a tmospheric, o ceanic and s pace s ciences university of michigan daniel j. gershman, james a....
TRANSCRIPT
ATMOSPHERIC, OCEANIC
AND SPACE SCIENCESUNIVERSITY of MICHIGAN
Daniel J. Gershman, James A. Slavin, Jim M. Raines, Thomas H. Zurbuchen,
Brian J. Anderson, Haje Korth, and Sean C. Solomon
Heavy ions in Mercury’s magnetotail
4th SERENA – HEWG MeetingKey Largo, FL
IntroductionZurbuchen 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+, He2+, O6++C5+ (i.e., <OC>) 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
108
104
106
Diff
eren
tial E
nerg
y F
lux
(cm
-2 s
-1 s
r-1 [k
eV/e
]-1)
Example Orbit: 12 Feb 2012 (668)
108
104
106
Diff
eren
tial E
nerg
y F
lux
(cm
-2 s
-1 s
r-1 [k
eV/e
]-1)(a)
(b)
Plasma Sheet Cusp MSHBSMP
Post-midnight Plasma Sheet
Example Orbit: 23 Feb 2012 (691)
108
104
106
Diff
eren
tial E
nerg
y F
lux
(cm
-2 s
-1 s
r-1 [k
eV/e
]-1)
(a)
(b)
MSH Plasma Sheet Cusp MSHBSMPMP
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 ±45o 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
nMB=0.02 cm-3, TMB=103.75 MK
(c) <OC>(b) He2+
nMB=0.23 cm-3, TMB=30.3 MK
Measured Data
Moment-derived Maxwellian1 FIPS Event
2 FIPS Events
(a) H+
nMB= 11.22 cm-3, TMB= 8.4 MK(with high energy tail)
Pre-midnight Plasma Sheet (10 Nov 2011 – 5 Dec 2011, 5 Feb 2012 – 2 Mar 2012)
Plasma Sheet: Planetary Ions
(a) Na+ group (b) O+ group (c) He+
nMB=0.34 cm-3, TMB=13.6 MK nMB=0.08 cm-3, TMB=12.6 MKnMB=0.03 cm-3, TMB=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 2012)
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 2012)
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
