Dipolarization Events With Inductive, Radial Electric Fields Observed by Van Allen Probes

Dipolarization events with inductive, radial electric fields are examined, using Van Allen Probes data between 2012 and 2019. Two cases are studied, followed by statistical analyses. These events were observed between evening and premidnight magnetic local times (MLTs) under moderate geomagnetic activities. Radial electric field variations, azimuthal magnetic field variations, and energetic protons were often observed when horizontal magnetic fields started to decrease in the dip region. Magnetic field lines were stretched with their motion similar to the gradient B/curvature drift velocities of energetic protons. Signs of electric fields changed when horizontal magnetic fields started to increase in the dipolarization front (DF). Electric field and magnetic field variations were correlated and in quadrature regarding the phase relation. Among other explanations, we interpret these observations in terms of energetic proton structures drifting toward the probe locations, while being accompanied by standing waves.