Decoupling the influence of solar cycle and seasons on Mars' dayside ionosphere: Insights from MAVEN observations during the declining phase of solar cycle 24

The Mars ionosphere, highly sensitive to solar activity and seasonal changes, exhibited noticeable variation in plasma density during the declining phase of solar cycle 24. This study aims to differentiate the impact of solar activity and changes in Sun-Mars distance (seasons) by comparing ionospheric density measurements from MAVEN NGIMS and LPW instruments during 2015-2018 period. We also investigated whether the ionosphere on opposite hemispheres show symmetry during equinoxes. In each case, we quantified the contribution of atmospheric contraction to observed variability by analyzing densities at fixed pressure levels. Our finding suggests that a shift in solar activity from medium to low results in a plasma density decrease of up to 40%, with an additional 5-20% attributed by atmospheric contraction. A 90 degrees phase change in solar longitude showed insignificant plasma density variation when the 10% effect from atmospheric expansion is accounted. However, transition from perihelion to aphelion position results in atmospheric contraction that contribute up to 50% of observed variability, along with similar to 35% decrease due to reduction in solar radiation. Under similar solar activity and equinox conditions, ionospheric densities are symmetric in opposite hemispheres, when data from regions with strong crustal magnetic fields are excluded.