High Time and Spatial Resolution Observations of the Topology of Mars' Crustal Magnetic Fields
Authors: Mitchell David , Xu Shaosui , Brain David , McFadden James , Mazelle Christian , Espley Jared 
Published: September 2022

&lt;p&gt;Mars has a patchwork of intense crustal magnetization that gives rise to magnetic fields that can dominate the plasma environment up to ~1000 km altitude in some locations. &amp;#160;The strongest crustal fields, in the southern hemisphere centered near 180 deg longitude, form an arcade of reversing polarities that extends &gt; 1000 kilometers east-west. &amp;#160;The topology of these fields can be complex and dynamic, with closed loops connecting either nearby and distant crustal sources, and magnetic reconnection with the IMF temporarily forming open field lines. &amp;#160;The boundary between closed magnetic loops and open field lines is expected to have a small spatial scale that has so far been unresolved by plasma instruments at Mars.&lt;/p&gt;
&lt;p&gt;Electron energy-pitch angle distributions measured by the MAVEN Solar Wind Electron Analyzer (SWEA) can be used to infer magnetic topology and thus reveal the large-scale configuration of the magnetic field from measurements at one location. &amp;#160;During normal operation, the instrument has a 2-second measurement cycle that provides ~8 km spatial resolution, which is too large to probe the transition region between closed and open field lines, for example. &amp;#160;(Topology changes appear sharp and instantaneous at that scale.) &amp;#160;However, it is possible to operate the instrument in a non-standard way to measure the electron angular distribution at a single energy with a time resolution of 0.03 seconds and a spatial resolution of 125 meters, which is comparable to the 50-eV electron gyroradius in a 100-nT field.&lt;/p&gt;
&lt;p&gt;High resolution observations from ~150 to 800 km altitude over strong crustal fields near the evening terminator were obtained on 12 orbits on January 27, 2019, and from May 27 to June 1, 2020. &amp;#160;A total of eight topological transitions were captured. &amp;#160;Six of these were transitions from open to closed, or the reverse. &amp;#160; For example, observations on January 27, 2019 revealed a transition region between closed and open field lines with a spatial scale of ~900 meters, which is equivalent to 5 electron gyroradii under the prevailing conditions. &amp;#160;The transition region was traversed in 290 millisec, which is equivalent to four mirror times (from the spacecraft to the reflection point and back). &amp;#160;Before the transition region, trapped mirroring electrons are observed, which is indicative of a closed crustal magnetic loop. After the transition region, a loss cone is present, which is formed when precipitating electrons are absorbed before they can reflect back up to the spacecraft. This indicates an open field line. &amp;#160;Within the transition region itself, the angular distribution is more isotropic, which indicates that a finite time is needed for the electron population to transition from one to the other. &amp;#160;The other two events show sharp transitions between neighboring closed crustal magnetic flux tubes with different sets of ionospheric footpoints. &amp;#160;This reveals a structured ionosphere controlled by the magnetic topology connecting crustal magnetic sources.&lt;/p&gt;