Ground based observations of structured Pc 1 pulsations. Since the magnetic field lines have minimums at the equator, the wave growth occurs there. The ring current ion source depends strongly on geomagnetic activity, and the pearl events occur typically during a recovery phase of a geomagnetic storm , while the IPDPs occur during the active phase of a substorm . However, the plasmapause is not the most important region for EMIC wave growth.
Individual bursts have even smaller extent.
The events are much more extended longitudinally, as the ions providing the energy drift around the Earth. The pearl pulsations appear as repetitive bursts of Pc 1 waves, formed by wave packets propagating along magnetic field lines between conjugate points and partially reflecting from the ionosphere. Accordingly, it has been shown that the bursts are in antiphase in the northern and southern hemispheres. For the wave growth to occur, the reflected wave's k-vector should be parallel to B , and this is possible only in the presence of a density gradient. Such a gradient occurs at the plasmapause and, indeed, all structured events have been found to occur just inside or near the plasmapause.