 August, similar to the pressure enhancement observed in the 14 July event. This increase was accompanied by a modest field rotation of À1 ~75° in the XJSO-YJSO plane. The angle of B with the Z axis, i.e., cos (BZ/B), changed by less than ~15° across the compression structure. Juno crossed the magnetopause at 19:36 UT as evidenced by the rotation in the magnetic field and the drop- out in continuum radiation (see Figure S2 in the supporting information for a close-up view). The spacecraft then briefly returned to the magnetosphere between 20:56 and 21:56 UT. Multiple magnetopause crossings over the course of ~3 h indicates motion of the boundary during this time period, i.e., magnetospheric compression and expansion or boundary wave motion. In contrast to the 14 July event, here there was only À a modest increase in density (n ~ 0.005 cm 3) immediately adjacent to the magnetopause inside the magne- tosphere. This narrow increase suggested that no comparable MPBL formed during this time period. We estimated a maximum Δβ ~ 8 across the magnetopause. Minimum variance analysis of the first crossing (from 19:36:00 to 19:37:06 UT) resulted in a normal vector

direction of [0.81,À0.56, À0.21]JSO similar to the previous event. In this case, however, the small normal component (<0.1 nT) when compared to the magnetic field strength inside the magnetosphere (4.4 nT) suggested that the magnetopause was a tangential discontinuity, i.e., “closed”. For this crossing, the local magnetic shear angle between the magnetosheath and magnetosphere fields was ~70°. The complete set

of eigenvectors and corresponding eigenvalues for this first crossing were ([0.81, À0.56, À0.21]JSO, 0.01), ([0.46,0.81,0.36]JSO, 0.06), and ([À0.37, À0.20,0.91]JSO, 2.31).

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4. Discussion The multiple crossings of the MPBL and magnetopause on 14 July and 1 August, respectively, indicate that Jupiter’s magnetosphere was adjusting in size and shape due to variations in solar wind [McComas et al., 2017] and magnetosheath properties during these times. The proximity of Juno to the magnetopause for each event and the natural variation of Jupiter’s magnetospheric cavity size suggest that the features observed in the instrument data were related to magnetopause dynamics rather than internal sources. We therefore discuss the MPBL and magnetic field strength enhancements in terms of magnetospheric driving by the solar wind.

4.1. Boundary Layer Formation The presence of magnetosheath material on closed field lines and magnetospheric material on open field lines in the MPBL, as observed here and by Voyager [Scudder et al., 1981] and [Galvin et al., 1993], is a direct consequence of mass transport across the magnetopause in both directions. The large normal magnetic field determined from MVA for the 14 July event suggested that magnetic reconnection was responsible for this plasma entry. Given the outward radial of the spacecraft of ~4 km/s, its initial entry into the MPBL at 10:52 UT, and assumption that the magnetopause was approximately at rest at the time of the partial crossing at 13:35 (i.e., it did not have sufficient speed to result in a full crossing), we estimated a

MPBL thickness of ~0.5 RJ. This thickness is several times larger than the ~0.15 RJ thick boundary layers previously reported at Jupiter [Sonnerup et al., 1981; Tsurutani et al., 1997]. This extended MPBL may have been responsible for the absence of a significant cushion region on 14 July, as empty flux tubes returning to the dayside were partially refilled with magnetosheath plasma. The rate of magnetic reconnection is reduced when the velocity of the reconnection X line is in the same direction as the reconnection outflow [Swisdak et al., 2003; Doss et al., 2015]. This limitation inhibits recon- nection between the draped magnetosheath fields and stretched magnetospheric fields along the magnetopause flank. At the magnetic equator, the magnetospheric fields are more vertical, which results

in reconnection outflows with increased ±ZJSO components [Ebert et al., 2017]. However, without a strong component of the magnetosheath flow normal to the magnetopause boundary, there is no force that initiates merging between magnetosheath and magnetospheric fields. Perturbations of the magneto- pause boundary such as global compression or boundary waves such as Kelvin-Helmholtz vortices are therefore required to enable this merging to occur. We note that Swisdak et al. [2003] predicted that reconnection could have occurred for the combination of shear angle and Δβ for the 14 July event, but not the 1 August event. Two reconnection events near along Jupiter’s dawnside magnetopause were recently analyzed by Ebert

