The lower dayside ionosphere of Mars from 14 years of MaRS radio science observations

This work uses a subset of "quiet" MaRS ionospheric dayside observations (MaRS<SUB>quiet</SUB>, 2004-2017) and a 1-D photochemical model (IonA-2) to investigate the potential formation processes of the excess electron densities merged with the base of the main ionosphere (Mm). 42% of the investigated MaRS observations contain identified Mm, which occur in a large variety of shapes ranging from smoothly decreasing electron densities to peak structures below the base of M1. The Mm appear over the full range of accessible solar zenith angles (50° - 90°) and are found between approximately 70 and 110 km altitude. Their base is found on average deeper in the atmosphere than the base of the averaged undisturbed MaRS electron density profiles. This indicates a dependence of the Mm formation on energy sources that penetrate deep into the atmosphere. This is supported by a strong positive correlation with increasing solar activity when solar flares, coronal mass ejections, and enhanced short solar X-ray and Ly-α intensities are more common. No relationship is found between the Mm occurrence rate and the magnitude/inclination of the weak crustal crustal magnetic field in MaRS<SUB>quiet</SUB>. <P />Investigations with the IonA-2 photochemical model for undisturbed and flare conditions show that the ionization of the local neutral atmosphere by solar X-ray radiation &lt;2 nm provides a satisfying explanation for detected Mm features with smoothly decreasing electron densities below the M1 base in combination with moderate slopes of the lower Mm region α<SUB>Mm</SUB> and altitudes of the lower boundary h<SUB>L,S</SUB>. While sufficient ionization energy reaches the region of interest during flares, no Mm features with peaks below the M1 base occur in any of the model electron density profiles. This supports the conclusion that the subgroup of merged excess electron densities with peaks or intermediate features (Mi) below the M1 base must have an origin different from the sole variability of solar X-ray radiation during undisturbed and solar flare conditions. The size of the identified Mm makes an exclusive meteoric origin of the Mm peak structures unlikely. <P />It is indicative from the IonA-2 model results that the general increase/decrease of solar X-ray &lt;2 nm leads to a correlated response of the Mm region. The sporadic occurrence of the merged excess electron densities in the MaRS observations is therefore assumed to be a combination of observational (increased observation noise level compared to the available amount of X-ray radiation &lt;2 nm, shift of the lower baseline by ionospheric deviations from radial symmetry) and environmental (e.g. variations in solar X-ray) factors.