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

Abstract

Abstract. A long series of polar patches was observed by ionosondes and an all-sky imager during a disturbed period (K p 7-and IMF B z < 0). The ionosondes measured electron densities of up to 9´10 11 m A3 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 B y > 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 B y ) 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.

A decade of the Super Dual Auroral Radar Network (SuperDARN): scientific achievements, new techniques and future directions

“…Early studies estimated the typical recurrence time between bursts as being *7-8 mins Provan et al 1998) closely matching what had been identified as the average recurrence time for magnetopause FTEs (Rijnbeek et al 1984;Lockwood and Wild 1993). Prikryl et al (1998Prikryl et al ( , 1999Prikryl et al ( , 2002 presented observations that implied that, at times, quasi-periodic ionospheric flow bursts were correlated with Alfvénic fluctuations observed in the upstream solar wind, with similar waveforms being observed in the IMF, in the magnetosheath, and in ionospheric convection flows. They suggested that compressional MHD waves in the solar wind were responsible for driving compressional waves in the magnetosphere, and further suggested that these ULF waves modulated reconnection at the dayside magnetopause into pulses, hence the correlated periodicity.…”

“…However, there have been attempts to characterize this 2-D signature. Although not making 2-D observations, Prikryl et al (1999) suggested that the Wild et al (2003), copyright by the European Geophysical Union Surv Geophys (2007) 28:33-109 49 ionospheric flow bursts could be associated with poleward progressing DPY currents (Hall currents modulated by variations in IMF B y ). Many of the recent SuperDARN 2-D observations of dayside flow transients appear to fit with a simple picture of polewardmoving FAC systems which are associated with PMAFs, poleward-moving local convection reversals, and other poleward-moving velocity structures (Chisham et al 2000b;Thorolfsson et al 2000;Lockwood et al 2001).…”

Climatology of GPS phase scintillation and HF radar backscatter for the high-latitude ionosphere under solar minimum conditions

“…This is consistent with expectation of largely precipitation-driven auroral irregularities and convection-driven cusp irregularities. In the cusp, L-shell aligned flow channel events frequently shape the density structure by producing density depletions with steep gradients perpendicularly to L-shell (Rodger et al, 1994;Prikryl et al, 1999).…”

“…Under the conditions of IMF B y > 0, the SuperDARN convection maps (Fig. 4) showed quasi-periodic dayside ionospheric convection enhancements (westward flow channels) as a response to solar wind coupling at the magnetopause (Prikryl et al, 1999; see their Fig. 1).…”

Coherence between radar observations of magnetospheric field line resonances and discrete oscillations in the solar wind

“…Similar observations have been made by Kepko et al (2002) relating oscillations in the solar wind to pulsations at geosynchronous orbit. Other authors (Potemra et al, 1989;Shimazu et al, 1995;Prikryl et al, 1998Prikryl et al, , 1999 have also reported ground-based observations of quasi-periodic phenomena associated with solar wind oscillations. There have also been observations of surface waves on the magnetopause correlated with solar wind oscillations at discrete frequencies (Sibeck, 1992).…”