Properties and Occurrence of Periodic Rising and Falling Tone ECH Waves: Statistical Results

Abstract:

Electrostatic Electron Cyclotron Harmonic (ECH) waves play an important role in driving diffuse aurora at large L shells (L > 8) via pitch angle scattering plasma sheet electrons into the upper atmosphere, which further influences the ionospheric conductance. Their generation, properties and effects have been studied extensively; however, the new feature of periodic rising and falling tone has not been evaluated in previous studies. In this work, by using Van Allen Probes observations, we performed a survey of ECH wave events with fine frequency structures of rising or falling tones, and identified more than twenty events in total. We found that most of these events occurred near the equator (< 4 degrees) over the 16–03 h MLT sector, which is mostly consistent with the statistical ECH wave distribution but in slightly earlier MLTs. The averaged wave electric amplitude is around 0.1 mV/m over L = 4.5–6.5. Events typically have a repetition period ranging from 1 to 2 minutes, and the period increases with increasing L shell at L < 6. We have also calculated wave element duration and element chirping rate, indicating the peak occurrences at ~40 seconds and ~50 Hz/s, respectively. Moreover, we analyzed the plasma and geomagnetic conditions during the presence of these waves to study the favorable condition for the generation of periodic rising and falling tone ECH waves. We found that most of these events occurred in relatively high plasma density regions compared to normal ECH waves and during geomagnetically quiet conditions (AL > -100 nT). Overall, our study reports the observations of periodic rising and falling ECH waves, and revealed their properties and dependences on the plasma and geomagnetic conditions, providing credible insights into their generation mechanisms.