Comparing the solar wind-magnetosphere interaction at Mercury and Saturn

1. Comparing the solar wind-magnetosphere interaction at Mercury and Saturn A. Masters Institute of Space and AstronauticalScience, Japan Aerospace Exploration Agency, Japan J. A. Slavin1, G. A. DiBraccio1, T. Sundberg2, R. M. Winslow3, C. L. Johnson3,4,B. J. Anderson5, H. Korth5 1Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, USA. 2Centerfor Space Physics, Boston University, 725 Commonwealth Avenue, USA. 3Department of Earth Ocean and Atmospheric Sciences, University of British Columbia, Canada.4Planetary Science Institute, Tucson, USA. 5Johns Hopkins University Applied Physics Laboratory, USA.

2. Introduction: The solar wind-magnetosphere interaction B1 v1 • Earth’s magnetosphere is a referencepoint 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 Interface v2 B2 time Magnetic reconnection Kelvin-Helmholtz (K-H) instability From Paschmann, GRL (2008)

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

4. Why compare the Mercury and Saturn interactions? Saturn’s magnetosphere • 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 Dawn North Sun Sun South Cassini crossings of Saturn’s magnetopause Dusk From Slavin et al., Science (2009) From Masters et al., JGR (2011)

5. Comparing solar wind conditions Mercury’s bow shock Saturn’s bow shock • Some important solar wind parameters for the interaction with the magnetosphere are dimensionless (e.g. Machnumbers, 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 planetarybow shocks is also consistent with the expected high Mach number at Saturn Downstream Upstream

6. Comparing magnetic reconnection at the magnetopause Saturn • 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) From Lai et al., (2012) Magnetosheath Magnetosphere Mercury • 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) Magnetosphere Magnetosheath From Slavin et al., GRL (2010)

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

8. Comparing the Kelvin-Helmholtz stability of the magnetopause Saturn From Masters et al., P&SS (2012) • 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 exclusivelyunder northward Interplanetary Magnetic Field(IMF), and have only been detected at dusk (Sundberg et al., JGR, 2012) Dawn Dusk From Sundberg et al., JGR (2012) Mercury North South • 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

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 eitherside of each boundary are needed for a solution to the problem • The dawn-dusk asymmetry in K-H vortices atMercury’s magnetopause has been interpretedas a kinetic effect (e.g. Nakamura et al., 2010) Jupiter From Khurana, JGR (2001) • The absence of a dawn-dusk asymmetry in the level of wave activity on Saturn’s magnetopause may result from a dawn-dusk asymmetryin the magnetospheric magneticfield, counteractingthediffering flow shears From Nakamura et al.(2010)

10. Summary • The interaction between the solar wind and a planetary magnetosphere is underpinned byfundamental 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 magnetopauseInterpretation: 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’sInterpretation: Likely due to differences in the typical magnetized plasma conditions adjacent to eachboundary, but requiring a detailed assessment • Further studies are ongoing to shed more light on these differences

12. Reconnectionprohibited From Krimigis et al., SSR (2004) ~Mercury Reconnectionpossible ~Earth ~Jupiter ~Saturn ~Uranus& ~Neptune