Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Geophysical Research Letters
Bow shocks are shock waves formed upstream of astrophysical objects when they interact with a supersonic plasma flow, such as the solar wind. These shocks can heat electrons through different processes, some of which involve smooth, gradual changes (adiabatic), while others involve more chaotic interactions (non-adiabatic). However, the relative contribution of adiabatic and non-adiabatic processes to electron heating has been a matter of debate.
Lalti et al. [2024] study data from 310 shock events observed by NASA’s Magnetospheric Multiscale (MMS) spacecraft at Earth’s bow shock, focusing on shocks with a normal vector almost perpendicular to the direction of the magnetic field. They develop a new method to measure how much of the heating is due to non-adiabatic processes by studying patterns in the electron energy distributions.
They found that the way electrons are heated is mainly controlled by the Alfvénic Mach number, a dimensionless parameter which describes how fast the shock is moving compared to a specific speed in the plasma, in a particular frame of reference (the deHoffman-Teller frame). When this Mach number is above 30, the heating changes from being mostly adiabatic to mostly non-adiabatic. When non-adiabatic heating is dominant, it matches well with a known process called stochastic shock drift acceleration.
Understanding the transition from adiabatic to non-adiabatic heating at Earth’s bow shock offers new insights into how particles are heated to high temperatures in our local cosmos. The findings are also directly applicable to any astrophysical shock in the same Mach number range. It is likely that the observed dominance of non-adiabatic heating for high Mach numbers will also hold for even higher Mach numbers characteristic of some astrophysical shocks.
Citation: Lalti, A., Khotyaintsev, Y. V., & Graham, D. B. (2024). Adiabatic and non-adiabatic electron heating at quasi-perpendicular collisionless shocks. Geophysical Research Letters, 51, e2024GL112547. https://doi.org/10.1029/2024GL112547
—Marit Oieroset, Former Editor, Geophysical Research Letters