[1] We show evidence that left-hand polarised electromagnetic ion cyclotron (EMIC) plasma waves can cause the loss of relativistic electrons into the atmosphere. Our unique set of ground and satellite observations shows coincident precipitation of ions with energies of tens of keV and of relativistic electrons into an isolated proton aurora. The coincident precipitation was produced by wave-particle interactions with EMIC waves near the plasmapause. The estimation of pitch angle diffusion coefficients supports that the observed EMIC waves caused coincident precipitation of both ions and relativistic electrons. This study clarifies that ions with energies of tens of keV affect the evolution of relativistic electrons in the radiation belts via cyclotron resonance with EMIC waves, an effect that was first theoretically predicted in the early 1970 0 s. Citation: Miyoshi,
With the deployment of the all‐sky imager array of the THEMIS mission, we were able to construct a preliminary database of auroral substorm expansion phase onsets, from which we have established a number of common features characterizing the first tens of seconds of the substorm auroral intensification. We find that the intensification occurs within ∼10 sec over an arc segment extending approximately 1 h MLT and featuring wave‐like formations distributed in longitude. The longitudinal wave number ranges between 100 and 300 such that the wavelength is comparable to the ion gyroradius in the central plasma sheet. The scale the intensification is about 10–30 sec. This study casts important observational constraints on substorm onset theories.
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