Evolution of partial ring current ion pitch angle distributions during the main phase of a storm on 17 March 2015

During a severe magnetic storm from 17 to 19 March 2015, three identically instrumented Time History of Events and Macroscale Interactions during Substorms probes crossed the magnetosphere in a string-of-pearls configuration enabling sequential observations of same regions of the magnetosphere with a 2 h time separation. These observations allow us to study the temporal evolution of ion pitch angle distributions (PADs) in the dusk-premidnight sector (between 17 and 20 h magnetic local time) during the storm's main phase. We found that the ion PAD evolved from pancake to isotropic in fewer than 2 h. Analysis of electromagnetic wave spectra revealed the presence of electromagnetic ion cyclotron (EMIC) waves at frequencies below the helium cyclotron frequency. We conclude that the observed pitch angle evolution was due to ion scattering by EMIC waves.