Characterizing Ion‐Scale Fluctuations Around Dipolarization Fronts in Earth's Magnetotail

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Abstract
In the Earth's magnetotail bursty bulk flows are often associated with dipolarizing magnetic flux bundles. The leading edges of such earthward‐moving flux bundles are called dipolarization fronts (DF). In the present study we investigate the characteristics of ion‐scale oscillations embedded inside DFs, using observations from NASA's Magnetospheric Multiscale mission (MMS) between 2017 and 2022. By applying wavelet and spectral analysis methods to multi‐point measurements, we obtain a statistical picture of the magnetic and electric field fluctuations from parameters such as the polarization, propagation direction and velocity in the central and outer plasma sheet. More than 96% of the observed events show enhanced power around the proton cyclotron frequency. The power maximum occurred either at the beginning or end of the DF. For a large portion of DF events, in particular those in the central plasma sheet, the power enhancement is due to the DF itself as expected from the ion scale‐thin DFs. However, ion‐scale fluctuations embedded in the DF are observed for the majority of the cases in the outer plasma sheet. While some limited cases are associated with electromagnetic ion cyclotron waves, most of these fluctuations can be interpreted as static co‐moving structures, generated by local Hall or field‐aligned currents. There are quite a number of fluctuations suggesting a rippled dipolarization front predominantly for the events in the outer plasma sheet.

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