The Alfvén-wave acceleration of auroral electrons: laboratory measurement

Since the 1970’s, Alfvén waves have been predicted to accelerate auroral electrons. Spacecraft measurements have shown that powerful descending Alfvén waves are common above the bright and active discrete auroras of geomagnetic storms. Theoretical and computational work has suggested that auroral electrons can be accelerated by Alfvén waves via Landau resonance. A direct test requires simultaneous measurements of electrons and the wave fields responsible for their acceleration. Spacecraft data have not provided such a direct test. Laboratory measurements using UCLA’s Large Plasma Device (LAPD) avoid many of the limitations of spacecraft data and offer increased control and repeatability. To produce conditions relevant to an altitude of 1-3 Earth radii where electron acceleration occurs, the LAPD is tuned so v_A>v_te. Inertial Alfvén waves are launched by an antenna, and the electron velocity distribution is measured using the resonant absorption of a small amplitude whistler-mode wave. Data are analyzed using the field-particle correlation technique. Experimental results show that electrons near the Alfvén wave phase velocity gain energy from the Alfvén wave, indicating electron acceleration by Landau resonance. The field-particle correlation produced by analytical kinetic theory agrees with experimental results. Two additional theoretical approaches are consistent with experimental results including Liouville mapping of a test-particle distribution through simulated Alfvén wave fields and a nonlinear gyrokinetic simulation. The agreement of direct measurements with theory and simulation shows for the first time that Alfvén waves can accelerate electrons that cause the aurora.