Whistler-mode chorus are electromagnetic waves generated outside the plasmapause boundary when plasma sheet electrons are injected into Earth’s inner magnetosphere, creating temperature-anisotropic electron distributions unstable to wave growth. Wave-particle interactions then lead to acceleration and/or precipitation of radiation belt electrons. While low amplitude chorus (< 10 mV/m) may interact quasi-linearly, recently observed high amplitude chorus whistler waves (up to 400 mV/m) may accelerate electrons to MeV energies in less than a second.
Despite their fundamental role in radiation belt physics, several key properties of chorus waves remain unexplained, in particular their tendency to appear in chains of localized wave packets. Various authors have attributed this packetization to mechanisms such as wave ducting or trapping by density wells/enhancements, lower-hybrid wave interactions, or ponderomotive wave growth. Coincident observations of chorus wave packets and fluctuations in spacecraft potential (interpreted as density fluctuations) have often served as evidence for these processes.
Using Van Allen Probes data, we find that the spacecraft potential fluctuations observed during high amplitude chorus are not due to true density fluctuations, but are instead produced by an enhancement in photoelectron escape caused by the chorus wave electric fields. We present Van Allen Probes data demonstrating this result, supported by laboratory investigations and simulations of spacecraft charging.
This result has far-reaching consequences for the interpretation of spacecraft potential fluctuations as plasma density structure in any environment where high amplitude electric fields are present and photoelectron currents dominate spacecraft charging.