Fundamental Poloidal Wave Event Extending From Midnight to Dawn

Abstract

Fundamental poloidal (P1) waves are a class of magnetospheric standing Alfvén waves characterized by field line oscillations in the meridional plane. P1 waves are infrequently excited in the ring current region and are believed to be the source of giant pulsations (Pgs) observed on the ground. Pgs are mostly detected on the morning side with a sharp cutoff near midnight. Motivated by this local time dependence, we examine a longitudinally extended P1 wave event observed on 24 March 2013 by Van Allen Probes (RBSP)‐A and RBSP‐B (midnight sector), Engineering Test Satellite (ETS)‐VIII (morning side), and Time History of Events and Macroscale Interactions during Substorms (THEMIS)‐D (dawn sector). At each spacecraft, the magnetic field perturbations exhibit a strong radial component and a finite compressional component consistent with previously reported P1 waves. A fortuitous conjunction of RBSP‐A and RBSP‐B allows us, for the first time, to determine the magnitude of the azimuthal wave number at midnight to be , if we assume the waves propagate westward. This value implies a strong ionospheric attenuation of the waves, explaining why few Pg events have been reported near midnight relative to other local time sectors. Ion flux data from RBSP and THEMIS indicate that the P1 waves were excited in a longitudinally extended region of a negative radial gradient of the ion phase space density, which is the condition for an instability to excite P1 waves through drift resonance.