Statistical Analysis of Subpacket Structure in Isolated and Overlapping Chorus Elements

The rising tone chorus elements show a fine structure consisting of multiple subpackets with varying amplitudes and durations. Using data from Van Allen Probe A (2012 −2019) and an automated ”isolation” algorithm, we identify 382594 chorus elements and classify them as isolated or overlapped. We find that during active conditions, these two types show opposite day-night asymmetry distribution in magnetic local time (MLT). The isolated chorus elements are observed more on the nightside and dawnside, corresponding to the shorter repetition time. Conversely, overlapped chorus elements dominate the dayside due to the smaller frequency difference between the overlapping segments, facilitating wave superposition inside packets. Additionally, we compare the properties of both types. We find the packets of isolated waves tend to exhibit longer duration and larger amplitude, and show good agreement with the nonlinear theory of chorus wave growth. However, the packets of overlapped waves are shorter and exhibit small frequency and amplitude differences confined in a narrow range, suggesting a contribution from wave superposition effects.