Statistical analyses of the multi-scale spatial and temporal variation of Martian bow shock and magnetosheath structure
Creators: name: Fruchtman, Jacob
Publisher: name: University of Iowa
Year: 2024
Subjects: Bow Shock
Magnetosheath
Mars
MAVEN
spatial analysis
Statistical physics

description: The solar wind is a plasma which constantly blows out from the sun at high speeds. It encounters Mars, producing the Solar wind-Mars interaction region. Here, the solar wind must slow down and be compressed due to conservation of energy-momentum in order to be deflected around a planet. The solar wind is traveling faster than the phase speed of the plasma - too fast to be compressed gradually, and so instead there is a sharp boundary where it transitions from fast and uncompressed to slowed down and compressed. The boundary surface is called the bow shock and the downstream compressed region is called the magnetosheath. The SW Mach numbers and plasma beta, and the amount of solar extreme ultraviolet irradiance, to name a few parameters, control the geometry of the system as well as the structures and dynamics at and downstream of the bow shock. Also important is the spatial position - of the shock with respect to Mars or the Mars-Sun line, of the equator with respect to the ecliptic plane, or of Mars with respect to the Sun. Variation in the irradiance and the geometry of the Martian orbit additionally introduce seasonal variation into the Martian atmosphere, which may drive corresponding variation in the bow shock and magnetosheath.
description: We measure and analyze these dependences in order to better understand how the bow shock and magnetosheath work and what happens within. Things vary enough in space and time between measurements that any in-depth observational study must be statistical in nature. By aggregating five years of in-situ Mars Atmosphere and Volatile EvolutioN (MAVEN) data, we can statistically analyze the system at different scales.
description: We start by analyzing the bow shock discontinuity, at kinetic (shock overshoot), MHD (Rankine-Hugoniot magnetic compression), and global spatial scales. We find a lack of seasonal dependence in the shock overshoot and magnetic compression. We also find identical dependence of the overshoot on upstream parameters as has been found at Earth. Single species Rankine-Hugoniot jump conditions are found to be insufficient to predict the magnetic compression at Mars. We also find that the overshoot and magnetic compression drop with bow shock size and SZA respectively. We use this data for calibration purposes in order to analyze the magnetosheath as a boundary value problem.
description: Statistical studies of the magnetosheath volume are generally done via analyses of spatial distributions of a data set. To do so, space is divided into 3D cells (or bins) with each cell filled with the average value of spacecraft measurements made within the volume. Because the magnetosheath system varies in size and shape, and magnetosheath processes are set by the conditions at the bow shock, the standard technique of utilizing a fixed Cartesian coordinate grid will unavoidably lead to cells sampling multiple different processes - and individual processes being sampled by multiple different cells - at different times, resulting in coarse grained distributions. We develop a new framework for constructing spatial distributions in order to attain higher precision. This `scale normalization' framework accounts for the system geometry and the compressible nature of the plasma by using a dimensionless shock based coordinate system, where the primary coordinate is the fractional distance from the bow shock surface to an inner boundary.
description: Our implementation of this framework allows us to resolve otherwise obfuscated spatial structures - such as the Larmor thickness shock overshoot - and determine how they vary with upstream conditions. Our technique also allows for improved precision in calculating forces. This allows us to distinguish the forces within the magnetosheath interior from those at the boundaries. We find that the magnetic pressure gradient at the shock vaguely agrees with theoretical power law scaling predictions. This agreement with predictions becomes significant once appropriate normalization conditions are introduced. We find that the ion pressure divergence at the shock is anisotropic, varying more significantly with IMF orientation than with Mach number. The variation in anisotropy and mirror instability throughout the magnetosheath is analyzed, and it is found that flow-lines starting at the BS flanks never reach mirror stability.