Excitation of electromagnetic ion cyclotron waves under different geomagnetic activities: THEMIS observation and modeling

Understanding excitation of electromagnetic ion cyclotron (EMIC) waves remains a considerable scientific challenge in the magnetospheric physics. Here we adopt correlated data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft under low (Kp = 1+) and medium (Kp = 4) geomagnetic activities to investigate the favorable conditions for the excitation of EMIC waves. We utilize a sum of bi‐Maxwellian components and kappa components to fit the observed ion (6–25 keV) distributions collected by the electrostatic analyzer (ESA) onboard the THEMIS spacecraft. We show that the kappa distribution models better and more smoothly with the observations. Then we evaluate the local growth rate and path‐integrated gain of EMIC waves by bi‐Maxwellian and kappa distributions, respectively. We demonstrate that the path‐integrated wave gain simulated from the kappa distribution is consistent with observations, with intensities 24 dB in H+ band and 33 dB in He+ band. However, bi‐Maxwellian distribution tends to overestimate the wave growth rate and path‐integrated gain, with intensities 49 dB in H+ band and 48 dB in He+ band. Moreover, compared to the He+ band, a higher proton anisotropy is needed to excite the H+ band waves. The current study presents a further observational support for the understanding of EMIC wave instability under different geomagnetic conditions and suggests that the kappa‐type distributions representative of the power law spectra are probably ubiquitous in space plasmas. Key Points kappa distribution yields a consistent wave gain to dataMaxwellian distribution tends to overestimate wave gainpeak wave frequency by both distributions agrees with data