Morphology of polar cap patch activity

Abstract

For the period July 1, 1989, to June 30, 1990, for high solar activity, ƒ0F2 data were recorded by the Digital Ionospheric Sounding Systems from two Qaanaaq and Sondrestromfjord; ionospheric UHF scintillation data were recorded from Thule; and airglow images were recorded from Qaanaaq. These data were used to study the morphology of polar cap patches. For this study the polar cap patch activity has been defined as excess electron density above a baseline. The megahertz scale ƒ0F2 for the strength of a patch matches well with the decibel scale of scintillation patches. On a daily basis the polar cap patch occurrence is very high (90%) for weak patches (≥3 MHz), and it decreases significantly (20%) for stronger patches (≥8 MHz). On a daily basis the occurrence of the number of patches per day shows a similar behavior, with as many as 10 weak (≥3 MHz) patches and as few as two strong (≥8 MHz) patches. On the average, patches are seen 50% of the time, whereas strong patches are seen only 5% of the time. Polar cap patches show a strong seasonal dependence, with the highest occurrence in the winter months. Average patch duration is about 55 min at the polar latitudes of Thule and Qaanaaq. The time interval between the patches is about 100 min. The patch occurrence and strength both show a diurnal behavior, with the highest occurrence around 2200–0200 UT and a minimum around 0600–1100 UT. These observations are consistent with the suggestion that the source for patches is a tongue of ionization formed by plasma convection carrying auroral/subauroral plasma into the nightside polar region. The observations support the mechanism of a chopping of a tongue of ionization for a formation of a patch close to auroral latitude, and the patch drifts across the polar cap. The patch activity seems to be weakly dependent on magnetic activity index Kp. The present study shows that the patch occurrence is 27% for By positive and 73% for By negative, and 32% for Bz positive and 68% Bz negative, supporting the present understanding that the polar cap patches predominantly occur with the southward turning of the interplanetary magnetic field.