Quantitative estimates of relationships between geomagnetic activity and equatorial spread-F as determined by TID occurrence levels

Using a world-wide set of stations for 15 years, quantitative estimates of changes to equatorial spread-F (ESF) occurrence rates obtained from ionogram scalings, have been determined for a range of geomagnetic activity (GA) levels, as well as for four different levels of solar activity. Average occurrence rates were used as a reference. The percentage changes vary significantly depending on these subdivisions. For example for very high GA the inverse association is recorded by a change of -33% for R-z greater than or equal to 150, and -10% for R-z < 50. Using data for 9 years for the equatorial station, Huancayo, these measurements of ESF which indicate the presence of TIDs, have also been investigated by somewhat similar analyses. Additional parameters were used which involved the local times of GA, with the ESF being examined separately for occurrence pre-midnight (PM) and after-midnight (AM). Again the negative changes were most pronounced for high GA in R-z-max years (-21%). This result is for PM ESF for GA at a local time of 1700. There were increased ESF levels (+31%) for AM ESF in R-z-min years for high GA around 2300 LT. This additional knowledge of the influence of GA on ESF occurrence involving not only percentage changes, but these values for a range of parameter levels, may be useful if ever short-term forecasts are needed. There is some discussion on comparisons which can be made between ESF results obtained by coherent scatter from incoherent-scatter equipment and those obtained by ionosondes

Maps of <I>fo</I>F2, <I>hm</I>F2, and plasma frequency above F2-layer peak in the night-time low-latitude ionosphere derived from Intercosmos-19 satellite topside sounding data

Maps of <I>fo</I>F2, <I>hm</I>F2, and plasma frequency, <I>fp</I>, in the topside ionosphere at low latitudes, derived from Intercosmos-19 satellite topside sounding data, obtained from March 1979 to January 1981 and covering all longitudes, are presented for quiet geomagnetic conditions in June and December solstices at solar maximum for several local time intervals during the night. Based on these maps, features of the equatorial anomaly (EA) at different longitudes and their change during the night are considered. The maps show that averaged <I>fo</i>F2, <I>hm</I>F2, and <I>fp</I> longitudinal variations are rather complicated, their structure looks wave-like with quasi-periods in longitude of about 75&ndash;100&deg;, similar to that on individual days revealed previously at low latitudes using Intercosmos-19 data. Positions of the structure extrema in certain longitude intervals are stable enough so that they are clearly seen in the maps after averaging over a large number of measurements made on different days and even in different years. Such structure seems to need at least five harmonics for its description. <br><br> The maps derived from Intercosmos-19 data were compared with the maps given by the IRI model. Along with general resemblance, essential distinctions between them were found. Intercosmos-19 maps show more complicated and pronounced longitudinal structure than IRI maps. They also show that at solar maximum, in general, at night, EA is stronger and persists for a longer time (on average, until 04:00 LT) than that presented in IRI model. Besides, much stronger asymmetry between the characteristics of the EA northern and southern crests in certain longitude intervals was revealed, most evident in <I>hm</I>F2 maps

Global pattern of the ionospheric response to large-scale internal gravity waves

The global pattern of the ionospheric response to large-scale acoustic gravity waves (LS AGW) has been constructed on the basis of an analysis of the large data set available during the 22 March 1979, magnetic storm. Ground-based ionospheric measurements and in-situ satellite measurements from Cosmos-900 were used in this study together with the Joule heating distribution in the high-latitude ionosphere specifically taken at the maxima of two substorms. The characteristics of the reconstructed planetary pattern of the LS AGW have been analysed in detail. It has been established that the LS AGW effects in the ionosphere in terms of both universal and local time were determined by the pattern of high-latitude atmospheric heating, and that the wave front of the LS AGW during both substorms covered practically all local times, i.e. all longitudes. In addition, it was established that one of the sources of the LS AGW was the thermospheric heating in the day-side cusp region. The local time dependence of the amplitude of the AGW effect in both maximum height, h(m)F2, and critical frequency, f(O)F2, has been reconstructed for the mid-latitude F2 layer. The AGW effects were clearly separated from the electric field effects related to turnings of the interplanetary magnetic field (IMF) B-Z. In the daytime, electric field effects prevailed over the AGW effects, but during the night-time the amplitudes of these two effects were comparable. In contrast to the common view, fOF2 variations after the AGW passage had a quasi-sinusoidal character both in the day-time and in the night-time. In the night-time ionosphere a high degree of symmetry was observed for the AGW effects in Northern and Southern hemispheres. During the day-time a significant asymmetry was observed in the American longitudinal sector which was related largely to the peculiarities of the heating pattern in the high-latitude ionospheres of the Northern and Southern hemispheres. These observations demonstrate the complexity of the response of the ionosphere at all latitudes to heating of the auroral region