The Martian Bow Shock Over Solar Cycle 23–24 as Observed by the Mars Express Mission

B E S Hall,J A Wild,B Sánchez‐Cano,M Holmström,M Lester

doi:10.1029/2018ja026404

Abstract

AbstractThe Martian bow shock position is known to be correlated with solar extreme ultraviolet irradiance. Since this parameter is also correlated with the evolution of the solar cycle, it is expected that the Martian bow shock position should also vary over such a period. However, previous reports on this topic have often proved contradictory. Using 13 years of observations of the Martian bow shock by the Mars Express mission over the period 2004 to 2017, we report that the Martian bow shock position does vary over the solar cycle. Over this period, our analysis shows the bow shock position to increase on average by 7% between the solar minimum and maximum phases of solar cycle 23–24, which could be even larger for more extreme previous solar cycles. We show that both annual and solar cycle variations play major roles in the location of the bow shock at Mars.

Highlights

The Martian bow shock develops to slow the supermagnetosonic flowing solar wind to subsonic speeds such that it can be diverted about the Martian plasma system
This period covers approximately 7 Martian years (MY, terrestrial days, see top of panel a), and a full solar cycle of variability, in particular the cycles referred to as solar cycle 23 and 24
In this paper we have used ~13 years of near-continuous observations of the Martian bow shock by the Mars Express mission to report that its average position is sensitive to solar activity, and varies over the solar cycle

Summary

Introduction

The Martian bow shock develops to slow the supermagnetosonic flowing solar wind to subsonic speeds such that it can be diverted about the Martian plasma system. Unlike celestial bodies that are enclosed within an intrinsic global magnetic field (e.g., the magnetosphere of Earth), Mars' ionosphere and extended exosphere instead acts as an obstacle to the solar wind flow. The location of the bow shock depends on several drivers that can be either external, such as the solar extreme ultraviolet (EUV) flux, the solar cycle phases, the solar wind dynamic pressure, the interplanetary magnetic field (IMF) orientation, or the magnetosonic Mach number; or internal, such as the crustal magnetic field sources location rotating with the planet. Both types of drivers produce different types of bow shock variability, which can manifest on short and long time-scales. More details can be found in e.g. Mazelle et al

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