et al. [2017]. Both events had reconnection exhausts in the +ZJSO direction, consistent with the merging of fields near the magnetic equator. The first event (on 25 June) had a magnetopause boundary normal in

the ÀYJSO direction. This boundary normal direction more closely resembled that expected along the magnetospheric flank than near the dawn terminator [Joy et al., 2002]. The second event (29 June) exhib- ited multiple magnetopause crossings, indicative of small-amplitude boundary waves. However, Waves data indicated that neither of these events exhibited prolonged increases of density associated with an extended MPBL. Juno observations on 14 July, evidenced an isolated magnetopause crossing with a sunward tilted

magnetopause boundary normal (i.e., |XJSO| > |YJSO|), and provided an “open” magnetopause. This cross- ing was surrounded by an extended MPBL. As shown in Figure 4, during a magnetospheric compression, a surface that connects the uncompressed magnetopause to the compressed magnetopause will propa-

gate antisunward at the magnetosheath flow speed. This compression imparts an increased +XJSO component to the boundary normal. The draping of the magnetosheath field on the magnetopause flank during a strong magnetospheric compression can therefore resemble that of the subsolar region, result- ing in a globally increased rate of magnetic reconnection and an extended MPBL. Such extreme modifi- cations of magnetopause geometry will be more common in highly compressible magnetospheres, i.e., at Jupiter and Saturn. Here the formation of an extended MPBL is attributed to a global increase in magnetic reconnection rates along the magnetopause rather than localized increases associated with smaller-scale boundary motion.

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Figure 4. An illustration of a magnetospheric compression at Jupiter in the equatorial (XJSO-YJSO) plane. As the magneto- pause is compressed by distance ΔRMP, a front with an increased +XJSO boundary normal direction propagates antisun- ward with the magnetosheath flow. Along this compression front, magnetic reconnection between magnetosheath and magnetospheric fields in the equatorial plane can generate ±ZJSO outflow jets, and results in mass transport across the magnetopause. In addition, MHD-scale perturbations will be continuously generated along the front and propagate into the magnetosphere. A superposition of fast-mode structures generated at times t0, t1, and t2 along the dayside magne- topause will travel ahead of the compression front, potentially leading to a long-duration (i.e., t2 À t0) increase in magnetic field strength at an observer inside the magnetosphere near the dawn terminator.

4.2. Large-Scale Strength Enhancements Localized increases in the total pressure are not uncommon in planetary magnetospheres. Such increases at Earth have been associated with fast magnetosonic fluctuations [Wolfe and Kaufmann, 1975], magne- tosheath jets [Dmitriev and Suvorova, 2012], Kelvin-Helmholtz vortices [Hasegawa et al., 2004], flux ropes [Rijnbeek et al., 1984], and and associated traveling compression regions [Slavin et al., 2003]. At Jupiter, as at Earth, many of these phenomena have been observed with timescales of minutes [Tsurutani et al., 1993; Russell et al., 1998; Vogt et al., 2014]. Temporal scales on the order of minutes

combined with flow velocities of ~10–100 km/s result in spatial structures ~1–10 RJ in size, a small fraction of the magnetosphere. In addition, distortions of the current sheet on the nightside have been observed to last ~1 day (i.e., over several Jupiter rotations) and are associated with loading and unloading of magnetic flux into Jupiter’s magnetosphere [Tao et al., 2005]. However, localized structures with increased magnetic field magnitude lasting ~2 h near dawn have not been reported. The field variation of these compression structures, in particular, for the 1 August event, did not exhibit bipolar signatures expected of flux ropes or boundary waves. A standing slow-mode structure embedded in the background magnetospheric flow (≪1000 km/s) could

have had a reasonable scale size <10 RJ for a localized structure. However, because the observed increase in magnetic pressure was not balanced by a decrease in plasma pressure, the structure could not have been an MHD slow mode. The total pressure (thermal + magnetic) can increase in fast-mode structures, but such structures must move at the fast magnetosonic speed, which exceeded ~1000 km/s in this region À (n ~ 0.005 cm 3, B ~ 5 nT, T ~ several keV). A structure moving at these that persisted for ~2 h in space-

craft observations would have had a scale size of ~100 RJ, i.e., it would have taken up a sizeable fraction of

