MAVEN and MEX Multi‐instrument Study of the Dayside of the Martian Induced Magnetospheric Structure Revealed by Pressure Analyses

Abstract

A combination of statistical studies and 18 case studies have been used to investigate the structure of the induced Martian magnetosphere. The different plasma and magnetic pressure forces on the dayside of the induced magnetosphere of Mars have been studied using 3.5 years of Mars Atmosphere and Volatile Evolution (MAVEN) and Mars Express (MEX) observations. We present estimates of typical values for the dominant pressure terms, that is, the thermal pressures of the ionosphere and the magnetosheath, the magnetic pressure of the magnetic pile-up region, and the solar wind dynamic pressure. For 18 typical orbits the altitudes and relative distances of the pressure balance boundaries, the photoelectron boundary, the ion composition boundary, and the induced magnetosphere boundary are estimated. The magnetic pile-up boundary is discussed but not further studied since earlier characterizations of the magnetic pile-up boundary do not agree with our results. This study focuses on the transition region between the ionosphere and the magnetosheath on the dayside of Mars. We show that earlier definitions of the photoelectron boundary, ion composition boundary, and induced magnetosphere boundary do not characterize the transition region well, mainly because each boundary is based on measurements from only one or two instruments. In order to characterize the transition region correctly, changes in magnetic field strength and fluctuations, dominant ion species, electron and ion densities and energy distributions need to be considered. This article confirms a complex interaction between Mars and the solar wind and can explain why previous studies have had difficulties to describe the force balance.

Mars' Ionopause: A Matter of Pressures

“…We note that all the calculated quantities with Equations – agree very well with the recent MAVEN estimates by Holmberg et al (2019).…”

“…Figures 5 and 6 indicate that the larger the solar wind dynamic pressure, the lower the ionopause altitude. For those cases without an ionopause, the pressure balance could occur at higher altitudes than those considered in this study, or even at the lower edge of the MPB which is typically found at~1,000 km (e.g., Dubinin et al, 2008;Holmberg et al, 2019). This should be a focus of a future study in which higher altitude pressure observations are considered.…”

“…Therefore, it should only be considered as a proxy for the dynamic pressure of the solar wind (e.g., Holmberg et al, 2019; Witasse et al, 2017) and its maximum magnitude per orbit should be smaller than the actual solar wind dynamic pressure measured by MEX, (e.g., Ma et al, 2017). Note that at the subsolar point, the solar wind dynamic pressure in the sheath has a drastic reduction (e.g., Dubinin et al, 2008), which is not the case toward the flanks (e.g., Holmberg et al, 2019). In our case, due to orbit evolution, SWIA observations in the sheath come from regions far from the subsolar point.…”

“…At the diffusion region, both the electron density and electron temperature profiles show large irregularities. If there is no ionopause detection (Figures 2b and 2d), the diffusion region expands up to the lower boundary of the magnetosheath, at ~1,000 km (e.g., Dubinin et al, 2008; Holmberg et al, 2019). However, if there is an ionopause detection (dashed line in Figures 2a and 2c), the ionopause marks the end of the ionosphere, and so, of the diffusion region.…”

The Magnetic Structure of the Subsolar MPB Current Layer From MAVEN Observations: Implications for the Hall Electric Force

“…These particles, being relatively slow, heavy and numerous compared to the solar wind, decrease the average speed of the solar wind (Szego et al, 2000) in areas where the influence of crustal magnetic fields is negligible (Connerney et al, 2001). This deceleration precedes a change in the composition of the plasma, from solar wind ions to heavy ions of planetary origin, at the ion composition boundary (ICB), which on the dayside is almost coincident with the MPB (Breus et al, 1991; Halekas et al, 2018; Holmberg et al, 2019; Matsunaga et al, 2017; Sauer et al, 1994).…”