Effect of solar wind density and velocity on the subsolar standoff distance of the Martian magnetic pileup boundary

Abstract

Using a 3D multispecies magnetohydrodynamic model, we investigated the effect of the solar wind dynamic pressure (Pd) with different densities and velocities on the subsolar standoff distance (r0) of the Martian magnetic pileup boundary (MPB). We fixed the solar maximum condition, the strongest crustal field located in the dayside region, and the Parker spiral interplanetary magnetic field at Mars. We simulated 35 cases with a Pd range of 0.1494 to 7.323 nPa (solar wind number density n ∈ [1, 9] cm−3, and solar wind velocity V ∈ [−258, −1344] km s−1). The main results are as follows. (1) r0 decreases with increasing Pd according to the power-law relations. For the same Pd, a higher solar wind velocity (lower density) results in a larger r0 of the Martian MPB. (2) A higher solar wind density leads to a lower ratio of the compressed magnetic field strength to the crustal field strength and a larger plasma β under the same Pd. This indicates that the thermal pressure at the Martian MPB plays a significant role for the compressed magnetic field. Because the magnetic pileup process is stronger for a higher solar wind velocity, the magnetic pressure at the Martian MPB is increased. As a result, the thermal pressure decreases and r0 of the Martian MPB increases. (3) We present a new formula of r0 with the parameters of the solar wind dynamic pressure, number density, and velocity.