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Proton cyclotron waves at Mars: Exosphere structure and evidence for a fast neutral disk

Mars is an unmagnetized planet whose hydrogen exosphere extends into the solar wind, creating proton cyclotron waves. Mars Global Surveyor data reveal the occurrence of waves to be extensive and often intermittent at large distance, indicating that the exosphere is time varying or non‐spherical. When the region of wave occurrence is examined in a magnetic‐electric coordinate system, a strong asymmetry in the occurrence of these waves is seen in the direction of the interplanetary electric field. The extensive, yet asymmetric and intermittent, occurrence of waves can be understood if, after protons are first picked up near Mars, the ions are neutralized by charge exchange and transported across field lines to distant regions, allowing the pickup process to extend far from Mars on only one side of the planet. Thus the exosphere of Mars appears to extend in a disk of fast hydrogen atoms both downstream and to the side of Mars in the direction of the interplanetary electric field.