Solar‐wind‐driven pulsed magnetic reconnection at the dayside magnetopause, Pc5 compressional oscillations, and field line resonances

Abstract

A series of observations by satellites of the solar wind and magnetosphere, by HF radars of the F region, and by ground‐based magnetometers of E region currents are presented to show the correlation between quasiperiodic fluctuations in the solar wind, magnetosphere, and ionosphere on January 24, 1996. The Iceland Stokkseyri SuperDARN radar observed quasiperiodic convection flow bursts in or near the ionospheric footprint of the cusp, identified by large spectral widths. Sometimes called flow channel events (FCEs), these enhanced convection flows are generally believed to be the ionospheric signatures of flux transfer events (FTEs) in the cusp. Initially, the flows pulsed with a periodicity of ∼12 min. When the “radar” cusp shifted to lower latitudes, possibly as a result of a series of FTEs, FCEs occurred every 10 min or less. The Wind spacecraft observed linearly polarized interplanetary magnetic field (IMF) oscillations with dominant periodicities of about 12 and 20 min (∼1.3 and 0.8 mHz) that were superposed on long‐period IMF fluctuations on the scale of a few hours. The pure state power spectra of the IMF components also showed several subsidiary peaks at higher frequencies (e.g., ∼1.7, 2.0, 2.4, and 2.9 mHz) or shorter periods (∼10, 8.5, 7, and 6 min). Similar oscillations were observed in the solar wind dynamic pressure which was anticorrelated with the IMF magnitude and included a strong oscillation with a period of ∼15 min (1.1 mHz). About 20 min later, in the postnoon magnetosheath, Geotail's magnetic and electric field sensors observed an ULF wave packet showing similar discrete spectra. The compressional MHD waves in the solar wind applied oscillating magnetic/electric fields and dynamic pressure on the dayside magnetopause driving multifrequency compressional oscillations in the magnetosphere. Discrete frequencies similar to those found in the solar wind and magnetosheath data were identified in the fast Fourier transform spectra of the ground‐based and space‐borne (GOES 8 and 9) magnetograms. A series of solar wind dynamic pressure pulses caused compressions and rarefactions of the magnetosphere observed by GOES 8 and GOES 9 magnetometers at geostationary orbit. The dynamic pressure driven magnetic perturbations, including those observed on the ground, propagated antisunward. In the noon sector the compressional waves coupled to shear modes resulting in propagation delays of the ground/ionospheric response. The convection flow bursts in the ionospheric footprint of the cusp were closely correlated with the low‐frequency oscillations of the IMF BZ and magnetosheath duskward electric field suggesting that the ULF waves in the solar wind modulated the magnetic reconnection at the dayside magnetopause into pulses. The higher‐frequency components of the IMF oscillations and/or sudden IMF changes excited field line resonances (FLRs) on the magnetic shells adjacent to the noon magnetopause. As the relative power of these higher‐frequency oscillations in the solar wind IMF increased and the cusp shifted to lower latitudes, the FCE rate also increased. A possibility of a reverse coupling (feedback) from the resonating magnetic shells to the reconnection region at the dayside magnetopause is suggested.