THE MARS/SOLAR WIND
INTERACTION
The interaction of the solar wind with Mars is, to first order,
determined by whether or not the planet has an intrinsic magnetic
field. From the very beginning of Mars exploration this question
has been hotly debated. When the first measurements were taken
near Mars by Mariner 4 in 1965, a bow shock was observed by the
magnetometer [Smith et al., 1965]. Its position was interpreted
by some to indicate the presence of a small planetary field, while
others argued against it [e.g., the review by Vaisberg, 1992, and
references therein].
The next seven missions to Mars space, including the Viking mission,
added only marginally to the understanding, primarily do to the lack of
appropriate instrumentation carried on these spacecraft, and unsuitable
trajectories. Then the Phobos mission was launched in 1988,
carrying a nearly complete suite of field and particle instruments on
board. Upon arrival in 1989, Phobos 2 entered a highly
elliptical, 77 h period orbit, with periapsis at 850km altitude.
After four orbits the spacecraft was maneuvered into an 8 h period
circular orbit of 6203 km altitude (2.8 Rm). Unfortunately, after
only three month contact with Phobos 2 was lost (Phobos 1 was lost
during cruise), ending much too prematurely an exiting and ambitious
mission. But during its time in orbit, Phobos measurements showed
that a complex and probably unique plasma environment existed around
Mars. However, it still could not been shown whether or not a
planetary magnetic field was present.
This question was finally answered by the Mars Global Surveyor (MGS)
mission. After the loss of Mars Observer during its preparation
for the Mars orbit insertion maneuver (1993), MGS was launched on
November 7, 1996. The primary objective of the MGS mission is to
conduct a global mapping mission of Mars by performing an extended
orbital study of the planet's surface, atmosphere, and gravitational
and magnetic fields.
The spacecraft’s planned mapping orbit was at low altitude (about 350
km, near-circular, near-polar orbit that is Sun-synchronous with the
day-side equatorial crossing at 2:00 PM local mean solar time. To
deliver the spacecraft into this orbit, aerobraking was used over a
period of about 18 month (this was longer than originally planned
because one of the solar panels was damaged during launch). The
extended aerobraking turned out to be a great bonus to the MAG/ER
investigation, because it carried the spacecraft to about 100 km
altitudes (many below 100 km) on each orbit, well below the ionospheric
peak. It was thus possible for the magnetometer to peek below the
hiding cover of the ionosphere and check directly for any planetary
magnetic field.
From these orbits MGS could show conclusively that no dipole-like,
dynamo generated planetary field exist on Mars at present.
However, large remnant magnetic fields were detected in localized
regions on the planet, indicating that a dynamo field did exist,
probably until some 4 billion years ago [Acuna et al., 1999; Acuna et
al., 1998; Connerney et al., 1999]. The absence of a planetary
dipole filed sets stringent conditions on the solar wind/Mars
interaction process, in fact it now clearly defines how we look at the
near Mars plasma environment.
We knew before that the interaction was not lunar-like, because of
Mars’ ionosphere, albeit the ionosphere was calculated too thin to
routinely stand-off the solar wind. We know now that the
interaction is not magnetospheric, Earth-like. However, the
magnetic anomalies, larger than any seen on Earth, add a dynamic
character to the interaction as they rotate into view of the solar wind
every Mars rotation period. There is thus a periodic addition of
significant magnetic pressure to help standoff the solar wind. So
we know that the interaction is not purely ionospheric,
Venus-like. Rather, it is clear now that the Mars interaction
with the solar wind is a complicated and dynamic mixture of all three
possibilities.
It can easily be envisioned that this geometry leads to many boundaries
within the Mars plasma environment. In fact the Phobos and MGS
literature is full of descriptions of boundaries found by individual
instrument. Many of the boundaries are controversial in that they
where not seen by the other instruments. Table 1 indicates the
name, definition, and location of some of them. The table
indicates the complicated nature of the interaction.
MGS carried only two magnetometers and an Electron Reflectometer, and
thus is blind to ion, plasma wave, and energetic particle events.
But it did gather the magnetic and electron data in great detail and
with very good statistics from an optimal orbit. Phobos 2 on the
other hand, had many field and particle instruments, but took
measurements at large distances and for a relatively short period of
time.
The workshop has “dovetailed” the observations by Phobos and MGS, and
theoretical and numerical work, to produce interpretations with greater
fidelity and clarity. Much work was expanded to integrate all the
observations into a coherent picture of the Mars/solar wind interaction
process.
• Papers and articles that have resulted from the workshop, published
in “Mars’ Magnetism and its Interaction with the Solar Wind”, and in
Space Science Reviews (Kluwer, 2004):
“Mars Crustal Magnetism”, J. E. P. Connerney, M. H. Acuña,
N. F. Ness, T. Spohn, and G. Schubert.
“The Plasma Environment of Mars”, A. F. Nagy, D. Winterhalter, K.
