A TMOSPHERIC, O CEANIC AND S PACE S CIENCES UNIVERSITY of MICHIGAN Daniel J. Gershman, James A. Slavin, Jim M. Raines, Thomas H. Zurbuchen, Brian J. Anderson, Haje Korth, and Sean C. Solomon 4 th SERENA – HEWG Meeting Key Largo, FL

Introduction Zurbuchen et al., Science, 2011 Central plasma sheet in Mercury’s magnetotail

Introduction  Motivating science questions:  How are solar wind ions and planetary ions ‘processed’ differently in Mercury’s magnetospheric system?  What kind of velocity distribution function is appropriate for describing heavy ions in Mercury’s magnetotail? (e.g., ring, shell, Maxwellian, etc…)  What is the contribution of heavy ions to the pressure and mass density in the plasma sheet?

FIPS Plasma Parameters  The Fast Imaging Plasma Spectrometer (FIPS) on MESSENGER is a time-of- flight mass spectrometer  FIPS measures ions with E/q between 50eV/e – 13keV/e  Ion species regularly measured by FIPS:  Solar wind: H +, He 2+, O 6+ +C 5+ (i.e., )  Planetary: He +, O + group (m/q 14-20), Na + group (m/q 21-30)

FIPS Plasma Parameters  Limited plasma parameters can be derived from FIPS measurements of partial velocity distribution functions Raines et al., PSS, 2011

Distribution Function Visualization: H + Magnetosheath Flank Plasma Sheet

Distribution Function Visualization: H + Magnetosheath Flank Plasma Sheet Measured Data Equivalent Isotropic Maxwellian 1 FIPS Event (assuming isotropy) 2 FIPS Events (assuming isotropy)

FIPS Plasma Parameters Differential Energy Flux (cm -2 s -1 sr -1 [keV/e] -1 )

Example Orbit: 12 Feb 2012 (668) Differential Energy Flux (cm -2 s -1 sr -1 [keV/e] -1 ) (a) (b) Plasma Sheet Cusp MSH BSMP Post-midnight Plasma Sheet

Example Orbit: 23 Feb 2012 (691) Differential Energy Flux (cm -2 s -1 sr -1 [keV/e] -1 ) (a) (b) MSH Plasma Sheet Cusp MSH BSMP Pre-midnight Plasma Sheet Na+ group ions are enhanced in the pre-midnight plasma sheet (Zurbuchen et al., Science, 2011; Raines et al., JGR, 2013)

Average Plasma Sheet Properties  We identify the plasma sheet from FIPS:  Two ranges encompassing 12-hr ‘hot season’ orbits: 10 Nov 2011 – 5 Dec 2011, 5 Feb 2012 – 2 Mar 2012  > 50 measured proton events per scan  Within ±45 o magnetic latitude on the nightside  We accumulate all events measured in the plasma sheet during several orbits to examine heavy ion distribution functions

Plasma Sheet: Solar Wind Ions n MB =0.02 cm -3, T MB = MK (c) (b) He 2+ n MB =0.23 cm -3, T MB =30.3 MK Measured Data Moment-derived Maxwellian 1 FIPS Event 2 FIPS Events (a) H + n MB = cm -3, T MB = 8.4 MK (with high energy tail) Pre-midnight Plasma Sheet (10 Nov 2011 – 5 Dec 2011, 5 Feb 2012 – 2 Mar 201 2)

Plasma Sheet: Planetary Ions (a) Na + group(b) O + group (c) He + n MB =0.34 cm -3, T MB =13.6 MK n MB =0.08 cm -3, T MB =12.6 MK n MB =0.03 cm -3, T MB =9.4MK Measured Data Moment-derived Maxwellian 1 FIPS Event 2 FIPS Events Pre-midnight Plasma Sheet (10 Nov 2011 – 5 Dec 2011, 5 Feb 2012 – 2 Mar 201 2)

Average Plasma Sheet Properties Solar wind-like abundances Relative abundances consistent with Raines et al., JGR, 2013 Planetary ions have similar energy distributions Energization process of planetary ions does not have a strong affect on bulk solar wind ions Pre-midnight Post-midnight Plasma Sheet (10 Nov 2011 – 5 Dec 2011, 5 Feb 2012 – 2 Mar 201 2)

 How are solar wind ions and planetary ions ‘processed’ differently in Mercury’s magnetospheric system?  Solar wind ions maintain expected upstream abundance ratios and experience mass proportional heating across the subsolar bow shock. These properties are preserved even after the solar wind gains access to Mercury’s magnetosphere through subsolar reconnection.  What kind of velocity distribution function is appropriate for describing heavy ions in Mercury’s magnetosphere? (e.g., ring, shell, Maxwellian, etc…)  The measured fluxes of all ion species in the tail are consistent with hot Maxwell-Boltzmann velocity distributions, more so than energy-localized ring or thin shell distributions. The absence of regularly measured flux at lower energies is most likely a result of the FIPS’ finite sensitivity.  What is the contribution of heavy ions to the pressure and mass density in the plasma sheet?  For the average post-midnight plasma sheet, we find that planetary ions contribute ~1% of the total plasma pressure and ~ 10% of the mass density in the nightside plasma sheet. All heavy ions (including those of solar wind origin) contribute 10% and ~15%, to the plasma pressure and mass density, respectively.  For the average pre-midnight plasma sheet, we find that planetary ions contribute ~5% of the total plasma pressure and ~ 40% of the mass density in the nightside plasma sheet. All heavy ions (including those of solar wind origin) contribute ~15% and ~50%, to the plasma pressure and mass density, respectively.  Do heavy ions at Mercury influence the plasma sheet dynamics? Conclusions

Questions?