A MAVEN case study of radial IMF at Mars: impacts on the dayside ionosphere

Abstract

<p>Mars is an unmagnetized planet meaning that it has no internally generated global dipole magnetic field; the solar wind interaction with Mars is thus highly dependent on upstream conditions that include the orientation of the Interplanetary Magnetic Field (IMF). This interaction controls the transfer of energy and momentum from the solar wind into the magnetosphere, ionosphere and atmosphere, driving structure and dynamics within each. The Mars-solar wind interaction has been studied in detail for “nominal” Parker Spiral IMF conditions, but few studies exist that investigate the interaction under less typical “radial IMF” conditions. We utilize in-situ measurements made by NASAs Mars Atmosphere and Volatile EvolutioN (MAVEN) mission during radial IMF conditions at Mars to identify several prominent features that arise during this interaction:</p><ul><li>Highly disturbed magnetic and plasma conditions exist over the nose of the planet, which are reminiscent of foreshock like conditions and indicative of radial IMF.</li> <li>Solar wind protons and alphas are observed to penetrate directly into the dayside ionosphere down to MAVEN periapsis altitudes (~230 km).</li> <li>The magnetic pileup region (or magnetic barrier) forms deep within the dayside ionosphere, directly exposing significant heavy planetary ion densities above to the solar wind flow.</li> <li>Planetary ions above the magnetic barrier are mass loaded by the solar wind flow, characterized by signatures consistent with VxB acceleration under near radial IMF conditions.</li> <li>Thermal ion distributions in the ionosphere are observed to possess significant suprathermal energetic tails, suggesting that additional energization mechanisms are present that heat the cold ion core. Electron and ion temperatures are 2-10 times higher than average values that occur under more typical IMF conditions.</li> <li>Significant erosion of the dayside upper ionosphere occurs and appears to drive substantial ion escape to space.</li> </ul>