Dayside pulsation events near the plasmapause

Abstract

Four dayside pulsation events were observed from DE- l near the equatorial plasmapause region on 23, 24 August, 30 September and 10 October 1982. The corresponding ground data from Siple for three of them are presented and compared to the DE-l data. The concept of field line resonance is applied to interpret the standing wave structure of the events. Most of the events exhibit complicated wave forms with transverse and compressional components seen simultaneously on the same field shell. Long period transverse waves (> 100s) were often seen as fundamental mode oscillations of magnetic shells and had superimposed on them short period waves (20–40 s) which are the higher harmonics excited simultaneously with the fundamental mode. For the long period waves a possible energy source is the Kelvin-Helmholtz instability at the magnetopause, while the radially polarized Pc 4 seen on 10 October was attributed to bounce resonance excitation. “Wave bursts” (short intervals of 5–10 or more cycles) were seen in almost all frequency ranges. The brief duration may be due to temporal or spatial variation of wave sources. A clear tendency of continuously decreasing wave period with decreasing L value was seen in many cases. This is attributed to the broad band frequency of the wave source which excites individual magnetic shells oscillating at their own eigenfrequencies.