Solar Wind Alfven Mach Number Research Articles

Properties of Electron Distributions in the Martian Space Environment

AbstractElectron and magnetic field measurements from the Mars atmosphere and volatile environment (MAVEN) mission are utilized to study the interaction between Mars and the solar wind. Instruments like the solar wind electron analyzer (SWEA) aboard MAVEN measure properties of the electron environment over a broad range of electron energies. Measurements at low electron energies include contributions from spacecraft photoelectrons and secondary electrons that must be accounted for to accurately characterize the environment. We developed an algorithm to identify and remove secondary electron contamination to improve estimates of electron densities and temperature. We then compiled global maps of average electron density, temperature, and temperature anisotropy under different conditions, considering quasi‐parallel and quasi‐perpendicular bow shocks and upstream solar wind Alfven Mach number. Higher temperature anisotropy is observed for quasi‐perpendicular shock crossings, as expected. We find significant electron temperature anisotropy upstream of the bow shock for quasi‐perpendicular shock crossings, suggesting a heating mechanism, such as that provided by electromagnetic waves. We analyzed the influence of hi and low Alfven Mach number conditions and found the electron plasma beta to be the only electron property significantly affected. We studied the relationship between the electron distribution function and the generation of instabilities and conclude that the upstream Alfven Mach number influences the stability of electron distributions in the Martian environment.

Understanding the response of the ionosphere-magnetosphere system to sudden solar wind density increases

[1] The different responses of the magnetosphere to sudden solar wind density enhancements are investigated. The majority of the responses are observed to have two phases: the first is due to an inductive E field propagating through the system, while the second is due to a vortex formation in the magnetosphere. However, during some events, the system displays a single-phase response. In order to understand the controlling parameters in the system response, statistical analyses of several solar wind external characteristics as well as the internal index Dst are conducted. Results of these analyses show that none of those characteristics seem to be associated with the way the geospace system responds to the abrupt solar wind density increase. Owing to the lack of statistics in the analysis, simulations investigating the effect of the solar wind Alfven Mach number on the magnetospheric response have been conducted. The simulation results indicate that the two-phase response is always present but that a low solar wind Mach number causes an indistinct two-phase response, which is difficult to identify as having two phases. Compression signals following the sudden compression at the magnetopause are carried by a fast mode wave in the magnetosphere, which propagates toward the Earth and is reflected back toward the magnetopause by the plasmapause or ionosphere. The wave is subsequently reflected back again, leading to a secondary Earthward propagation. These multiple bounces of the fast mode wave result in two groups of the two-phase responses in the ionosphere, although the second group is extremely difficult to identify in ionospheric data.

Interaction of interplanetary shocks and rotational discontinuities with the Earth's bow shock

The magnetohydrodynamic (MHD) aspects of the interaction between the Earth's bow shock and interplanetary shocks or rotational discontinuities are systematically studied in this paper by MHD simulations. The interplanetary shocks under consideration include forward fast shocks, reverse fast shocks, forward slow shocks, and reverse slow shocks. As an incident forward shock transmits through the bow shock, a fast shock, a slow expansion wave, a slow shock, and a contact discontinuity are generated downstream of the bow shock. At the same time, the bow shock is modified and moves earthward. If the incident shock is a reverse shock, the generated fast shock becomes a fast expansion wave, and the bow shock moves away from the Earth. The generated fast shock or fast expansion wave carries most of the total pressure variation imposed by the incident shock. The contact discontinuity also carries a significant part of the pressure variation. The slow expansion wave and the slow shock are both generated with a small variation in plasma density and pressure but a large variation in magnetic field. When the solar wind Alfven Mach number is small, the density variations associated with the slow shock or slow expansion wave can be significant. Through the interaction between an incident rotational discontinuity and the bow shock, a plateau in the plasma density and thermal pressure is formed. The magnetic pressure is depressed accordingly and anticorrelated with the thermal pressure. If the incident rotational discontinuity propagates toward the Earth (Sun) in the solar wind frame, the leading (trailing) edge of the plateau consists of an intermediate shock or a time‐dependent intermediate shock and a slow shock, while the trailing (leading) edge is mainly a slow shock. The generated structure with enhanced plasma density and thermal pressure and a depressed magnetic pressure agrees very well with the observed slow‐mode structure in the magnetosheath. Strong dynamic pressure variations associated with this structure may impinge on the magnetopause as a strong pressure pulse.

A study of multiple X line reconnection at the dayside magnetopause

Interaction between the interplanetary magnetic field and the earth's geomagnetic field at the dayside magnetopause is studied by a global two‐dimensional incompressible MHD simulation. It is found that for a large magnetic Reynolds number (Rm ≥400), the multiple X line reconnection (MXR) becomes the major dayside reconnection process, while for a small magnetic Reynolds number (Rm ≤100), the classical single X line reconnection takes place. Since the magnetic Reynolds number at the dayside magnetopause is probably very high (Rm >10³), it may be suggested that MXR will dominate the dayside reconnection pattern. As a result of the MXR process, magnetic islands are formed on the dayside magnetopause surface. The dependence of the dayside reconnection geometry on the solar wind Alfvén Mach number is also examined.