Alfvén wave-particle acceleration near the ionosphere: a rocket-borne investigation

Earth’s geomagnetic field interacts with the external magnetic fields carried by the solar wind, a hot plasma emanating from the sun. Normally, our terrestrial magnetic shield impedes much of the Sun’s influence from reaching the surface of Earth. However, when conditions are just right, the sun’s influence can trickle down into our atmosphere to be seen as bright and beautiful aurora. Understanding this energy cascade, which starts at the sizes of planets down to a few tens of kilometers is complex and highly variable in space and time. Despite this, commonalities exist between solar “storms” that produce predictable behavior near Earth’s electrically conductive atmosphere, called the Ionosphere. When disturbances occur in Earth’s outer magnetic field, they can propagate down to the Ionosphere as plasma waves and interact with ambient particles. When certain conditions are met, these particles literally “surf” the wave and gain energy. Just like humans surfacing an ocean wave, these electrons must have speed that’s close to the wave speed in order to be picked up and accelerated, otherwise the wave just passes by. In this manuscript, we use data from a scientific sounding rocket to show that the processes which produce aurora also produce a greater number of electrons that can surf these plasma “Alfvén” waves compared to the ambient atmospheric particles, which are not associated with aurora. This work constitutes direct evidence for theoretically and numerically modelled predictions from the previous two decades, for which fundamental plasma experiments have only recently been capable of reproducing in highly-controlled laboratory experiments.