UPSTREAM WAVES AND FIELD LINE RESONANCES - SIMULTANEOUS PRESENCE AND ALTERNATION IN PC3 PULSATION EVENTS

Based on a detailed study of Pc3 events at an array between L = 1.5 an d 3 in Central Europe, the authors found quick changes between upstrea m waves (UW, i.e. pulsation directly driven by UW) and field line reso nance (FLR, i.e. azimuthal oscillations of geomagnetic field lines). T he alternation of the two types is especially characteristic (and the UW part stronger) if the interplanetary magnetic field (IMF) is highly variable. Events due to field line resonance may have a structure con sisting of multiple lines with frequencies differing by about 10%, cor responding to neighbouring shells of field lines separated by about 10 0 km at the surface. This coincides with previous findings (about 10% at a meridional distance of 80 km). The frequency of the UW type is we ll correlated with the frequency of waves in the interplanetary medium . Additionally, there are signals of unidentified origin which also se em to be influenced by IMF.