Seasonal variability of the Martian dayside ionosphere from Mars Express observations
Authors: González-Galindo Francisco , Eusebio David , Němec Frantisek , Peter Kerstin , Kopf Andrew , Tellman Sylvia , Pätzold Martin 
Published: October 2020

&#160;Introduction&#160;The Martian dayside ionosphere features a well-defined maximum in the electron density profile placed at about 130 km from the surface and produced by the interaction of the incoming UV solar radiation with the upper atmosphere. The ionosphere of Mars is thus strongly affected by changes in the underlying neutral atmosphere and in the amount of solar radiation getting to the planet. Being one of the main reservoirs for atmospheric escape to space, it is important to characterize the variability of the ionosphere and its potential implications for the long-term evolution of the Martian climate.&#160;The major variability factor affecting the ionospheric main peak region is the change in the solar zenith angle (SZA), which produces an increase in the peak altitude and a decrease in the peak electron density when approaching the terminator (Withers, 2009). It is also well known that an increase in the solar activity would produce an increase in the peak electron density due to the enhanced ionization. Less attention has been devoted to the ionospheric seasonal variability. The significant eccentricity of the Martian orbit is expected to induce changes in the ionosphere. The changing Sun-Mars distance during the Martian year affects the amount of solar radiation getting to the planet, and thus is expected to modify the peak electron density. The changes in the temperature of the lower/middle atmosphere driven by the orbital eccentricity are also expected to affect the altitude of the peak. While previous works have confirmed these expectations (e.g. Zou et al., 2005; Morgan et al., 2008; N&#283;mec et al., 2011), a complete characterization of the seasonal variability has not yet been performed due to the limited seasonal coverage of the existing datasets. In this work we take advantage of the large ionospheric dataset collected by two instruments on Mars Express, the radar MARSIS on its Active Ionospheric Sounding (AIS) mode, and the radio-occultation experiment MaRS, during the more than 16 years (and still running) of the mission.&#160;Results&#160;In order to isolate the seasonal variability, we first remove the main variability factors, the change in the SZA and the change in the intrinsic solar activity, by using the well-known and thoroughly tested expressions derived from Chapman theory. Then we average the obtained corrected peak electron densities and peak altitudes in bins of 5 degrees of Ls. Finally we fit a sinusoidal function to the obtained seasonal variability. The main results we obtain are:&#160;1. The seasonal variability of both the peak electron density and the peak altitude can be well reproduced by sinusoidal functions maximizing around the date of the perihelion. The fitted amplitudes are, for the peak electron density, about 9% of the annually-averaged value, and for the peak altitude about 9 km.&#160;2. We find hints of latitudinal differences in the seasonal evolution. However the large spread of the data does not allow for a detailed study of these latitudinal effects.&#160;3. By separating the variability obtained during Mars Year 28 (MY28) from the rest of the years we find that the global dust storm during MY28 produced an increase in the peak altitude of about 10-15 km, in line with previous works (Girazian et al., 2020)&#160;4. We do not find an increase in the peak electron density around Ls=30-70, such as previously found for the TEC (S&#225;nchez-Cano et al., 2018)Fig. 1. Seasonal variability of the latitudinally-averaged, SZA and solar activity corrected peak electron densityFig. 2. Seasonal variability of the latitudinally-averaged, SZA corrected peak altitudeReferencesGirazian, Z., et al., Variations in the Ionospheric Peak Altitude at Mars in Response to Dust Storms: 13 Years of Observations From the Mars Express Radar Sounder. Journal of Geophysical Research (Planets), 125 (5), e06092. doi: 10.1029/2019JE006092, 2020Morgan, D. D., et al., Variation of the Martian ionospheric electron density from Mars Express radar soundings. Journal of Geophysical Research (Space Physics), 113 , 9303. doi: 10.1029/2008JA013313, 2008&#160;N&#283;mec, F., et al., Dayside ionosphere of Mars: Empirical model based on data from the MARSIS&#160;instrument. Journal of Geophysical Research (Planets), 116 (E7), E07003. doi:10.1029/2010JE003789, 2011&#160;S&#225;nchez-Cano, B., et al., Spatial, Seasonal, and Solar Cycle Variations of the Martian Total Electron Content (TEC): Is the TEC a Good Tracer for Atmospheric Cycles? Journal of Geophysical Research (Planets), 123 (7), 1746-1759. doi: 10.1029/2018JE005626, 2018JE005626, 2018&#160;Withers, P.. A review of observed variability in the dayside ionosphere of Mars. Advances in Space Research, 44 , 277-307. doi: 10.1016/j.asr.2009.04.027, 2009&#160;AcknowledgementsF.G.-G. is funded by the Spanish Ministerio de Ciencia, Innovaci&#243;n y Universidades, the Agencia Estatal de Investigaci&#243;n and EC FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award to the Instituto de Astrof&#237;sica de Andaluc&#237;a (SEV-2017-0709). F. N&#283;mec was supported by MSMT Grant LTAUSA1707