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. 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.2Center for 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.

Introduction: The solar wind-magnetosphere interaction 2

• 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

Magnetic reconnectionKelvin-Helmholtz (K-H) instability

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e

v1

v2

InterfaceB1

B2

From

Paschm

ann, GR

L (2008)

Why compare solar wind-magnetosphere interactions? 3F

rom W

alker and Russell, in

“Introduction to Space P

hysics” (1995)

• 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

Slavin &

Holzer, JG

R, (1981)

Fujim

oto et al., SS

R, (2007)

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 Sun

Dawn

Dusk

North

South

Comparing solar wind conditions

• 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

5

Ups

tre

am

Dow

nst

ream

Mercury’s bow shock Saturn’s bow shock

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)

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Lai et al., (2012)

Mag

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Mag

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• 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

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

Mercury

Saturn

• Mercury’s weaker bow shock produces lower plasma β conditions inthe magnetosheath More favorable conditions for onset, anda higher reconnection rate(Slavin & Holzer, JGR, 1979)

• Saturn’s stronger bow shock produces higher plasma β conditions inthe magnetosheath Less favorable conditions for onset, anda lower reconnection rate

• Higher magnetic shearsoccur more frequently athigher latitudes Is low-latitude reconnection rare?

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 exclusivelyunder northward Interplanetary Magnetic Field (IMF), and have only been detected at dusk (Sundberg et al., JGR, 2012)

8From 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

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)

9F

rom K

hurana, 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 asymmetryin the magnetospheric magnetic field, counteractingthe differing flow shears

From Nakamura et al. (2010)

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

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From Krimigis et al., SSR (2004)Reconnectionprohibited

Reconnectionpossible

~Mercury

~Earth

~Saturn

~Jupiter

~Uranus& ~Neptune