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Jupiter’s dayside magnetosphere. We therefore conclude that the observed pressure increase arose from a continuous generation process rather than from a single, impulsive event. Magnetospheric compressions will continuously generate MHD fluctuations along the magnetopause [Tamao, 1975; Wilken et al., 1982]. As illustrated in Figure 4, MHD structures with scale sizes on the order of

ΔRMP are generated at a compression front as it propagates along the dawn flank. Although ideal Alfvén waves are incompressible, as they traverse the strong curvature in the stretched magnetodisc fields, they can develop a compressional component [Wentzel, 1974]. In addition, fast-mode structures that were directly generated at the magnetopause near the equator can propagate deeper into the magnetosphere. Moving at the fast magnetosonic speed (>1000 km/s), these structures can travel ahead of the magnetopause compres- sion front that moves at the magnetosheath flow speed. An observer at dawn adjacent to the magnetopause (but inside the magnetosphere) would have measured a superposition of these pressure structures that mapped to sources all along the dayside magnetopause. With a magnetosheath flow speed of ~100 km/s, a ~2 h duration event would therefore have mapped to MHD structures being continuously generated along

~50 RJ of arc length on the magnetopause. While more data are needed to definitively identify the origin of these large-scale compressions structures, because of their proximity to the magnetopause, they indicated the transport of energy from the solar wind into the magnetosphere. Jupiter’s unique dawnside magnetospheric geometry may have enabled such signatures to exist in a way not possible at other planetary magnetospheres. Furthermore, these compression structures should have mapped, via field-aligned currents, to Jupiter’s polar [Vogt et al., 2011]. However, because flux tubes along the dawn-side have been depleted of plasma due to reconnection in the magnetotail [Cowley et al., 2003; Kivelson and Southwood, 2005], it was unlikely that these magneto- spheric structures caused observable features in Jupiter’s relative to other local times.

5. Concluding Remarks We have resolved the structure of Jupiter’s dawn magnetopause using particle and fields instrumentation on the Juno spacecraft. During a magnetopause crossing on 14 July, we identified a thick boundary layer of mixed magnetosheath-magnetosphere material adjacent to the magnetopause, estimated to be several times thicker than previously reported. The strong sunward component of the magnetopause boundary normal, large normal component of the magnetic field, and an extended MPBL suggested that a strong magnetospheric compression enabled increased merging between magnetosheath and magnetospheric fields and subsequent mass transport across the magnetopause. In addition, ~2 h increases in total magneto-

spheric pressure were observed on 14 July and 1 August adjacent to the magnetopause, where ΔRMP/RC ~1 and strong Alfvén-magnetosonic mode coupling was expected. These structures provided evidence of energy transport into the magnetosphere via magnetosonic structures that were generated along the

dayside magnetopause during times of strong magnetospheric compression. With the largest ΔRMP and ΔRMP/RC in the solar system, Jupiter therefore provides an end-member in parameter space for the study of solar wind-magnetosphere coupling under extreme driving conditions.

Acknowledgments References All Juno data presented here is publicly ’ available from NASA’s Planetary Data Achilleos, N., C. S. Arridge, C. Bertucci, C. M. Jackman, M. K. Dougherty, K. K. Khurana, and C. T. Russell (2008), Large-scale dynamics of Saturn s System as part of the JNO-SS-3-FGM- magnetopause: Observations by Cassini, J. Geophys. Res., 113, A11209, doi:10.1029/2008JA013265. CAL-V1.0, JNO-J/SW-JAD-3- Achilleos, N., N. Andre, X. Blanco-Cano, P. C. Brandt, P. A. Delamere, and R. Winglee (2015), 1. Transport of mass, momentum and energy in – CALIBRATED-V1.0, and JNO-E/J/SS- planetary magnetodisc regions, Space Sci. Rev., 187, 229 299, doi:10.1007/s11214-014-0086-y. WAV-3-CDR-SRVFULL-V1.0 data sets for Bagenal, F., et al. (2014), Magnetospheric science objectives of the Juno mission, Space Sci. Rev., doi:10.1007/s11214-014-0036-8. – the MAG, Waves, and JADE instruments, Bolton, S. J. (2010), The Juno mission, in IAU Symposium, vol. 269, edited by C. Barbieri et al., pp. 92 100, Cambridge Univ. Press, Cambridge, U. K. ’ respectively. This work was supported Caudal, G. (1986), A self-consistent model of Jupiter s magnetodisc including the effects of centrifugal force and pressure, J. Geophys. Res., – by the Juno mission. 91(A4), 4201 4221, doi:10.1029/JA091iA04p04201. Caudal, G., and J. E. P. Connerney (1989), Plasma pressure in the environment of Jupiter, inferred from magnetometer observations, J. Geophys. Res., 94, 15,055–15,061, doi:10.1029/JA094iA11p15055. Connerney, J. E. P., M. H. Açuna, and N. F. Ness (1981), Modeling the Jovian current sheet and inner magnetosphere, J. Geophys. Res., 86, 8370–8384. Connerney, J. E. P., et al. (2017), The Juno magnetic field investigation, Space Sci. Rev., doi:10.1007/s11214-017-0334-z. Cowley, S. W. H., E. J. Bunce, T. S. Stallard, and S. Miller (2003), Jupiter’s polar ionospheric flows: Theoretical interpretation, Geophys. Res. Lett., 30(5), 1220, doi:10.1029/2002GL016030. Delamere, P. A., and F. Bagenal (2010), Solar wind interaction with Jupiter’s magnetosphere, J. Geophys. Res., 115, A10201, doi:10.1029/ 2010JA015347.

GERSHMAN ET AL. JUPITER’S DAWNSIDE MAGNETOPAUSE 8 Geophysical Research Letters 10.1002/2017GL073132

