Statistical analysis of Pi 2 pulsations observed by the AMPTE CCE Spacecraft in the inner magnetosphere

Abstract

The spatial variation of the properties of magnetospheric Pi 2 pulsations is studied using magnetic field records acquired simultaneously by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) satellite at radial distances less than 6.6 Earth radii and at geomagnetic latitudes from −16° to 16° and at the Kakioka ground station located at magnetic shell of L = 1.23. Pi 2 magnetic pulsations are identified from the Kakioka data acquired within 3 hours of midnight, but no restriction is imposed on the local time of CCE. An automated Pi 2 selection procedure resulted in 249 events from the Kakioka data. We have characterized magnetic field variations in the radial Bx, azimuthal By, and compressional Bz components at CCE in terms of their spectral density, coherence, and phase relative to those of the Pi 2 pulsation in the horizontal H component of the Kakioka data and then examined how these parameters depend on the location of CCE. It is found that high‐coherence events (coherence between CCE and Kakioka > 0.6) are observed primarily when CCE is on the nightside and at L < 4. For these events the magnetic field perturbations at CCE are dominated by the poloidal components Bx and Bz, and these components exhibit a ground‐to‐satellite cross phase of either ∼0 or ∼180°, depending on the location of the satellite. The spatial phase structure is consistent with the eigenmode structure of a compressional cavity‐mode‐type resonance excited between two reflecting boundaries. We find no evidence supporting the view that ground Pi 2 are midlatitude toroidal field line resonances excited in response to source waves on auroral zone field lines. Rather, the results imply that midlatitude (2 <L< 5) Pi 2 pulsations observed on the ground originate from a cavity‐mode‐type resonance excited in the inner magnetosphere bounded below by the ionosphere and at high altitudes by an Alfvén velocity gradient. The cavity resonance is probably excited by earthward propagating fast mode waves launched at substorm onset by the large‐scale magnetic reconfiguration associated with cross‐tail current disruption.