Almost-Necessary Conditions for a Solar Active Region to Produce a Subsequent Polar Cap Absorption Event

摘要

Between 1955 and 1976, 133 polar cap absorption (PCA) events with unambigious solar flare associations were observed. Over one-fourth of these events (35/133) originated in solar active regions that had already produced a PCA. More significantly, these 'subsequent' PCA's included 35 percent (26/74) of all the principal greater than or = 2.0 dB) PCA's in this sample. In this paper, we establish quantitative criteria to distinguish between those solar active regions that are likely to produce such subsequent absorption events and those that pose no further threat to high latitude HF or VLF communications. These criteria are based on flare solar disk position and the greater than or = 8800 MHz burst peak flux density.