Solar cycle dependence of polar cap patch activity



[1] Ionospheric foF2 data from two stations, Sondrestromfjord and Qaanaq, Greenland, for 4 years corresponding to four different levels of sunspot activity (sunspot number (SSN) = 150 ± 50, 125 ± 50, 40 ± 20, and 10 ± 10) are used to study the dependence of occurrence and intensity of polar cap patches on solar cycle activity. At high sunspot activity, on the average, Sondrestromfjord sees six patches per day, and Qaanaaq sees eight patches per day. At low sunspot activity the number drops to three per day at both stations. It is found that on a daily basis the polar cap patch activity is independent of the sunspot number for weaker patches (≥3 MHz) but reduces with sunspot number for stronger patches (≥6 MHz). Only at high sunspot activity, stronger patches are seen for 20% of the time. The duration of polar cap patches reduces from 50% at high sunspot activity (SSN = 150) to 15% at low sunspot activity (SSN = 10). It exhibits a seasonal dependence with a maximum in winter months. The diurnal maximum in polar cap patch activity at both stations occurs close to local magnetic noontime. Assuming that polar cap patches are generated at high latitude (Sondrestromfjord) and drift to polar latitude (Qaanaaq), only about 30% of patches seen at Qaanaaq can be accounted for as originating at Sondrestromfjord. A reduction in patch activity with reduction in sunspot number is directly related to a reduction in the strength of the source: the tongue of ionization. The polar cap patches exhibit a weak dependence on the interplanetary magnetic activity index Kp. The distribution of the interplanetary magnetic field (IMF) for the patch occurrence very much resembles that of bx, by, bz of the IMF database. Over the solar cycle all these changes cover more than an order of magnitude of range. The effect of polar cap patch activity on communications operating at high-latitude regions is briefly discussed.