Morphology of Southern Hemisphere Riometer Auroral Absorption

摘要

A morphology of riometer auroral absorption is derived from hourly values, determined at several Southern Hemisphere stations, located both near the centre of the auroral absorption zone and at higher and lower latitude fringes of the zone. Since spatial coverage of the data precludes a detailed morphology determination, use was made as guideline at all stages of an auroral absorption model derived for the Northern Hemisphere (NH). The day-to-day variability was first determined from cumulative amplitude-probability distributions calculated for a given hour at a given location for a range of geophysical conditions. These distributions are found to be well represented by log-normal distributions for most locations, times-of-day and solar and geomagnetic activity levels, over the range of absorption for which values are more accurate (typically 0.3 dB or above). Furthermore, parameters of log- normal distributions are found to be related following a known simple expression, so as to permit full specification of any distribution by a single parameter. Both these results are found to be consistent with the NH counterparts. Then time-of-day dependencies were determined for several distribution parameters. These were also found to show the same features than in the NH except for locations near the so-called South Atlantic Anomaly of the geomagnetic field. Determination of latitude and longitude dependencies are only possible when due account is taken of the widening and equator ward movement of the auroral absorption zone with increasing geomagnetic activity level. Assuming Gaussian latitude dependencies apply for all hours, longitude factors are determined so as to be consistent with observed geomagnetic activity level dependencies of absorption distribution parameters for the different longitudes.