Abstract
Abstract. A long series of polar patches was observed by ionosondes and an all-sky imager during a disturbed period (Kp = 7- and IMF Bz < 0). The ionosondes measured electron densities of up to 9 × 1011 m-3 in the patch center, an increase above the density minimum between patches by a factor of \\sim4.5. 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 (flow 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 field line resonances (FLRs) were observed by magnetometers, riometers and VHF/HF radars. The AC electric field associated with the FLRs resulted in a poleward-progressing zonal flow 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 Alfvén 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 Alfvén waves coupling to the dayside magnetosphere. During a period of southward IMF the dawn-dusk electric field associated with the Alfvén waves modulated the subsolar magnetic reconnection into pulses that resulted in convection flow bursts mapping to the ionospheric footprint of the cusp.Key words. Ionosphere (polar ionosphere). Magneto- spheric physics (magnetosphere-ionosphere interactions; polar wind-magnetosphere interactions).
Highlights
The regions of electron density enhancements known as polar patches drifting antisunward in the polar cap ionosphere can be observed by various techniques, principally radio or optical (Crowley, 1996)
We suggest that footprint Pc5 ®eld line resonances (FLRs) associated precipitation in auroral arcs (PMAFs) drifting into the cusp region, combined with the cusp precipitation and density depletions in regions of strong electric ®eld, can build up signi®cant absolute and/or relative densities in addition to those caused by solar EUV ionization
Elongated ionization patches were observed with Canadian Advanced Digital Ionosonde (CADI) ionosondes and an all-sky imager in the polar cap
Summary
The regions of electron density enhancements known as polar patches drifting antisunward in the polar cap ionosphere can be observed by various techniques, principally radio or optical (Crowley, 1996). Lockwood and Carlson (1992), using a new concept ofow excitation developed by Cowley and Lockwood (1992), proposed that time-dependent magnetic reconnection and convection produce polar cap patches from pre-existing enhancements of the electron density and associated density gradients produced by solar photoionization near the terminator They invoked transient bursts of magnetic reconnection, known asux transfer events (FTEs), in their mechanism of patch production. It is suggested that the electron precipitation associated with ®eld line resonances (FLRs) equatorward of the cusp can contribute signi®cantly to patch formation This plasma is restructured into patches as it convects through the cusp footprint and is drawn into the polar cap. A multi-instrument data base is used to identify the patches and a variety of phenomena that are involved in their formation
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