Observations of polar patches generated by solar wind Alfve n wave coupling to the dayside magnetosphere

A long series of polar patches was observed by ionosondes and an all-sky imager during a disturbed period (Kp ˆ 7and IMF Bz < 0). The ionosondes measured electron densities of up to 9 ́ 10 m in the patch center, an increase above the density minimum between patches by a factor of 4.5. Bands of F-region irregularities generated at the equatorward edge of the patches were tracked by HF radars. The backscatter bands were swept northward and eastward across the polar cap in a fan-like formation as the afternoon convection cell expanded due to the IMF By > 0. Near the north magnetic pole, an all-sky imager observed the 630-nm emission patches of a distinctly band-like shape drifting northeastward to eastward. The 630-nm emission patches were associated with the density patches and backscatter bands. The patches originated in, or near, the cusp footprint where they were formed by convection bursts ( ̄ow channel events, FCEs) structuring the solar EUV-produced photoionization and the particle-produced auroral/cusp ionization by segmenting it into elongated patches. Just equatorward of the cusp footprint Pc5 ®eld line resonances (FLRs) were observed by magnetometers, riometers and VHF/HF radars. The AC electric ®eld associated with the FLRs resulted in a poleward-progressing zonal ̄ow pattern and backscatter bands. The VHF radar Doppler spectra indicated the presence of steep electron density gradients which, through the gradient drift instability, can lead to the generation of the ionospheric irregularities found in patches. The FLRs and FCEs were associated with poleward-progressing DPY currents (Hall currents modulated by the IMF By) and riometer absorption enhancements. The temporal and spatial characteristics of the VHF backscatter and associated riometer absorptions closely resembled those of poleward moving auroral forms (PMAFs). In the solar wind, IMP 8 observed large amplitude Alfve n waves that were correlated with Pc5 pulsations observed by the ground magnetometers, riometers and radars. It is concluded that the FLRs and FCEs that produced patches were driven by solar wind Alfve n waves coupling to the dayside magnetosphere. During a period of southward IMF the dawn-dusk electric ®eld associated with the Alfve n waves modulated the subsolar magnetic reconnection into pulses that resulted in convection ̄ow bursts mapping to the ionospheric footprint of the cusp.