SHARAD Mapping of Mars Dayside Ionosphere Patterns: Relationship to Regional Geology and the Magnetic Field.

The electron density of the Martian ionosphere is modulated by solar wind forcing and crustal magnetic fields. Sounding observations from the orbital Shallow Radar (SHARAD) map the ionospheric total electron content (TEC) at a spatial resolution of ∼35 km. Averaging over a 250‐km diameter window from data collected weeks to years apart yields the first map of long‐term stable dayside martian TEC features. An extensive region of suppressed TEC in the southern highlands correlates with strong radial magnetic fields, but in other areas no simple correlation is observed. The TEC maps do follow the outlines of exposed Noachian crust and patches of magnetization in Tharsis not reset by volcanic activity. SHARAD TEC mapping may capture magnetic field strength at an intermediate height between the surface and the altitudes of orbital measurements underlying spherical harmonic models. Existing and future data will allow SHARAD TEC mapping to ∼100 km spatial resolution. Plain Language Summary: The upper atmosphere of Mars hosts a layer of charged particles that form the ionosphere. The vertical distribution of electrons is affected by the stream of solar wind and by magnetic fields imprinted on rocks that formed about 4 b.y. ago when the interior of Mars was hot enough to sustain a dynamo like that of present‐day Earth. We use a novel technique based on radar echoes from the surface observed by the Shallow Radar (SHARAD) on the NASA Mars Reconnaissance Orbiter to measure the total electron content (TEC) of the atmosphere over the past 15 years. Averaging many observations at a location reduces the solar wind effect and leaves a map of persistent features of the ionosphere. Long‐term stable TEC features correlate with the most ancient crust, but also occur in areas covered by younger lava flows where volcanic heating has not totally erased the magnetic field of the rocks. This method provides a new way of mapping regional geologic and magnetic field correlations that complement results from orbiter and lander magnetometer instruments. Key Points: MRO extended mission Shallow Radar measurements map stable Total Electron Content (TEC) patterns in the dayside ionosphere of MarsTEC maps align with regional geology and partially match crustal magnetic field models based on orbital dataTEC mapping offers a new and complementary method for delineating magnetized crust [ABSTRACT FROM AUTHOR]