A Systematic Study in Characteristics of Lower Band Rising‐Tone Chorus Elements

Abstract

Chorus waves are usually generated outside the plasmapause in the equatorial region of the magnetosphere. The discrete characteristics of chorus elements are quantified by the three parameters: lasting time, frequency bandwidth, and repetition period. A systematic study in the lasting time and frequency bandwidth in terms of background plasma and magnetic fields has not been performed in the past. Here we use burst mode waveform data from the Time History of Events and Macroscale Interaction during Substorms (THEMIS) probes and the random forest method of machine learning and Pearson's correlation analysis to investigate which background plasma and magnetic field parameter is dominant over the lasting time and frequency bandwidth. We find that the temperature is the most important parameter that controls the lasting time. The lasting time is shorter when this temperature is higher. We also find that the normalized bandwidth by the local electron cyclotron frequency is controlled by the number density of energetic electrons. The normalized bandwidth is wider when this number density is larger. These results can be well explained by the threshold and optimum wave amplitudes for the nonlinear generation of chorus waves (Omura & Nunn, 2011, https://doi.org/10.1029/2010JA016280). The findings derived from this analysis can be used to serve as a guideline for a deep understanding of the generation mechanism of chorus elements and help choose input for a modeling of wave‐particle interactions in the radiation belts.

Rapid Frequency Variations Within Intense Chorus Wave Packets

“…(2017) and Shue et al. (2019) based on limited statistics of long packets (most with β > 50–100) from measurements of the Van Allen Probes and THEMIS, respectively.…”

“…Shue et al. (2015, 2019) analyzed the 2008–2010 waveform data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, examining different characteristics of chorus elements as a function of background plasma and magnetic field. In particular, Shue et al.…”

“…In particular, Shue et al. (2019) examined characteristics of 1,329 rising‐tone chorus elements, mostly long wave packets lasting > 50–100 wave periods. They found that wave packet duration and normalized bandwidth (i.e., final frequency minus initial frequency) are well correlated with energetic electron temperature and density, respectively.…”

Particle‐in‐Cell Simulations of Characteristics of Rising‐Tone Chorus Waves in the Inner Magnetosphere

“…Statistical analysis based on Van Allen Probes data suggests the chorus duration might be affected by the background magnetic field inhomogeneity (Teng et al, 2017). Recently, with the random forest method of machine learning and Pearson's correlation analysis, Shue et al (2019) investigated the characteristics of chorus waves by using THEMIS data and indicated that the chorus duration is inversely correlated with the temperature and the chorus frequency span is positively correlated with the number density of energetic electrons. In this paper, by performing a series of one-dimensional (1-D) particle-in-cell (PIC) simulation runs in a mirror magnetic field, we investigate the dependence of these properties of rising-tone chorus waves on the number density and the temperature anisotropy of energetic electrons.…”

ULF modulation of energetic electron precipitation observed by VLF/LF radio propagation

“…Rising-tone chorus emissions were found in the frequency range of 4.5 kHz to 6.5 kHz during the energetic electron precipitation. The typical duration, frequency bandwidth, and repetition period of the rising-tone emissions were 0.5 s, 1.0 kHz, and 0.5 s, respectively, similar to lower-band whistler-mode chorus waves [48,49]. Using Tsyganenko 2001 and 2004 magnetic fi eld models [50,51], half of the electron gyrofrequency at the magnetic equator, the magnetic footprint of which is just above ATHA, was estimated to be ~4 kHz.…”