Extent of ECH Wave Emissions in Earth's Magnetotail

The exact role of electron cyclotron harmonic (ECH) waves in driving diffuse aurora has been controversial for many years. Using THEMIS observations from five magnetotail seasons, we investigate the occurrence rate distribution of ECH waves and the extent of individual wave intensifications under various plasma sheet conditions. Both are critical for modeling these waves’ contributions to electron scattering and to diffuse auroral emissions. Single-spacecraft data analysis shows that ECH waves occur frequently in the midnight and post-midnight magnetotail and that their occurrence rates decrease with increasing radial distance from Earth. More than 50% of ECH wave events (continuous intervals of wave activity) occur during local plasma sheet activations, evidenced by energetic electron injections and dipolarization fronts. Excluding ECH wave emissions concurrent with such local plasma sheet activations (known to peak in occurrence rate near pre-midnight), we find that quiet plasma sheet ECH wave emissions exhibit preference for dawn. This preference suggests a close relationship between these waves and the drift paths of injected electrons. Dual-spacecraft data analysis shows that the Z-extent is ~0.5 RE, the Y-extent is ~2 RE, and the X-extent is at least 4 RE. During locally quiet plasma sheet conditions, ECH waves exhibit a smaller Z- and X-extent, but the Y-extent is similar during all conditions. In order to investigate the mechanism leading to the different Z-extents under various plasma sheet conditions, we use ray-tracing to model the wave power distribution as a function of distance from the neutral sheet. We find a linear correlation between the wave Z-extent and field line curvature radius (Rc). A Z-extent of ~0.2 RE (consistent with observations) corresponds to Rc ~ 0.9 RE. Our results suggest that ECH intensification following dipolarizing flux bundles is in part due to increased Rc, which enables intensification to higher amplitudes over a larger volume, explaining the increased occurrence rate and extent of active-time wave events.