Sauer, T. E. Cravens, S. Brecht, C. Mazelle, D. Crider, E. Kallio, A.
Zakharov, E. Dubinin, M. Verigin, G. Kotova, W. I. Axford, C. Bertucci,
and J. G. Trotignon.
“Bow Shock and Upstream Phenomena at Mars”, C. Mazelle, D.
Winterhalter, K. Sauer, J.G. Trotignon, M.H. Acuña, K.
Baumgärtel, C. Bertucci, D.A. Brain, S.H. Brecht, M. Delva, E.
Dubinin, M. Øieroset, and J. Slavin.
“The Magnetic Field Pile-Up And Density Depletion in the Martian
Magnetosheath: A Comparison with the Plasma Depletion Layer Upstream of
the Earth’s Magnetopause”, Marit Øieroset, David L. Mitchell,
Tai D. Phan, Robert. P. Lin, Dana H. Crider and Mario H. Acuña.
“Mars Global Surveyor Observations Of Solar Wind Magnetic Field Draping
Around Mars”, Dana H. Crider, David A. Brain, Mario H. Acuña,
Didier Vignes, Christian Mazelle, Cesar Bertucci.
“Magnetic Flux Ropes in the Martian Atmosphere: Global
Characteristics”, D. Vignes, M.H. Acuña, J.E.P. Connerney, D.H.
Crider, H. Reme and C. Mazelle.
“Unusually Distant Bow Shock Encounters at Mars: Analysis of the March
24, 1989 Event”, M. I. Verigin, J. Slavin, A. Szabo, G. A. Kotova, A.
P. Remizov, H. Rosenbauer, S. Livi, K. Szegö, M.
Tátrallyay, K. Schwingenschuh, and T.-L. Zhang.
“MarsExpress and MARSIS”, Erling Nielsen.
• Team members:
Mario Acuna, Goddard Space Flight Center/NASA, Greenbelt, USA;
Ian Axford, Max Planck Institut, Lindau, Germany;
Klaus Baumgaertel, Max Planck Institut, Lindau, Germany;
Siegfried Bauer, University of Graz, Graz, Austria;
Cesar Bertucci, Centre d'Etude Spatiale des Rayonnements/CNRS,
Toulouse, France;
David Brain, University of Colorado, Boulder, USA;
Stephen Brecht, Bay Area Res Corp, Orinda, CA 94563, USA;
Tamara Breus, Space Research Institute, Moscow, Russia;
Paul Cloutier, Rice University, Houston, USA;
Jack Connerney, Goddard Space Flight Center/NASA, Greenbelt, USA;
Tom Cravens, University of Kansas, Lawrence, USA;
Dana Crider, Goddard Space Flight Center/NASA, Greenbelt, USA;
Magda Delva, Österr. Akademie der Wissenschaften, Graz, Austria;
Eduard Dubinin, Max Planck Institut, Lindau, Germany;
Mats Holmstrom, Swedish Institute of Space Physics, Kiruna, Sweden;
Galina A. Kotova, Space Research Institute, Moscow, Russia;
Esa Kallio, Finnish Meteorological Institute, Geophysical Research,
Helsinki, Finland;
Alexander M. Krymskii, Rostov State University, Rostov-on-Don, Russia;
Robert Lin, University of California, Berkeley, USA;
Kiyoshi Maezawa, Institute of Space and Astronautical Science, Tokyo,
Japan;
Christian Mazelle, Centre d'Etude Spatiale des Rayonnements/CNRS,
Toulouse, France;
David Mitchell, University of California, Berkeley, USA;
Diedrich Möhlmann, Deutsches Zentrum für Luft- und Raumfahrt,
Cologne, Germany;
Andy Nagy, University of Michigan, Ann Arbor, USA;
Norman Ness, Bartol Research Institute, Newark, USA;
Erling Nielsen, Max Planck Institut, Lindau, Germany;
Marit Oieroset, University of California, Berkeley, USA;
Henri Reme, Centre d'Etude Spatiale des Rayonnements/CNRS, Toulouse,
France;
Konrad Sauer, Max Planck Institut, Lindau, Germany;
Gerald Schubert, University of California, Los Angeles, USA;
Konrad Schwingenschuh, Österr. Akademie der Wissenschaften, Graz,
Austria;
Jim Slavin, Goddard Space Flight Center/NASA, Greenbelt, USA;
Tilman Spohn, Westfälische Wilhelms-Universität,
Münster, Germany;
Jean-Gabriel Trotignon, LPCE/CNRS, Orleans, France;
Misha I. Verigin, Space Research Institute, Moscow, Russia;
Didier Vignes, Goddard Space Flight Center/NASA, Greenbelt, USA;
Daniel Winterhalter, Jet Propulsion Laboratory/Caltech, Pasadena, USA;
Alexander Zakharov, Space Research Institute, Moscow, Russia;