Piercing the Martian Veil: A Statistical Study of Interplanetary Magnetic Field Reach Through Ionospheric Pressure Balance

Abstract

Mars, being a small planet with a tenuous atmosphere, does not have a sharp boundary between regions dominated by solar wind plasma and planetary plasma. Instead, this transition is typically extended, allowing the interplanetary magnetic field (IMF) to penetrate into the Martian ionosphere. However, the depth of this penetration is not well understood. Using 6 years of MAVEN data, we statistically assess locations where a transition exists between the dominance of magnetic versus cold (1 eV), thermal plasma pressure to better understand the reach of the IMF. We identify the presence or absence of pressure transitions from 200 to 800 km altitude for each MAVEN orbit and find a clear transition in $\sim$ 55% of cases. The pressure transition locations are mapped in different coordinate systems that provide insight into the solar and planetary driving conditions that cause a detected transition region. Transitions are more likely to occur under weak-to-nominal solar wind conditions, away from strong crustal magnetic fields, near the terminator, on the dusk side of the planet compared to the dawn side, and in the negative solar wind motional electric field hemisphere. We speculate on possible causes for asymmetries that arise in the mapped locations of these pressure transitions and the effect that penetrated IMF may have on driving plasma dynamics in the Martian ionosphere.