Plasma and magnetic-field structures near the Martian induced magnetosphere boundary

I. Plasma depletion region and tangential discontinuity

¹ State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macao, PR China
e-mail: xjxu@must.edu.mo
² Institute of Earth Science, Academia Sinica, Nankang, Taipei, Taiwan
³ School of Space and Environment, Beihang University, Beijing 100191, PR China
⁴ Planetary Environmental and Astrobiological Research Laboratory (PEARL), School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai, PR China

Received: 28 June 2019
Accepted: 21 July 2020

Abstract

The interaction between the solar wind and the Martian induced magnetosphere can lead to the formation of various regions with different plasma and magnetic-field characteristics. In this paper, these structures are investigated based on the plasma and magnetic-field measurements from Mars Atmosphere and Volatile EvolutioN (MAVEN). We find that the structures upstream of Mars are similar to those around Earth: both have a bow shock, magnetosheath, magnetopause, and a magnetosphere or induced magnetosphere. The inner part of Martian magnetosheath is called a plasma depletion region (PDR), similar to the plasma depletion layer upstream of the Earth’s magnetopause, in which the magnetosheath magnetic fields are piled up and the magnetosheath plasmas (including ions and electrons) are partially depleted. Several cases of PDRs are examined in detail. The hotter plasmas in PDRs are squeezed out along the enhanced magnetic field, resulting in the decrease of the plasma beta, the plasma density, and the ion temperature. The boundary between the magnetosheath and the induced magnetosphere is called the magnetopause, which can be identified as a magnetohydrodynamic discontinuity, either tangential discontinuity (TD) or rotational discontinuity, where the magnetic field changes its orientation. Tangential discontinuities with an insignificant normal component (B_N ≈ 0) of the magnetic field are the focus of this study. This discontinuity separates the magnetosheath H⁺ ions from the heavy ions (e.g. O⁺, O₂⁺) in the induced magnetosphere. Inside a TD, ions from both sides are mixed. There are 3332 boundary crossings by MAVEN in 2015, 1075 cases of which are identified as the TD (including the potential TD). Tangential discontinuities at Mars are at higher locations in the southern hemisphere and have an average thickness of ~200 km, mostly ranging from 50 to 400 km. The sample of TD is a decreasing function of θ (θ is the magnetic field rotation angle on the two sides of the TD). The PDRs in front of TDs are thicker in the northern hemisphere. From the sub-solar point to the Mars tail, PDR thickness increases and the proton number density and temperature decrease.