M3 electron density layer in the dayside ionosphere of Mars: Analysis of MAVEN ROSE observations

Abstract

We investigate the characteristics of the topside M3 layer in the Martian dayside electron density profiles obtained from the Radio Occultation Science Experiment (ROSE) onboard the MAVEN spacecraft. M3 layer is a local enhancement in the electron density above the prominent ionospheric M2 peak. ROSE measurements have a wider latitudinal and solar zenith angle (SZA) coverage as compared to the spacecraft observations used by previous studies for characterizing the M3 layer. We analyzed 179 dayside (SZA <85∘) electron density profiles from July 2016 to December 2020. The analysis show that the typical altitude of M3 layer is ∼180 (±10) km, with a density of ∼8 (±3) x 103 cm−3, and occur ∼43 (±8) km above the M2 peak. These values are consistent with those reported by an earlier study using MGS RO data. The density of M3 layer seems to be insensitive to changes in SZA in the range of 55∘ to 85∘. The increasing trend in the M3 peak altitude with increasing SZA might be indicative of M3 forming at constant pressure levels, similar to M2. In the northern hemisphere, both the M3 peak density and altitude seem insensitive to latitudinal changes. The most commonly cited reasons for the production of the M3 layer are the enhancement in the electron temperature and the converging vertical plasma motion caused by the crustal magnetic field. Our simulations using a one-dimensional photochemical model suggest that decreased dissociative recombination rate due to electron temperature enhancements may not contribute to the formation of M3 layer. We conclude that the presence of crustal magnetic field also could not wholly explain the formation of M3 layer.