Laboratory measurements of the physics of auroral electron acceleration by Alfv\xe9n waves

J. W. R. Schroeder,F. Skiff,S. Vincena,C. A. Kletzing,G. G. Howes,T. A. Carter,S. Dorfman

doi:10.1038/s41467-021-23377-5

Abstract

While the aurora has attracted attention for millennia, important questions remain unanswered. Foremost is how auroral electrons are accelerated before colliding with the ionosphere and producing auroral light. Powerful Alfvén waves are often found traveling Earthward above auroras with sufficient energy to generate auroras, but there has been no direct measurement of the processes by which Alfvén waves transfer their energy to auroral electrons. Here, we show laboratory measurements of the resonant transfer of energy from Alfvén waves to electrons under conditions relevant to the auroral zone. Experiments are performed by launching Alfvén waves and simultaneously recording the electron velocity distribution. Numerical simulations and analytical theory support that the measured energy transfer process produces accelerated electrons capable of reaching auroral energies. The experiments, theory, and simulations demonstrate a clear causal relationship between Alfvén waves and accelerated electrons that directly cause auroras.

Highlights

Statistical studies of the spatial distribution of Alfvén wave power and precipitating electron flux using FAST and Polar measurements have shown that the spatial distribution in magnetic local time and latitude of Alfvén wave Poynting flux is well correlated with the distribution of accelerated electrons and the auroral emission[22,23,24], with modeling suggesting the accelerated electrons primarily arrive from an altitude of 2 RE ≲ z ≲ 3 RE22
While the characteristic parameters of the Large Plasma Device (LAPD) experiments differ by orders of magnitude from those in the auroral magnetosphere, a similarity analysis can be exploited to demonstrate that the key dimensionless parameters governing the physics of auroral electron acceleration can be reproduced in the laboratory[35,37]
Our experiments achieve a Whistler Wave Absorption Diagnostic (WWAD) probes (W1, W2). b Contour plot of B⊥(x, y) of the inertial Alfvén wave in the plane perpendicular to B0 measured at E2

Summary

Introduction

Statistical studies of the spatial distribution of Alfvén wave power and precipitating electron flux using FAST and Polar measurements have shown that the spatial distribution in magnetic local time and latitude of Alfvén wave Poynting flux is well correlated with the distribution of accelerated electrons and the auroral emission[22,23,24], with modeling suggesting the accelerated electrons primarily arrive from an altitude of 2 RE ≲ z ≲ 3 RE22. The Alfvén wave Poynting flux was found to be sufficient to power ~1/3 of the global auroral luminosity, and up to 50% in the pre-midnight sector where substorm driven auroral events occur[23,24], and Alfvén waves may be the dominant mechanism of electron acceleration during geomagnetic storms[20,25,26] Together, these conjunction and statistical studies build a strong case for the Alfvén-wave acceleration of auroral electrons. It is scientifically desirable to confirm definitively the kinetic physics of Alfvénic electron acceleration by simultaneously measuring the Alfvén wave fields and the resulting changes to the electron velocity distribution. Both measurements are essential to diagnose directly the transfer of energy between the waves and electrons. While it was previously known that Alfvén waves are often coincident with auroras, this direct measurement demonstrates a causal relationship between Alfvén waves and accelerated electrons that produce auroras

Methods

Results

Conclusion