Space-time variability of polar cap patches: Direct evidence for internal plasma structuring

摘要

Coordinated observations of ionospheric variability near the geomagnetic pole using the Resolute Bay Incoherent Scatter Radar (RISR-N), Super Dual Auroral Radar Network (SuperDARN) High Frequency (HF) radars, and all-sky imagers have clarified the relative contribution of structuring mechanisms operating on polar plasma patches. From the multipoint RISR-N observations, a three dimensional image can be constructed of the plasma parameters. The colocated coherent echoes from the SuperDARN radars provide information on field aligned irregularities, and from all-sky imagers located in Resolute Bay, Canada and Qaanaaq, Greenland, information is obtained on the emission brightness at different wavelengths. A good correlation is found between the location of the coherent, incoherent and optical signals of patches. From the SuperDARN radar data it is evident that plasma irregularities seem to be present throughout the region of enhanced electron density. The patches are observed to be formed in the cusp region due to bursty flux transfer events and are then transported across the polar cap. During the time period of about 10 minutes when a patch drifted through the RISR-N field of view, the patch seemed to undergo significant deformation in all three spatial dimensions, with density fluctuations of about 10% and spatial variations leading to stretching and tilting of the patch. The findings show that plasma structuring can likely occur within polar cap patches, which support previous suggestions that a patch is highly variable as it drifts across the polar cap, with a faster spread of irregularities throughout the patch as a result.