A statistical survey of electrostatic electron cyclotron harmonic waves based on THEMIS FFF wave data

Abstract

AbstractBased on the high‐resolution FFF wave spectral data obtained from the three innermost Time History of Events and Macroscale Interactions during Substorms spacecraft, electrostatic electron cyclotron harmonic (ECH) emissions are identified, using automatic selection criteria, for the period from May 2010 to December 2015. A statistical analysis of wave spectral intensity, peak wave frequency, and wave occurrence rate is performed for the first harmonic ECH waves that are predominantly strongest among all harmonic bands, in terms of dependence on L shell, magnetic local time (MLT), magnetic latitude, and the level of geomagnetic activity. Our results indicate that ECH emissions are preferentially a nightside phenomenon primarily confined to the MLT interval of 21–06 and that the most intense ECH waves are commonly present at L = 5–9 and MLT = 23–03 within 3° of the magnetic equator. As the geomagnetic activity intensifies, averaged nightside ECH wave amplitude can increase from a few tenth mV/m to well above 1 mV/m. The presence of >0.1 mV/m ECH emissions extends from L < 10 to L > ~12 with a broad MLT coverage from the evening to postdawnside at the occurrence rate above 20% for the equatorial emissions and at a rate up to ~7% for higher‐latitude waves. Overall, the average peak wave frequency of the first harmonic ECH waves is located ~1.5 fce (where fce is the electron gyrofrequency) for L < 10 and becomes smaller at higher L shells. It also exhibits a tendency to shift to lower frequencies with increasing geomagnetic activity level. By finalizing a numeric table that gives the statistically average values of wave amplitude and peak wave frequency for different ranges of L shell, MLT, and geomagnetic activity level, our detailed investigation provides an improved statistical model of ECH wave global distribution in the Earth's inner and outer magnetosphere, which can be readily adopted as critical inputs in diffusion codes to evaluate the rates of ECH wave‐driven pitch angle scattering and to determine the precise contributions of ECH waves to the plasma sheet electron dynamics and diffuse auroral electron precipitation.