On the relationship of electrostatic cyclotron harmonic emissions with electron injections and dipolarization fronts

Electron cyclotron harmonic (ECH) waves in Earth's magnetotail have long been considered a potential driver of diffuse aurora. Because of observational constraints prohibiting theoretical progress, however, no consensus on the plasma conditions that enable excitation and observation of these waves has emerged, especially in the outer magnetosphere. Intense ECH waves are often observed upon arrival of fast earthward flows, which in turn are correlated with particle injections, and dipolarization fronts (DFs) in the plasma sheet. Using THEMIS observations, we investigate the relationship between such waves and electron injections, relevant for linear growth rates, and DFs, relevant for wave propagation. We find that >70% of ECH waves are correlated with injections and >50% of the waves are correlated with DFs. The median time lag for ECH wave onsets relative to local particle injection and DF onset is similar to 500s and similar to 60s, respectively. When ECH waves are correlated with both DFs and injections, injections are observed to occur first, then DFs, shortly followed by ECH waves. We discuss possible mechanisms leading to the intensification of ECH waves under different dynamic conditions, which helps to elucidate excitation of these waves in the outer magnetosphere.