Dynamical response of the magnetosphere‐ionosphere system to a solar wind dynamic pressure oscillation

Abstract

On 28 February 1998, four quasi‐periodic pressure pulses with an amplitude of a few nPa detected by ACE gave rise to periodic compressions of the magnetosphere with period of about 14 min. In concert with periodic compressed and expanded states of the magnetosphere forced directly by the pressure variation, a coherent geomagnetic field fluctuation with the same period appeared on a global scale and was recorded at stations located from polar to equatorial regions. Most ground‐level geomagnetic field signatures on the dayside can be interpreted as the result of a global ionospheric current system, like the global Pc5 event examined by Motoba et al. [2002]. In the afternoon polar ionosphere covered with the dense magnetometer stations, a vortical current structure associated with pressure‐induced field‐aligned currents (FACs) is centered at 72° ± 1° and consists of a counterclockwise (clockwise) vortex in response to positive (negative) changes of solar wind pressure oscillation. Although the vortical current signatures are unclear in the morning sector, each afternoon vortex could pair with the morning one with opposite rotation. During this event, the interplanetary magnetic field (IMF) remained steady with a strong southward orientation (−10 nT or less). In addition to the pressure‐induced FAC system, the steady southward IMF drives the dayside Region 1 (R1) current system, resulting in the familiar large‐scale two‐cell convection pattern in the ionosphere observed by SuperDARN radars. The SuperDARN convection patterns indicated that the ionospheric convection reversal boundary (CRB) in the afternoon was located in the range of 73° ∼ 77°N around 15 MLT. The ionospheric footprint of the pressure‐induced FAC in the afternoon was found to be 1.5° ± 1.1° ∼ 4.0° ± 1.4° equatorward of the CRB. This suggests that the pressure‐induced FAC is started inside the R1 current system originating from the outer magnetospheric boundary layer. We argue that the paired FAC system responsible for the global geomagnetic fluctuations on the ground arises from the oscillatory large‐scale dynamical convection originating well inside the closed field lines in direct response to the quasi‐periodic pressure variations, not from the localized undulations on the magnetopause nor from global eigenmode oscillations of the magnetospheric cavity.