RELATIONS OF FIELD LINE RESONANCES AND UPSTREAM WAVES AND THE WINTER ATTENUATION OF PULSATIONS

Using data on the occurrence frequency of geomagnetic pulsations of di fferent periods from three observatories in Central Europe, conclusion s are drawn about the occurrence of field line resonances and pulsatio ns directly driven by upstream waves at L-values below 3. It was found that both types occur during the interval studied (first 6 months of the year 1991), but both the occurrence frequency of the two types and the characteristic period of the field line resonance change signific antly as compared to other intervals. During Northern winter, pulsatio n activity is severely damped in solar maximum years, including the ye ar 1991. The decrease in the activity of the pulsations is more signif icant at shorter periods.