36th DPS Meeting, 8-12 November 2004
Session 37 Mars Atmosphere
Poster II, Thursday, November 11, 2004, 4:15-7:00pm, Exhibition Hall 1A

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[37.19] Observations of Open and Closed Magnetic Field Lines at Mars: Implications for the Upper Atmosphere

D.A. Brain, D.L. Mitchell, R. Lillis, R.P. Lin (UC Berkeley Space Sciences Lab)

Mars lacks a global dynamo magnetic field, and therefore the solar wind interacts directly with the Martian atmosphere. However, many of the crustal fields observed by the Mars Global Surveyor (MGS) Magnetometer and Electron Reflectometer (MAG/ER) instrument are sufficiently strong that they shield local regions of the atmosphere from the solar wind. Like Earth's polar cusp regions, Martian crustal fields do not act as perfect shields for the atmosphere. Field lines connected at opposite ends to Mars and the solar wind provide conduits for particle exchange between the solar wind and lower ionosphere. These open field lines have been observed by spacecraft and predicted by models. The amount and variability of both open and closed field lines has implications for atmospheric escape to space and upper atmospheric energetics.

As MGS orbits the planet at 400 km altitudes it samples both open and closed magnetic field lines. The angular distribution of electrons observed by ER at a given energy can be used to distinguish between three different field topologies: open, closed (connected to Mars at both ends), and unconnected. Using four years of data from the MGS mapping orbit, we have classified ER observations of electrons at 190 eV according to the topology of the magnetic field. This extended data set allows us to explore a number of parameters that control magnetic field topology at 400 km. For this work we will show where open and closed field lines are likely to occur, and how changes in the direction of the interplanetary magnetic field affect the locations of open field regions. Our goal is to determine the fraction of Mars' atmosphere at 400 km that is protected by crustal magnetic fields, and use this result to quantify the effect on the efficiency of solar wind related atmospheric loss processes.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.