1. Comparing the solar wind-magnetosphere interaction at Mercury and Saturn A. Masters Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Japan J. A. Slavin 1, G. A. DiBraccio 1, T. Sundberg 2, R. M. Winslow 3, C. L. Johnson 3,4, B. J. Anderson 5, H. Korth 5 1 Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, USA. 2 Center for Space Physics, Boston University, 725 Commonwealth Avenue, USA. 3 Department of Earth Ocean and Atmospheric Sciences, University of British Columbia, Canada. 4 Planetary Science Institute, Tucson, USA. 5 Johns Hopkins University Applied Physics Laboratory, USA.

2. Introduction: The solar wind-magnetosphere interaction 2 Earth’s magnetosphere is a reference point for all solar wind-magnetosphere interactions How does the solar wind interact with a planetary magnetosphere? - Compression/expansion of the system - Direct entry at the cusps - Fluid instabilities at the magnetopause - Kinetic instabilities at the magnetopause Magnetic reconnection Kelvin-Helmholtz (K-H) instability time v1v1 v2v2 Interface B1B1 B2B2 From Paschmann, GRL (2008)

3. Why compare solar wind-magnetosphere interactions? 3 From Walker and Russell, in “Introduction to Space Physics” (1995) The interaction involves a number of fundamental space plasma processes Solar wind parameters vary with heliocentric distance, so each magnetosphere allows us to study these processes in a different region of parameter space, often beyond the present reach of simulations We expect the interaction to vary between magnetized planets, so comparing interactions has potentially broad implications Slavin & Holzer, JGR, (1981) Fujimoto et al., SSR, (2007)

4. Why compare the Mercury and Saturn interactions? 4 From Slavin et al., Science (2009) MESSENGER at Mercury and Cassini at Saturn provide us with orbiters around planets at more diverse heliocentric distances than ever before Data returned by MESSENGER and Cassini suggest that how each magnetosphere interacts with the solar wind is one of the many differences between them From Krimigis et al., SSR (2004) Mercury’s magnetosphere Saturn’s magnetosphere Cassini crossings of Saturn’s magnetopause From Masters et al., JGR (2011) Sun Dawn Dusk North South

5. Comparing solar wind conditions Some important solar wind parameters for the interaction with the magnetosphere are dimensionless (e.g. Mach numbers, plasma β) Both spacecraft are three-axis-stabilized  Limited plasma instrument pointing Reported observations are consistent with expected parameter differences (Crary et al., Nature, 2005) (Achilleos et al., JGR, 2006) (Gerschman et al., JGR, 2012) Comparing the magnetic overshoot of the planetary bow shocks is also consistent with the expected high Mach number at Saturn 5 Upstream Downstream Mercury’s bow shockSaturn’s bow shock

6. Comparing magnetic reconnection at the magnetopause MESSENGER has revealed a significant amount of evidence for magnetic reconnection at Mercury’s magnetopause: - Flux transfer events (FTEs) (e.g. Slavin et al., JGR, 2012) - Finite-normal components of the magnetic field at the magnetopause (e.g. DiBraccio et al., JGR, 2013) 6 From Slavin et al., GRL (2010) Magnetosphere Magnetosheath From Lai et al., (2012) Magnetosphere Magnetosheath Cassini has revealed far more limited evidence for magnetic reconnection at Saturn’s magnetopause - No confirmed FTEs (Lai et al., JGR, 2012) - Only one crossing at low latitude with clear signatures of reconnection, but no resolvable finite-normal magnetic field component (McAndrews et al., JGR, 2008) - Indications of reconnection at higher latitudes (Badman et al., GRL, 2013) Mercury Saturn

7. Interpretation of reconnection comparison The diamagnetic suppression condition for reconnection onset may explain the difference (Swisdak et al., JGR, 2003) 7 From Masters et al., GRL (2012)From DiBraccio et al., JGR (2013) From Phan et al., ApJ (2010) Solar wind current sheets Mercury Saturn Mercury’s weaker bow shock produces lower plasma β conditions in the magnetosheath  More favorable conditions for onset, and a higher reconnection rate (Slavin & Holzer, JGR, 1979) Saturn’s stronger bow shock produces higher plasma β conditions in the magnetosheath  Less favorable conditions for onset, and a lower reconnection rate Higher magnetic shears occur more frequently at higher latitudes  Is low-latitude reconnection rare?

8. Comparing the Kelvin-Helmholtz stability of the magnetopause MESSENGER has seen clear magnetic signatures of K-H instability-driven vortices at Mercury’s magnetopause (Slavin et al., Science, 2008) Such K-H vortices occur almost exclusively under northward Interplanetary Magnetic Field (IMF), and have only been detected at dusk (Sundberg et al., JGR, 2012) 8 From Masters et al., P&SS (2012) From Sundberg et al., JGR (2012) No similar magnetic evidence for vortices has been seen by Cassini at Saturn’s magnetopause Waves on the magnetopause (many of which are K-H waves) occur roughly equally at both dawn and dusk Dawn Dusk North South Mercury Saturn

9. Interpretation of Kelvin-Helmholtz stability comparison It appears that K-H growth rates are higher at Mercury’s magnetopause than at Saturn’s Typical magnetized plasma parameters either side of each boundary are needed for a solution to the problem The dawn-dusk asymmetry in K-H vortices at Mercury’s magnetopause has been interpreted as a kinetic effect (e.g. Nakamura et al., 2010) 9 From Khurana, JGR (2001) Jupiter The absence of a dawn-dusk asymmetry in the level of wave activity on Saturn’s magnetopause may result from a dawn-dusk asymmetry in the magnetospheric magnetic field, counteracting the differing flow shears From Nakamura et al. (2010)

10. Summary The interaction between the solar wind and a planetary magnetosphere is underpinned by fundamental space plasma processes At each magnetosphere these processes operate in a different region of parameter space, which we expect to result in different types of solar wind-magnetosphere interaction MESSENGER at Mercury and Cassini at Saturn provide us with orbiters around planets at more diverse heliocentric distances than ever before Signatures of magnetic reconnection are observed far more commonly by MESSENGER at Mercury’s magnetopause than by Cassini at Saturn’s magnetopause Interpretation: Differences in bow shock Mach number  Different plasma β conditions  More/less favorable conditions for onset, different reconnection rates Evidence for K-H perturbations has been observed at both planetary magnetopauses, but the growth rates appear to be higher at Mercury’s magnetopause than at Saturn’s Interpretation: Likely due to differences in the typical magnetized plasma conditions adjacent to each boundary, but requiring a detailed assessment Further studies are ongoing to shed more light on these differences 10

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12. 12 From Krimigis et al., SSR (2004) Reconnection prohibited Reconnection possible ~Mercury ~Earth ~Saturn ~Jupiter ~Uranus & ~Neptune