Relationship of dayside main layer ionosphere height to local solar time on Mars and implications for solar wind interaction influence

To understand the influence of solar wind on the daytime main layer ionosphere of Mars, we investigated the local solar time (LST) variations of three characteristic heights of the ionosphere, namely, the heights of the 1 MHz and 1.5 MHz reflection points (i.e., 1.24 × 1010 m−3 and 2.79 × 1010 m−3 isodensity contours, respectively) and the density peak. We used a total of 19,996 Mars Advanced Radar for Subsurface and Ionosphere Sounding observations distributed on the northern hemisphere, with solar zenith angle ≤80°, acquired from June 2005 to October 2013. We exploited the kernel partial least squares regression method to extract the nonlinear relationships of the heights to LST and a few other variables. The average height of the 1 MHz reflection point decreased from ~218 km at 10:00 A.M. to ~206 km at 16:00 P.M. local time; the height of the 1.5 MHz reflection point decreased simultaneously from ~190 km to ~181 km. These decreasing trends are in contrast to the LST variation of the density peak height, which increased from ~128 km to ~137 km over the same LST interval. Based on these findings and previous results, we suggest that the solar wind may penetrate the Martian ionosphere down to altitudes of about 50 km above the main density peak and may, in conjunction with the asymmetric draping of the interplanetary magnetic field, compress the upper part of the main ionosphere layer on the P.M. side ~10 km more than on the A.M. side.