Large-Amplitude Magnetic Compression of a Collision-Free Plasma. II. Development of a Thermalized Plasma

Abstract

Numerical calculations are made of a strong one-dimensional disturbance traveling perpendicular to a magnetic field in a fully ionized and collisionless plasma. When the Alfvén-Mach number Mh is greater than 2, orbit crossings of the ions occur, which rapidly leads to thermalization perpendicular to the magnetic field if the crossings are extensive (Mh > 3). The thermalization approximates the behavior of a classical hydromagnetic shock, with a shock-front thickness roughly equal to the distance the shock front travels in one-half an ion gyration time. This length is somewhat greater than the ion gyration radius. The structure of the front is found to be strongly time dependent, and undergoes large fluctuations in an ion gyration period. It is argued that when the ion-electron mass is large, the magnetic forces tend to suppress electron orbit crossings. This results in relatively cold electrons in the shocked region, with the ions obtaining nearly all of the thermal energy. A relationship between the longitudinal electrostatic potential difference across the shock front and mass flow in the plane of the front is derived on the basis of a simplified model and is found to be in qualitative agreement with the numerical results. The range of applicability of the calculations to real plasmas is discussed.

Bow Shock Generator Current Systems: MMS Observations of Possible Current Closure

“…But also during rather stable conditions, it can be both nonuniform and nonstationary. For example, ripples can propagate on the surface and it has been suggested that the shock can exhibit cyclical reformation (e.g., Auer et al, ; Eastwood et al, ; Johlander et al, ). As shown in section 3, the events included in our database are observed Earthward of the nominal subsolar position of the bow shock, indicating that they correspond to rather elevated upstream solar wind conditions.…”

Observations of Magnetic Reconnection in the Transition Region of Quasi‐Parallel Shocks

“…In order to reduce flows from supersonic to subsonic speeds, collisionless shocks must dissipate energy by particle processes, that is, they are by necessity kinetic plasma structures. Understanding these microphysical processes is critical for understanding particle heating and acceleration (Auer et al, 1962;Gosling & Robson, 1985;Morse et al, 1972). The family of kinetic plasma processes responsible for energy dissipation is strongly dependent on shock parameters such as the Mach number, plasma beta, and the angle, Bn , between upstream magnetic field and shock normal Burgess and Scholer (2015).…”

Solar-Wind Flow Past Objects in the Solar System

“…This is not a unique property of these data, but has been observed repeatedly by other spacecraft in association both with the bow wave and with propagating interplanetary shock waves. The origin of these irregulari ties is not known definitely, but it has been suggested (8, 15, 96) that they may be a basic property of collisionless shock waves, as indicated by their resemblance to irregularities that appear in the calculated properties of such shock waves (112)(113)(114)(115) ; or (8, 15) that they be the result of great amplifica tion of small disturbances in the incident solar wind, as indicated by the corresponding gasdynamic or magneto hydrodynamic theory of the interac tion of small irregularities, such as sound waves, with a strong shock wave. Since the solar wind is rarely, if ever, truly steady, it appears likely that at least part of the irregularities observed downstream of the bow wave may be due to amplification, with a residual part, understandable in terms of the properties of collisionless shock waves, that would remain even if the solar wind were perfectly steady and uniform.…”