Evidence for Weak Crustal Magnetic Fields Over the Hellas Basin

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. A map of the PEB altitude closely resembles maps of the crustal magnetic field intensity measured at 400 km by the MGS Magnetometer. As expected, the best correlation is between the PEB altitude and the horizontal magnetic field component, which provides vertical pressure support. We have analyzed more than 4.8 million electron spectra obtained in the mapping orbit, covering over 1.5 Martian years. 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. 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 Magnetometer measurements. Weak crustal magnetic fields within the Hellas basin suggest that a weak Martian dynamo was still present when that basin cooled. No detectable PEB or magnetic signature is observed over the younger Argyre basin.