Delamere P. A., R. J. Wilson, S. Eriksson, and F. Bagenal (2013), Magnetic signatures of Kelvin-Helmholtz vortices on Saturn’s magnetopause: Global survey, J. Geophys. Res. Space Physics, 118, 393–404, doi:10.1029/2012JA018197. Delamere, P. A.,F.Bagenal, C. Paranicas,A. Masters,A. Radioti, B.Bonfond,L.Ray, X. Jia,J.Nichols,andC. Arridge (2015a),Solarwind and internally driven dynamics: Influences on Magnetodiscs and Auroral responses, Space Sci. Rev., 187,51–97, doi:10.1007/s11214-014-0075-1. Delamere, P. A., A. Otto, X. Ma, F. Bagenal, and R. J. Wilson (2015b), Magnetic flux circulation in the rotationally driven giant magnetospheres, J. Geophys. Res. Space Physics, 120, 4229–4245, doi:10.1002/2015JA021036. Desroche, M., F. Bagenal, P. A. Delamere, and N. Erkaev (2012), Conditions at the expanded Jovian magnetopause and implications for the solar wind interaction, J. Geophys. Res., 117, A07202, doi:10.1029/2012JA017621. DiBraccio, G. A., J. A. Slavin, S. A. Boardsen, B. J. Anderson, H. Korth, T. H. Zurbuchen, J. M. Raines, D. N. Baker, R. L. McNutt Jr., and S. C. Solomon (2013), MESSENGER observations of magnetopause structure and dynamics at Mercury, J. Geophys. Res. Space Physics, 118, 997–1008, doi:10.1002/jgra.50123. Dmitriev, A. V., and A. V. Suvorova (2012), Traveling magnetopause distortion related to a large-scale magnetosheath plasma jet: THEMIS and ground-based observations, J. Geophys. Res., 117, A08217, doi:10.1029/2011JA016861. Doss, C. E., C. M. Komar, P. A. Cassak, F. D. Wilder, S. Eriksson, and J. F. Drake (2015), Asymmetric magnetic reconnection with a flow shear and applications to the magnetopause, J. Geophys. Res. Space Physics, 120, 7748–7763, doi:10.1002/2015JA021489. Ebert, R. W., et al. (2017), Accelerated flows at Jupiter’s magnetopause: Evidence for magnetic reconnection along the Dawn flank, Geophys. Res. Lett., 44, 4401–4409, doi:10.1002/2016GL072187. Fairfield, D. H. (1971), Average and unusual locations of the Earth’s magnetopause and , J. Geophys. Res., 76(28), 6700–6716, doi:10.1029/JA076i028p06700. Galvin, A. B., C. M. S. Cohen, F. M. Ipavich, R. von Steiger, J. Woch, and U. Mall (1993), Boundary layer ion composition at Jupiter during the inbound pass of the Ulysses flyby, Planet. Space Sci., 41, 869–876, doi:10.1016/0032-0633(93)90094-I. Gershman, D. J., J. M. Raines, J. A. Slavin, T. H. Zurbuchen, T. Sundberg, S. A. Boardsen, B. J. Anderson, H. Korth, and S. C. Solomon (2015), MESSENGER observations of multiscale Kelvin-Helmholtz vortices at Mercury, J. Geophys. Res. Space Physics, 120, 4354–4368, doi:10.1002/ 2014JA020903. Gledhill, J. A. (1967), Magnetosphere of Jupiter, Nature, 214, 155. Hasegawa, H., M. Fujimoto, T.-D. Phan, H. Reme, A. Balogh, M. W. Dunlop, C. Hashimoto, and R. TanDokoro (2004), Transport of solar wind into Earth’s magnetosphere through rolled-up Kelvin-Helmholtz vortices, Nature, 430, 755. Hospodarsky, G. B., et al. (2017), Jovian bow shock and magnetopause encounters by the Juno spacecraft, Geophys. Res. Lett., 44,7, doi:10.1002/2017GL073177. Huddleston, D. E., C. T. Russell, M. G. Kivelson, K. K. Khurana, and L. Bennett (1998), Location and shape of the Jovian magnetopause and bow shock, J. Geophys. Res., 103(E9), 20,075–20,082, doi:10.1029/98JE00394. Joy, S. P., M. G. Kivelson, R. J. Walker, K. K. Khurana, C. T. Russell, and T. Ogino (2002), Probabilistic models of the Jovian magnetopause and bow shock locations, J. Geophys. Res., 107(A10), 1309, doi:10.1029/2001JA009146. Kavosi, S., and J. Raeder (2015), Ubiquity of Kelvin-Helmholtz waves at Earth’s magnetopause, Nat. Commun., 6, 7019, doi:10.1038/ ncomms8019. Kivelson, M. G. (2014), Planetary magnetodiscs: Some unanswered questions, Space