PUBLICATIONS Geophysical Research Letters RESEARCH LETTER Juno observations of large-scale compressions 10.1002/2017GL073132 of Jupiter’s dawnside magnetopause 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 , David J. McComas6,8,9 , Fran Bagenal10 , Key Points: Steve Levin11 , and Scott J. Bolton6 • Jupiter’s dawnside magnetosphere is highly compressible and subject to 1Department of Astronomy, University of Maryland, College Park, College Park, Maryland, USA, 2NASA Goddard Spaceflight strong Alfvén-magnetosonic mode Center, Greenbelt, Maryland, USA, 3Universities Space Research Association, Columbia, Maryland, USA, 4Space Research coupling 5 • Magnetospheric compressions may Corporation, Annapolis, Maryland, USA, Department of Physics 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 Plasma Physics Laboratory, Princeton University, Princeton, New Jersey, USA, • Total pressure increases inside the 10Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA, 11Jet Propulsion magnetopause with durations of hours are indicative of strong solar Laboratory, Pasadena, California, USA wind-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 solar wind 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 [email protected] transport across the magnetopause via magnetic reconnection. 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 solar system, due to its strong internal magnetic eld (magnetic Accepted 19 JUN 2017 moment ~20,000 times that of Earth). 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 planet. 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 dawn 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 magnetospheres. 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 magnetosheath 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. Kelvin-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 Mercury [Winslow et al., 2013], Earth [Fairfield, 1971], Saturn 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 planets (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, electron, and ion data to map Juno’s trajectory through the plasma sheet (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-Sun-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 magnetometer (MAG) instru- ment [Connerney et al., 2017], the plasma waves 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 magnetometers (FGM) sensors each mounted on an ultrastable optical bench with two star 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