Poloidal ULF Waves During the August 2015 Geomagnetic Storm

Abstract:

Previous satellite studies showed that stormtime poloidal ULF waves in the ring current region have an antisymmetric (second harmonic) mode structure about the magnetic equator. We found that this is not the case for poloidal waves detected by Van Allen Probes in the postnoon sector during the geomagnetic storm on 26-30 August 2015. We can distinguish between symmetric and antisymmetric waves based on the model prediction of equatorial nodes and antinodes. Symmetric waves have a node of B_perp and an antinode of B_para and E_perp, where “E” and “B” are the wave electric and magnetic fields and “para” and “perp” refer to the components parallel and perpendicular to the background magnetic field. The node and antinode switch for antisymmetric waves. The E- and B-field perturbations observed by the Van Allen Probes spacecraft in the main and early recovery phases of the storm are consistent with symmetric waves. Antisymmetric waves were detected only very late in the recovery phase. The symmetric waves occurred at L = 3.5-5 outside the plasmasphere and had frequencies of 4-10 mHz, which are lower than the fundamental field line resonance frequencies in the same L range determined using the European quasi-Meridional Magnetometer Array. We discuss possible generation mechanisms of the symmetric waves based on the properties of ions detected in the ring current energy range.