In Situ Observations of Harmonic Alfven Waves and Associated Heavy Ion Heating

Resonant ion heating by high-frequency Alfven waves has long been believed to be the primary dissipation mechanism for solar coronal heating, and these high-frequency Alfven waves are considered to be generated via cascade from low-frequency Alfven waves. In this study, we report an unusual harmonic Alfven event from in situ observations by the Van Allen Probes in the magnetosphere, having an environment similar to that in the solar corona. The harmonic Alfven waves, which propagate almost along the wave vector of the fundamental waves, are considered to be generated due to the interaction between quasi-parallel Alfven waves and plasma density fluctuations with almost identical frequency. These high-frequency harmonic Alfven waves can then cyclotron resonantly heat the heavy ions. Our observations provide an important insight into solar corona heating by Alfven waves.