A numerical simulation of the Pi2 pulsations associated with the substorm current wedge

Abstract

The present paper deals with the transient behavior of MHD perturbations in the inner magnetosphere induced by an impulsive localized eastward current (source current) as a model of Pi2 pulsations in the magnetosphere. The magnetospheric model consists of a dipole magnetic field, plasmasphere, ionosphere with Pedersen conductivity, and a free outer boundary. The source current is an impulsive magnetospheric current at the onset of the substorm current wedge and is distributed around the equatorial plane of L = 10 with ±2 hour longitudinal extent around midnight. The numerical results allow us to track variation in the expected Pi2 pulsation signals in both local time and L. The poloidal‐mode wave exhibits plasmasphere virtual resonance, resulting in large amplitudes around midnight, weakening toward dayside. The toroidal‐mode wave is excited as a field line resonance immediately after the wave front of the poloidal‐mode wave crosses regions where the radial gradient of VA is steep. The toroidal‐mode wave has largest amplitude at the local time of the east/west edge of the source current. The duration of this wave is ∼5 min. In the middle plasmasphere where the radial gradient of the VA is smaller, the poloidal‐mode wave tends to predominate over the toroidal‐mode wave. These numerical results are consistent with satellite observations, in so far as the day‐night asymmetry of Pi2 pulsations and the observation of transient toroidal waves.