Multi-Spacecraft Observations of Whistler-Mode Wave-Particle Interactions: Temporal or Spatial Structures?

Abstract:

Wave-particle interactions that generate whistler-mode chorus emissions and modify energetic electron distributions are thought of as highly localized phenomena in both space and time. We will use data from the two satellite Van Allen Probe constellation to investigate such events. Individual chorus elements have very short time durations and interaction regions that span small distances at the magnetic equator. Recent observations by the Van Allen Probes satellites imply in contrast that the interactions sometimes form relatively long-lived signatures in both electrons and waves that drift past the satellites. Fennell et al [GRL, 2014] described an event with upper-band chorus emissions that correlated well with simultaneously measured bursts of electron flux at 30 keV in narrow ranges of pitch angle. Both satellites, separated by 0.3 hours in the morning local time sector near L~6, observed similar quasiperiodic trains of correlated flux bursts and waves. Detailed comparison of the data from the two spacecraft shows that the sequence of wave and particle pulses match almost exactly if Probe A is shifted in time by two minutes relative to Probe B. This implies that the interactions produced a set of spatial structures that drifted from one satellite to the other at a speed of ~25 km/s. The result is consistent with the idea of both waves and electrons propagating and being observed at distant locations. But the high correlation of waves and electrons at near-zero delay time at both locations is surprising since the waves and electrons nominally travel at different speeds. Several additional wave-particle interaction events observed by both Van Allen Probes are analyzed to provide additional information on the spatial scale size and propagation speed of wave emissions and particle features.