Type IPDP magnetic pulsations and the development of their sources

Abstract

Intervals of pulsations of diminishing periods (IPDP) magnetic pulsation data have been analyzed from several recording campaigns and from stations that are widely spaced in both meridional and azimuthal directions. On the basis of their local time occurrence it seems useful to divide the events into at least two main categories: 1400 < LT < 1800 and 1800 < LT < 2400. Those events which are only observed at auroral latitudes typically develop slowly and extend at the most to 1 Hz, whereas those events that are observed at a low‐latitude station may extend to several Hz. Simultaneous observations at several meridians extending over 100° show that the IPDP source region moves from east to west at a speed of 2–5° min −1 . There are necessarily no remarkable changes in frequency‐time characteristics of IPDP pulsations. In the radial direction, IPDP wave frequency seems to be related to the He + gyrofrequency in the equatorial plane. Observations are shown to be in general agreement with the evolution of the ion cyclotron instability in the plasma convection system of the magnetosphere. Several mechanisms seem to contribute simultaneously to the frequency shift of pulsations during IPDP events.