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

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.

The global morphology of wave Poynting flux: powering the aurora

Large-scale, electric currents flowing along magnetic field lines into the polar regions of Earth are thought to be the main contributors of the energy that powers the ionospheric aurora. However, we have found evidence for global contributions from electromagnetic waves (Alfvén waves). Data that were collected from the Polar spacecraft over the course of 1 year show that the flow of wave electromagnetic energy at altitudes of 25,000 to 38,000 kilometers delineates the statistical auroral oval. The Poynting flux of individual events distributed along the auroral oval was larger than 5 ergs per square centimeter per second, which is sufficient to power auroral acceleration processes. This evidence suggests that in addition to magnetic field-aligned currents, the dayside and nightside aurora is globally powered by the energy flow of these high-altitude Alfvén waves.