Sci. Rev., 187,5–21, doi:10.1007/s11214-014-0046-6. Kivelson, M. G., and D. J. Southwood (2005), Dynamical consequences of two modes of centrifugal instability in Jupiter’s outer magnetosphere, J. Geophys. Res., 110, A12209, doi:10.1029/2005JA011176. Khurana, K. K., M. G. Kivelson, V. M. Vasyliunas, N. Krupp, J. Woch, A. Lagg, B. H. Mauk, and W. S. Kurth (2004), The configuration of Jupiter’s magnetosphere, in Jupiter: The Planet, Satellites, and Magnetosphere, pp. 593–616, edited by F. Bagenal, T. E. Dowling, and W. B. McKinnon, Cambridge Univ. Press, New York. Kurth, W. S., et al. (2002), The dusk flank of Jupiter’s magnetosphere, Nature, 415, 991–994. Kurth, W. S., G. B. Hospodarsky, D. L. Kirchner, B. T. Mokrzycki, T. F. Averkamp, W. T. Robison, C. W. Piker, M. Sampl and P. Zarka (2017), The Juno Waves investigation, Space Sci. Rev., doi:10.1007/s11214-017-0396-y, in press. Ma, X., A. Otto, and P. A. Delamere (2014a), Interaction of magnetic reconnection and Kelvin-Helmholtz modes for large magnetic shear: 1. Kelvin-Helmholtz trigger, J. Geophys. Res. Space Physics, 119, 781–797, doi:10.1002/2013JA019224. Ma, X., A. Otto, and P. A. Delamere (2014b), Interaction of magnetic reconnection and Kelvin-Helmholtz modes for large magnetic shear: 2. Reconnection trigger, J. Geophys. Res. Space Physics, 119, 808–820, doi:10.1002/2013JA019225. Ma, X., B. Stauffer, P. A. Delamere, and A. Otto (2015), Asymmetric Kelvin-Helmholtz propagation at Saturn’s dayside magnetopause, J. Geophys. Res. Space Physics, 120, 1867–1875, doi:10.1002/2014JA020746. Masters, A., et al. (2010), Cassini observations of a Kelvin-Helmholtz vortex in Saturn’s outer magnetosphere, J. Geophys. Res., 115, A07225, doi:10.1029/2010JA015351. Mauk, B. H., and S. M. Krimigis (1987), Radial force balance within Jupiter’s dayside magnetosphere, J. Geophys. Res., 92, 9931–9941, doi:10.1029/JA092iA09p09931. McComas, D. J., et al. (2013), The Jovian Auroral Distributions Experiment (JADE) on the Juno mission to Jupiter, Space Sci. Rev., doi:10.1007/ s11214-013-9990-9. McComas, D. J., et al. (2017), Plasma environment at the dawn flank of Jupiter’s magnetosphere: Juno arrives at Jupiter, Geophys. Res. Lett., doi:10.1002/2017GL072831. Rijnbeek, R. P., S. W. H. Cowley, D. J. Southwood, and C. T. Russell (1984), A survey of dayside flux transfer events observed by ISEE 1 and 2 magnetometers, J. Geophys. Res., 89(A2), 786–800, doi:10.1029/JA089iA02p00786. Russell, C. T., K. K. Khurana, D. E. Huddleston, and M. G. Kivelson (1998), Localized reconnection in the near Jovian magnetotail, Science, 280, 1061, doi:10.1126/science.280.5366.1061. Scudder, J. D., E. C. Sittler Jr., and H. S. Bridge (1981), A survey of the plasma electron environment of Jupiter: A view from Voyager, J. Geophys. Res., 86(A10), 8157–8179, doi:10.1029/JA086iA10p08157. Slavin, J. A., E. J. Smith, J. R. Spreiter, and S. S. Stahara (1985), Solar wind flow about the outer planets: Gas dynamic modeling of the Jupiter and Saturn bow shocks, J. Geophys. Res., 90(A7), 6275–6286, doi:10.1029/JA090iA07p06275. Slavin, J. A., R. P. Lepping, J. Gjerloev, D. H. Fairfield, M. Hesse, C. J. Owen, M. B. Moldwin, T. Nagai, A. Ieda, and T. Mukai (2003), observations of magnetic flux ropes in the plasma sheet, J. Geophys. Res., 108(A1), 1015, doi:10.1029/2002JA009557. Smith, E. J., L. Davis Jr., D. E. Jones, P. J. Coleman Jr., D. S. Colburn, P. Dyal, C. P. Sonett, and A. M. A. Frandsen (1974), The planetary magnetic field and magnetosphere of Jupiter: , J. Geophys. Res., 79, 3501–3513, doi:10.1029/JA079i025p03501. Smith, E. J., L. Davis Jr., and D. E. Jones (1976), Jupiter’s magnetic field and magnetosphere, in Jupiter, pp. 788–829, Univ. of Ariz. Press, Tucson.

