HR: 0830h
AN: GP21C-0042    [PDF]
TI: The Crustal Magnetic Signature of Hellas Planitia
AU: * Lee, C O
EM: clee@ssl.berkeley.edu
AF: Space Sciences Laboratory, University of California at Berkeley, 7 Gauss Way, Berkeley, CA 94720 United States
AU: Mitchell, D L
EM: mitchell@ssl.berkeley.edu
AF: Space Sciences Laboratory, University of California at Berkeley, 7 Gauss Way, Berkeley, CA 94720 United States
AU: Lillis, R J
EM: rlillis@ssl.berkeley.edu
AF: Space Sciences Laboratory, University of California at Berkeley, 7 Gauss Way, Berkeley, CA 94720 United States
AU: Lin, R P
EM: rlin@ssl.berkeley.edu
AF: Space Sciences Laboratory, University of California at Berkeley, 7 Gauss Way, Berkeley, CA 94720 United States
AU: Acuna, M H
EM: mha@lepmom.gsfc.nasa.gov
AF: NASA, Goddard Space Flight Center, Planetary Magnetospheres Branch Code 695, Greenbelt, MD 20771 United States
AB: The Electron Reflectometer (ER) onboard Mars Global Surveyor (MGS) detected a plasma boundary between the ionosphere and the solar wind as the latter is diverted around and past the planet [Mitchell et al., GRL, 27, 1871, 2000; Mitchell et al., JGR, 106, 23419, 2001]. Above this boundary the 10-1000 eV electron population is dominated by solar wind electrons, while below the boundary it is dominated by ionospheric photoelectrons. This "photoelectron boundary", or PEB, is sensitive to pressure variations and moves vertically in response to changes in the ionospheric pressure from below and the solar wind pressure from above. The PEB is also sensitive to crustal magnetic fields, which locally increase the total ionospheric pressure and positively bias the PEB altitude. We have empirically modeled and removed systematic variations in the PEB altitude associated with the solar wind interaction, thus isolating perturbations caused by crustal magnetic fields. A map of the PEB altitude perturbations closely resembles maps of the horizontal component of the crustal magnetic field measured at 400 km by the MGS Magnetometer (MAG). We find a PEB altitude bias over the Hellas basin that is consistent with a horizontal magnetic field with an intensity of several nanotesla at 400 km altitude. This is compatible with upper limits to the horizontal crustal field strength set by MGS MAG measurements from the 400-km-altitude mapping orbit. If there are crustal sources within Hellas that give rise to a several-nanotesla horizontal field at 400 km altitude, then these same sources should give rise to crustal fields of at least several tens of nanotesla at 100 to 200 km aerobraking altitudes. Although low-altitude MAG measurements over Hellas are sparse [Acuna et al., Science, 284, 790, 1999], there was no evidence for crustal sources of this predicted field strength. Furthermore, electron reflection data (with more complete sampling) detected only a few isolated sources within Hellas [Lee et al., EGS-AGU-EUG Joint Assembly, Abstract EAE03-A-07938, 2003], which are far too weak to account for the observed PEB bias. We explore two possibilities for the observed PEB bias over Hellas: 1) horizontal fields over Hellas arise from magnetic sources around the basin perimeter, and 2) the solar wind flow is perturbed by crustal sources in the southern hemisphere such that the effective solar wind pressure over Hellas is systematically reduced. No detectable PEB or magnetic signature is observed over the younger Argyre and Isidis Basins.
DE: 5420 Impact phenomena (includes cratering)
DE: 5440 Magnetic fields and magnetism
DE: 6225 Mars
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting