Scientists identify the trigger that switches how electrons heat up in space shocks
Researchers studying Earth's bow shock have pinpointed exactly when electrons shift from one heating mechanism to another—a transition controlled by magnetic field strength. The discovery resolves a decades-old physics question and could improve models used to predict space weather events that damage satellites and power grids.
Originaltitel: Adiabatic and Non-Adiabatic Electron Heating at Quasi-Perpendicular Collisionless Shocks
<p>The relative contribution of adiabatic and non-adiabatic processes to electron heating across collisionless shocks remains an open question. We analyze the evolution of suprathermal electrons across 310 quasi-perpendicular shocks with Alfvénic Mach numbers in the normal-incidence frame (M<sub>A-NIF</sub>) ranging from 1.7 to 48, using in situ measurements of Earth's bow shock by the Magnetospheric Multiscale (MMS) spacecraft. We introduce a novel non-adiabaticity measure derived from the electron distribution function and based on Liouville's theorem. Our results reveal, for the first time, that the electron heating mechanism is governed by the Alfvénic Mach number in the de Hoffman-Teller frame (M<sub>A-HT</sub>), with a transition from predominantly adiabatic to non-adiabatic heating occurring at MA-HT greater than or similar to 30. Furthermore, by examining the spectral index of the suprathermal electron distribution, we find that for shocks exhibiting dominant non-adiabatic electron dynamics, the observed electron heating is consistent with the predictions of the stochastic shock drift acceleration (SSDA) mechanism.</p>