In-situ and optical observations of sub-ion magnetic holes

The magnetotail dipolarization during substorms is a mesoscale dynamical phenomenon, well documented by in-situ spacecraft observations, numerical simulations, and ground-based optical measurements. The magnetic energy release during substorms results in significant plasma heating and the formation of strong temperature and density gradients behind the dipolarization front. Such gradients are the source of many plasma instabilities that are responsible for further transport of energy to smaller spatial scales. In this study, we investigate an example of in-situ and optical observations of the nonlinear stage of one class of such instabilities, associated with the formation of sub-ion scale magnetic holes in the equatorial dipolarized magnetotail. THEMIS in-situ measurements demonstrate that these holes propagate dawnward and are associated with strong gradients of electron thermal pressure ((similar to)100 - 200 km in scale-size). Optical observations of THEMIS all sky imagers show comparably small-scale structures (accounting for magnetic mapping), just poleward of the active region associated with the dipolarization front and plasma flows. Comparison of in-situ and optical observations confirms their association and agreement in propagation direction. Our results demonstrate that sub-ion (electron dominated) magnetic field structures can be a source of small-scale aurora, and thus can host magnetosphere-ionosphere interactions at small scales.