46th Lunar and Planetary Science Conference (2015)
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MAVEN OBSERVATIONS OF THE MARTIAN MAGNETOSPHERE AND ITS RESPONSE TO SOLAR
WIND DRIVERS. J. S. Halekas1, D. L. Mitchell2, J. P. McFadden2, D. Larson2, J. E. P. Connerney3, J. Espley3, R.
E. Ergun4, L. Andersson4, J. G. Luhmann2, R.J. Lillis2, D. A. Brain4, S. Ruhunusiri1, Y. Harada2, T. Hara2, S. Curry2,
G. DiBraccio3, 1Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242 (jasperhalekas@uiowa.edu), 2Space Sciences Laboratory, University of California, Berkeley CA 94720, 3NASA Goddard
Space Flight Center, Greenbelt, MD 20771, 4Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, 80303.
Introduction: The MAVEN mission provides a
sphere through which escape takes place is controlled
comprehensive view of the Martian upper atmosphere,
by ionospheric production by solar UV, the impact of
ionosphere, and magnetosphere. MAVEN carries the
solar wind plasma and energetic particles, the intermost complete aeronomy payload yet sent to Mars, and
planetary magnetic field (IMF), and the Martian phase
has an orbit designed to sample both the low-altitude
(and thus the orientation of its remanent magnetic
ionosphere and the upstream solar wind, providing
fields). As the MAVEN orbit precesses with time, we
near-simultaneous measurements of solar energy inwill sample all parts of this coupled system and deterputs to the system, the reservoir of atmospheric gases,
mine its structure and dynamics (and escape from it) as
and the escaping particles. These measurements proa function of these parameters.
vide constraints not only on the structure and dynamics
To make these measurements, MAVEN carries a
of (and escape from) the current system, but also their
comprehensive Particle and Fields package, consisting
dependence on external drivers, allowing us to address
of a magnetometer (MAG), a Langmuir probe/waves
the history of the loss of gas from Mars to space.
instrument (LPW) which also houses the EUV sensor,
Background: Measurements by previous orbiters,
two ion instruments (SWIA and STATIC), an electron
including Phobos 2, Mars Global Surveyor, and Mars
instrument (SWEA), and an energetic particle instruExpress, have built our understanding of the Martian
ment (SEP), which work in concert to measure plasma
magnetosphere. The Martian magnetosphere is small
processes in the Martian environment.
compared to that of the Earth, and forms as a result of
The Solar Wind Ion Analyzer (SWIA) is designed
mass-loading and induced current systems in the ionoto measure the solar wind plasma, both upstream from
sphere [1,2,3] rather than an intrinsic magnetic field
the Martian bow shock and inside the magnetosphere
(Mars does have strong localized remanent magnetic
as it is mass-loaded, decelerated, and deflected around
fields [4]). Escape from
the Martian atmosphere
follows many channels,
including photochemical
escape, Jeans escape,
solar wind sputtering,
and non-thermal ion escape processes [5]. Phobos 2 and Mars Express
have measured and characterized many ion escape processes [6,7,8,9],
albeit at higher altitudes
than MAVEN will access
at periapsis. Nonetheless,
work remains to organize
ion escape processes by
the magnetic field orientation and determine how
they depend on the energy inputs from the sun.
MAVEN Observations: The structure of
Figure 1: Martian magnetosphere observed under low solar wind dynamic pressure and +By
the Martian magnetoIMF. Dashed lines indicate major boundaries and current sheets.
46th Lunar and Planetary Science Conference (2015)
1379.pdf
Mars’ ionosphere. SWIA produces two different kinds
pressed in the sheath. Given the favorable orientation
of 3-d distributions, coarse and fine, tailored respecof the convection electric field, we observe an energettively for the magnetosphere and the solar wind, as
ic plume of escaping pickup ions at high altitudes
well as onboard-computed bulk moments.
around apoapsis (circled portions of spectra in Fig. 2),
Figs. 1 and 2 show SWIA coarse and fine energy
accelerated outward by the penetration of this electric
spectra, together with MAG, STATIC, and SWEA data
field. The inbound flank boundary appears rather diffor context. Fig. 1 shows a typical orbit for low solar
ferent than in the previous case, with much less eviwind dynamic pressure and a +By IMF. For the current
dence of mixing and instabilities. However, we still see
mission phase, MAVEN reaches its lowest altitude
evidence of significant ion acceleration at the
near the north geographic pole – therefore, given +By,
plasmasheet, and at the outbound boundary, suggesting
periapsis lies in the +E hemisphere (that toward which
significant escape.
the vxB convection electric field points). The orbit
Conclusions: MAVEN has already observed a
travels through the undisturbed solar wind, the sheath,
highly structured magnetosphere and ionosphere at
a boundary layer, the draped tail lobes, a region of
Mars, clearly controlled in part by the solar wind and
crustal magnetic fields and dense ionospheric plasma,
the interplanetary magnetic field. In the coming year,
and then back into the sheath and solar wind. The solar
as our orbit covers more of the Martian system,
wind for this orbit has a proton density of ~2 cm-3,
MAVEN observations will allow us to determine how
velocity of ~320 km/s, temperature of ~4 eV, and an
the structure and dynamics of the magnetosphere, and
alpha content of ~4%. This flow is shocked and heated
escape from it, depend on external drivers.
by an order of magnitude in the sheath region. Between the sheath and the lobe, the –E flank of the
References: [1] Nagy A. F. et al. (2003) Space Sci.
magnetosphere contains highly mixed plasma, with
Rev., 111, 33-114. [2] Lundin R. et al. (1990) Geophys.
evidence of significant instabilities in both the ion enRes. Lett., 17, 873-876. [3] Mazelle C. et al. (2003)
ergy spectra and the magnetic fields, indicating the
Space. Sci. Rev., 111, 115-181. [4] Acuña M. H. et al.
likely presence of Kelvin-Helmholtz and/or inter(1998) Science, 279, 1676-1680. [5] Lammer H. et al.
change instabilities. The tail lobe contains highly
(2008) Space Sci. Rev., 139, 399-436. [6] Lundin R. et
draped magnetic fields with a plasmasheet between
al. (1989) Nature, 341, 609. [7] Barabash S. et al.
them showing evidence of accelerated heavy ions, in
(2007) Science, 315, 501. [8] Fedorov A. et al. (2011)
agreement with previous observations [7,8]. The periJ. Geophys. Res., 116, A07220. [9] Dubinin E. et al.,
apsis for this orbit, in the ionosphere, is dominated by
(2011) Space. Sci. Rev., 162, 173-211.
crustal magnetic fields.
At the outbound boundary between the ionosphere and the sheath, we
see significant acceleration of heavy ions, indicating pickup and/or other acceleration processes.
Fig. 2 shows an orbit
a day after that of Fig. 1,
with moderate solar wind
dynamic pressure and a
–By IMF (therefore, periapsis is in the -E hemisphere). The solar wind
for this orbit is variable,
with a proton density of
~5-20 cm-3, velocity of
~320-350 km/s, temperature of ~2 eV, and an
alpha content of ~3%. As
in the first orbit the flow
is shocked and signifiFigure 2: Martian magnetosophere observed under moderate solar wind dynamic pressure and
cantly heated and com–By IMF. Energetic pickup ion populations are indicated by black circles.