Spacecraft detect electron phase space holes undergoing strong wave-particle interactions

A fundamental way energy is transferred in collisionless plasmas is via wave-particle interaction. As a strong representation of this interaction, electron phase space holes (EHs) are nonlinear structures commonly observed through electromagnetic fields as bipolar electric field signatures in both laboratory and space plasma.

Norgren et al. made direct millisecond measurements of electron particle flux associated with individual EHs. The scientists were able to observe the continuous spatial-temporal variations in the electron distribution function.

“EHs are among the smallest structures that can form in a plasma, and measuring electrons associated with them is notoriously hard,” said author Cecilia Norgren. “A single EH is typically observed over only a few milliseconds, so historically, they’ve been identified through their electric field structure.”

The research was facilitated by technology onboard NASA’s Magnetospheric Multiscale Mission satellites. The Fast Plasma Investigation measures electrons and ions with unprecedented time resolution, while the Electron Drift Instrument measures displacement of weak beams of test electrons emitted and returned to spacecraft after one or more gyrations.

The study recorded observations of intense EHs in the plasma sheet boundary layer – a temporally variable transition between the magnetotail lobes and central plasma sheet – along with an electron beam moving toward a reconnection X line.

“For the first time, we measured continuous fluctuations in electron flux associated with electrostatic solitary waves, directly quantifying the strong EH-electron interaction,” said Norgren.

The observations provide evidence of strong nonlinear wave-electron interaction within EHs, a crucial ingredient for heating plasma irreversibly. In magnetic reconnection regions, where large amounts of energy are deposited into the plasma, the interaction contributes to transforming drift energy into thermal energy.

Source: “Millisecond observations of nonlinear wave-electron interaction in electron phase space holes,” by C. Norgren, D. B. Graham, M. R. Argall, K. Steinvall, M. Hesse, Yu. V. Khotyaintsev, A. Vaivads, P. Tenfjord, D. J. Gershman, P.-A. Lindqvist, J. L. Burch, and F. Plaschke, Physics of Plasmas (2022). The article can be accessed at https://doi.org/10.1063/5.0073097 .

This paper is part of the Special Topic section in Plasma Physics from the Magnetospheric Multiscale Mission, learn more here .