Observations of the Latitude and Frequency Distributions of Equatorward‐Propagating Chorus Waves

Abstract

Rising‐tone chorus elements are traditionally known to grow from low frequencies and extend to high frequencies. Using high‐resolution waveform measurements from the Van Allen Probes, we report a novel observation: equatorward‐propagating chorus waves at a few degrees in latitude are observed at high frequencies only. As these rising‐tone chorus waves propagate equatorward and approach the equator, they intensify and extend to lower frequencies. A multi‐year statistical analysis of Van Allen Probe A observations shows a systematic feature: at higher latitudes of source region, the equatorward‐propagating elements progressively appear at higher frequencies, exhibit narrower bandwidths and weaker amplitudes than the chorus wave elements propagating away from the equator. Such a feature challenges the conventional simple understanding that chorus waves grow from low frequencies and extend to high frequencies at the equator. These observations potentially supports the recently proposed nonlinear upstream–downstream generation framework described by the Trap–Release–Amplify (TaRA) model.