A Systematic Study in Characteristics of Lower Band Rising‐Tone Chorus Elements. Issue 11 (26th November 2019)

• Record Type: Journal Article
• Authors: Shue, Jih‐Hong; Nariyuki, Yasuhiro; Katoh, Yuto; Saito, Shinji; Kasahara, Yoshiya; Hsieh, Yi‐Kai; Matsuda, Shoya; Goto, Yoshitaka
• Is Part Of: Journal of geophysical research. Volume 124:Issue 11(2019)
• Page Start: 9003
• Page End: 9016
• Publication Date: 2019-11-26
• Subjects: chorus waves -- lasting time -- frequency shift -- temperature -- energetic particles; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2019JA027368
• Languages: English
• ISSNs: 2169-9380

Abstract

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. Plain Language Summary: Chorus waves are a series of electromagnetic fluctuations at kHz‐order frequencies generated in the Earth's magnetosphere. The fluctuations resemble dawn chorus of bird songs and whistles when converted to sound. Each chorus wave has a unique tone and a duration of a few hundreds of milliseconds. The present study examines the cause for the diversity of chorus waves. We detect the duration and tone shifting of all well‐defined chorus waves in high‐resolution waveform data obtained from the Time History of Events and Macroscale Interaction during Substorms (THEMIS) probes, and compare them with the background physical quantities around the probes. Our analysis shows that the local background quantities have a potential to control the features of chorus waves. The duration of chorus waves depends on electron temperature and the tone shifting depends on both the number density of energetic electrons and the magnetic field intensity. Our results suggest that the background plasma condition predicts duration and tone shifting of chorus waves. Understanding of the relationship between background quantities and chorus properties would be important to predict a rapid flux enhancement of killer electrons that influences on satellite operations and astronaut activities in space. Key Points: The lasting time and frequency bandwidth of chorus elements are separately analyzed in terms of the background plasma and magnetic fields The lasting time of chorus elements depends upon the temperature of energetic electrons The normalized frequency bandwidth of chorus elements is controlled mainly by the number density of energetic electrons