Spatial extent of quasi-periodic emission simultaneously observed by Arase and Van Allen Probes on November 29, 2018.

Extremely Low Frequency (ELF) and Very Low Frequency (VLF) emissions in the 3Hz to 30 kHz frequency range are plasma waves naturally present in the magnetosphere. ELF/VLF waves interact through cyclotron resonance with electrons in the radiation belts playing a fundamental role in the dynamics of relativistic electrons. Quasi-periodic emissions (QP) are a kind of ELF/VLF waves whose intensity is modulated in time with fairly regular periods, from tens of seconds to several minutes. Recently, the availability of a considerable amount of satellite and ground-based data allowed us to use rare ELF/VLF conjugated events to increase our understanding on these type of waves. In this study, we report the first QP emission simultaneously observed by the Arase (ERG) satellite, Van Allen Probes A, and B (RBSP). This conjugated event was observed on November 29, 2018 from 12:05 to 14:10 UT by the three spacecraft during geomagnetically quiet times. We estimated the size of the spatial extent of the observed QP emission to be rather large: a radial distance of up to 1.21 Re and a longitudinal extent of up to 2.26 MLT. At ~12:20 UT, changes in the QP element frequency sweep rate were observed associated with magnetic disturbances. Using electric field and magnetic field data, we also discuss the possible mechanisms behind these changes and the possible relationship between ULF pulsations and QP features. We found that the end of the observations at each satellite seems to coincide with the time when the frequency of the waves becomes lower than the lower hybrid resonance frequency.