Responses of Electrostatic Electron Cyclotron Harmonic Waves to Solar Wind Parameters and Geomagnetic Activity

As one of the typical electrostatic waves in the terrestrial magnetosphere, electron cyclotron harmonic (ECH) waves are capable of scattering hundreds of eV to several keV electrons and precipitating them into the atmosphere. In this study, using combined observations from the Van Allen Probes, Arase, and MMS missions spanning 2012–2023, we present a comprehensive statistical survey of electrostatic electron cyclotron harmonic (ECH) waves in Earth’s magnetosphere. ECH waves are observed over a broad region covering L = 3–15, MLAT < 40°, and nearly all magnetic local time sectors, exhibiting pronounced spatial and regional variations. In the inner magnetosphere (L < ~6), wave power preferentially peaks from premidnight to noon, whereas in the outer magnetosphere (L > 6), ECH waves occur most frequently on the dayside. Moreover, ECH waves are predominantly confined near the magnetic equator (MLAT < 5°) at L < ~8, while showing a much broader latitudinal extent (up to MLAT < 35°) at higher L. Furthermore, we investigate the dependence of ECH waves on solar wind conditions and geomagnetic activity indices, revealing pronounced day–night differences in the wave responses to solar wind driving and geomagnetic disturbances. These results suggest different generation and modulation processes of ECH waves in the dayside and nightside magnetosphere. In addition, based on the multi-satellite statistical results, we construct a global empirical model of ECH wave distribution, providing a quantitative framework for incorporating ECH waves into radiation belt and space weather studies.