Laboratory measurements of the physics of auroral electron acceleration by Alfvén waves

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.

A User’s Guide to the Magnetically Connected Space Weather System: A Brief Review

“…Because these electrons and protons are very energetic, when they collide with atoms and molecules of oxygen, nitrogen, etc. in Earth's upper atmosphere, they excite these particles, which causes them to radiate (Schroeder et al, 2021). This process is described in Russell et al (2016) and an example of the results can be seen in Figure 12B, where the aurora is composed of many different colors, which correspond to what type of atom or molecule was excited.…”

Auroral Beads in Conjunction With Kinetic Alfvén Waves in the Equatorial Inner‐Magnetosphere

“…For inertial Alfvén waves, Schroeder et al. (2021) demonstrated Alfvénic accelerated electron beams in lab plasmas at $${v}_{Te}/{v}_{A}$$ ∼ 0.3. Based on these simulations and observations, it is clear that the 100–400 eV electron beams will be further accelerated by Alfvén waves at lower altitudes, and that Alfvénic acceleration can produce keV electrons as demonstrated in simulations (Artemyev et al., 2015; Damiano et al., 2018; Schroeder et al., 2021; Watt & Rankin, 2010) and observations (Wygant et al., 2002).…”

Observing Particle Energization above the Nyquist Frequency: An Application of the Field-Particle Correlation Technique

“…Specifically we use the Astrophysical Gyrokinetics Code (AstroGK), 21 which has successfully modeled heliospheric plasmas in a variety of previous applications. 14,17,[22][23][24] The simulation is designed with high-resolution in both velocityspace and time to facilitate identification of phase-space energization signatures using field-particle correlations. Then, the dataset is systematically reduced in time resolution and the FPC technique applied again at each step.…”

“…Signatures of Landau damping have been observed using this technique in both simulations 7,9,10,13,14 , spacecraft observations, 15,16 and laboratory experiments. 17 However, finite spacecraft sampling rates put a limit on the study of the highest frequencies in the solar wind. This limit is expressed by the Nyquist-Shannon sampling theorem, which was first implied in the literature by Nyquist in the 1920s 18 and expressly written by Shannon two decades later.…”