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

¹ State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, PR China
² Institute of Space Weather, Nanjing University of Information Science and Technology, Nanjing, PR China
³ CNSA Macau Center for Space Exploration and Science, Macau, PR China
⁴ State Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing, PR China
e-mail: xielianghai@nssc.ac.cn
⁵ Joint Research and Development Center of Chinese Science Academy and Shen county, Shandong, PR China
⁶ Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
e-mail: louclee@earth.sinica.edu.tw
⁷ Department of Physics and Space Science, Royal Military College of Canada, Kingston, ON, Canada
⁸ Planetary Environmental and Astrobiological Research Laboratory (PEARL), School of Atmospheric Sciences, Sun Yatsen University, Zhuhai, PR China

Received: 8 February 2021
Accepted: 15 April 2021

Abstract

Using a 3D multispecies magnetohydrodynamic model, we investigated the effect of the solar wind dynamic pressure (P_d) 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 P_d 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 P_d according to the power-law relations. For the same P_d, 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 P_d. 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.