Van Allen Probes Observations of Symmetric Stormtime Compressional ULF Waves.

Previous spacecraft studies showed that stormtime poloidal ultralow-frequency (ULF) waves in the ring current region have an antisymmetric (second harmonic) mode structure about the magnetic equator. This paper reports Van Allen Probes observations of symmetric ULF waves in the postnoon sector during a moderate geomagnetic storm. The mode structure is determined from the presence of purely compressional magnetic field oscillations at the equator accompanied by strong transverse electric field perturbations. Antisymmetric waves were also detected but only very late in the recovery phase. The symmetric waves were detected outside the plasmasphere at L = 3.0-5.5 and had peak power at 4-10 mHz, lower than the frequency of the local fundamental toroidal standing Alfvén wave. During the wave events, the flux of protons was enhanced at energies below ∼5 keV, which appears to be a prerequisite for the waves. The protons may provide free energies to waves through drift resonance instability or drift compressional instability, which occur in the presence of radial gradients of plasma parameters.

On some features characterizing the plasmasphere-magnetosphere-ionosphere system during the geomagnetic storm of 27 May 2017.

This paper presents how the magnetosphere-plasmasphere-ionosphere system was affected as a whole during the geomagnetic storm peaking on 27 May 2017. The interplanetary conditions, the magnetospheric response in terms of the magnetopause motion, and the ionospheric current flow pattern were investigated using data, respectively, from the WIND spacecraft, from GOES15, GOES13, THEMIS E, THEMIS D and THEMIS A satellites and from the INTERMAGNET magnetometer array. The main objective of the work is to investigate the plasmaspheric dynamics under disturbed conditions and its possible relation to the ionospheric one; to reach this goal, the equatorial plasma mass densities derived from geomagnetic field line resonance observations at the European quasi-Meridional Magnetometer Array (EMMA) and total electron content values obtained through three GPS receivers close to EMMA were jointly considered. Despite the complexity of physical mechanisms behind them, we found a similarity between the ionospheric and plasmaspheric characteristic recovery times. Specifically, the ionospheric characteristic time turned out to be ~ 1.5 days, ~ 2 days and ~ 3.1 days, respectively, at L ~ 3, L ~ 4 and L ~ 5, while the plasmaspheric one, for similar L values, ranged from ~ 1 day to more than 4 days.