GERSHMAN ET AL. JUPITER’S DAWNSIDE MAGNETOPAUSE 9 Geophysical Research Letters 10.1002/2017GL073132

Sonnerup, B. U., and L. J. Cahill Jr. (1967), Magnetopause structure and attitude from observations, J. Geophys. Res., 72(1), 171–183, doi:10.1029/JZ072i001p00171. Sonnerup, B. U., E. J. Smith, B. T. Tsurutani, and J. H. Wolfe (1981), Structure of Jupiter’s magnetopause: Pioneer 10 and 11 observations, J. Geophys. Res., 86(A5), 3321–3334, doi:10.1029/JA086iA05p03321. Southwood, D. J., and M. A. Saunders (1985), Curvature coupling of slow and Alfvén MHD waves in a magnetotail field configuration, Planet. Space Sci., 33, 127. Sundberg, T., S. A. Boardsen, J. A. Slavin, B. J. Anderson, H. Korth, T. H. Zurbuchen, J. M. Raines, and S. C. Solomon (2012), MESSENGER orbital observations of large-amplitude Kelvin-Helmholtz waves at Mercury’s magnetopause, J. Geophys. Res., 117, A04216, doi:10.1029/ 2011JA017268. Swisdak, M., B. N. Rogers, J. F. Drake, and M. A. Shay (2003), Diamagnetic suppression of component magnetic reconnection at the magnetopause, J. Geophys. Res., 108(A5), 1218, doi:10.1029/2002JA009726. Szego, K., N. Achilleos, C. Arridge, S. V. Badman, P. Delamere, D. Grodent, M. G. Kivelson, and P. Louarn (2015), Giant planet magnetodiscs and aurorae: An introduction, Space Sci. Rev., 187,1–3, doi:10.1007/s11214-014-0131-x. Tamao, T. (1975), Unsteady interactions of solar wind disturbances with the magnetosphere, J. Geophys. Res., 80(31), 4230–4236, doi:10.1029/ JA080i031p04230. Tao, C., R. Kataoka, H. Fukunishi, Y. Takahashi, and T. Yokoyama (2005), Magnetic field variations in the Jovian magnetotail induced by solar wind dynamic pressure enhancements, J. Geophys. Res., 110, A11208, doi:10.1029/2004JA010959. Tsurutani, B. T., et al. (1997), Plasma wave characteristics of the Jovian magnetopause boundary layer: Relationship to the Jovian aurora?, J. Geophys. Res., 102(A3), 4751–4764, doi:10.1029/96JA02785. Tsurutani, B. T., D. J. Southwood, E. J. Smith, and A. Balogh (1993), A survey of low-frequency waves at Jupiter: The ULYSSES encounter, J. Geophys. Res., 98, 21,203–21,216, doi:10.1029/93JA02586. Vogt, M. F., M. G. Kivelson, K. K. Khurana, R. J. Walker, B. Bonfond, D. Grodent, and A. Radioti (2011), Improved mapping of Jupiter’s auroral features to magnetospheric sources, J. Geophys. Res., 116, A03220, doi:10.1029/2010JA016148. Vogt, M. F., C. M. Jackman, J. A. Slavin, E. J. Bunce, S. W. H. Cowley, M. G. Kivelson, and K. K. Khurana (2014), Structure and statistical properties of plasmoids in Jupiter’s magnetotail, J. Geophys. Res. Space Physics, 119, 821–843, doi:10.1002/2013JA019393. Went, D. R., M. G. Kivelson, N. Achilleos, C. S. Arridge, and M. K. Dougherty (2011), Outer magnetospheric structure: Jupiter and Saturn compared, J. Geophys. Res., 116, A04224, doi:10.1029/2010JA016045. Wentzel, D. G. (1974), Coronal heating by Alfvén waves, Sol. Phys., 39, 129, doi:10.1007/BF00154975. Wolfe, A., and R. L. Kaufmann (1975), MHD wave transmission and production near the magnetopause, J. Geophys. Res., 80(13), 1764–1775, doi:10.1029/JA080i013p01764. Wilken, B., C. K. Goertz, D. N. Baker, P. R. Higbie, and T. A. Fritz (1982), The SSC on July 29, 1977 and its propagation within the magnetosphere, J. Geophys. Res., 87(A8), 5901–5910, doi:10.1029/JA087iA08p05901. Winslow, R. M., B. J. Anderson, C. L. Johnson, J. A. Slavin, H. Korth, M. E. Purucker, D. N. Baker, and S. C. Solomon (2013), Mercury’s magnetopause and bow shock from MESSENGER magnetometer observations, J. Geophys. Res. Space Physics, 118, 2213–2227, doi:10.1002/jgra.50237.

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