Local Electron Density Depletions in the Martian Upper Ionosphere Obtained From MARSIS: Comparison With ASPERA‐3, and MAVEN, and the Connection With Crustal Magnetic Fields

Abstract

Steep electron density depletions in the Martian ionosphere are important, especially because they could be regions of significant plasma escape. Local electron density profiles of the Martian ionosphere obtained from the radar sounder onboard the Mars Express spacecraft often show large fluctuations. In some cases, these fluctuations are observed as deep depletions in the electron density. Investigation over the past 12 years of Mars Advanced Radar for Subsurface and Ionospheric Sounding data have revealed over 200 cases of density depletions where the lowest density is less than one fifth of the density in the surrounding regions. These density dips occur both on the dayside and the nightside, with an abundance between solar zenith angles of 50–70° and 100–120°. They occur at all altitudes in the ionosphere. Many of the depletions occur in the southern hemisphere where the crustal magnetic field is stronger and there are more regions where the crustal magnetic field is mostly vertical. In more than half of the cases, the depletion in the local electron density corresponds to a decrease in the electron flux measured with the Analyzer of Space Plasmas and Energetic Atoms electron spectrometer. Several different mechanisms could be responsible for the formation of the depletions, including regions of localized electric fields or magnetic fields creating areas of increased field pressure, escape of the plasma along magnetic field lines depleting density locally, and the density decrease around the photoelectron boundary followed by an increase in the plasma density due to plasma clouds.

Climatology of Martian Ionospheric Disturbances Deduced From MAVEN Data in 2014–2023

“…(2015), which were interpreted that precipitation of solar wind electrons is not an essential prerequisite for ionospheric structuring at Mars, and also with Duru et al. (2023), where suprathermal electron flux can either increase or decrease around plasma depletions in the Martian ionosphere. For completeness, the author has redrawn Figure 7 using only (a) dayside or (b) nightside SW electron data (the two figures not shown here), but the endeavor did not significantly improve the correlation between Martian ionospheric disturbances and SW electron density/temperature.…”

“…Lately, Duru et al. (2023) analyzed in situ plasma density depletions encountered by MEX/MARSIS at altitudes between 276 and 1,111 km. They concluded that the plasma density depletions prefer crustal magnetic anomaly, but are not well correlated with co‐located suprathermal electron flux.…”

“…The occurrence rates of the blackouts were higher for higher solar activity (than lower solar activity), nighttime (than daytime), and higher flux (than lower flux) of >70 keV electrons: on the other hand, the effect of crustal magnetic anomaly was unclear. Lately, Duru et al (2023) analyzed in situ plasma density depletions encountered by MEX/MARSIS at altitudes between 276 and 1,111 km. They concluded that the plasma density depletions prefer crustal magnetic anomaly, but are not well correlated with co-located suprathermal electron flux.…”