Observational Properties of Harmonic EMIC waves: Statistical Study

TL;DR

This study provides the first comprehensive statistical analysis of harmonic EMIC waves observed by the Van Allen Probes, categorizing their types and identifying their occurrence conditions and associated plasma parameters.

Contribution

It introduces a novel statistical classification of harmonic EMIC waves and details their occurrence patterns and plasma environment dependencies.

Findings

Harmonic EMIC waves mainly occur on the dayside outside the plasmasphere.

ES and EM harmonic EMIC waves are associated with low $f_{pe}/f_{ce}$ and high proton $eta_H$.

Harmonic EMIC waves are linked to a strong fundamental EMIC mode.

Abstract

Electromagnetic ion cyclotron (EMIC) waves are discrete electromagnetic emissions separated by multiple ion gyrofrequencies. Harmonic EMIC waves are defined as waves with a strong electric or magnetic field (or both) at the harmonics of the fundamental EMIC mode. In this paper, for the first time, we present a statistical study on harmonic EMIC waves by the Van Allen Probes. The EMIC waves are categorized into three types based on their harmonics: (1) fundamental mode only (without higher harmonics), (2) electrostatic (ES) harmonics, and (3) electromagnetic (EM) harmonics. Our statistical study shows that ES and EM harmonic EMIC waves predominantly occur on the dayside, outside the plasmasphere with $L >5$ and are associated with a low $f_{pe}/f_{ce}$, a high proton $\beta_H$, and a strong fundamental EMIC mode. The results will advance our understanding of harmonic EMIC waves and their generation mechanisms.