GIANT PULSATIONS - AN EXPLANATION FOR THEIR RARITY AND OCCURRENCE DURING GEOMAGNETICALLY QUIET TIMES

It is generally agreed that giant pulsations (Pgs) are the result of a particle instability that occurs inside the magnetosphere rather than the consequence of an external stimulus. Previous studies have sugges ted that protons with energies similar to 5-30 keV play a role in Pg e xcitation. It is shown that protons with energies similar to 5-30 keV, injected into the inner magnetosphere on the nightside, will only dri ft westward around the Earth on enclosed paths if the ExB drifts due t o the magnetospheric convection and corotation electric fields are sma ll. This is the case when the magnetosphere is quiet. If the ExB drift s are large, as is the case for more disturbed times, then their influ ence may overcome that of the gradient-curvature drift for these lower energy protons, detrapping them from their enclosed paths and allowin g them to follow convective paths to the dayside magnetopause. At thes e times, the lower energy protons which may be an important factor in Pg generation will not reach the early morning sector where Pgs occur. This phenomena can explain the rarity and occurrence during quiet tim es of Pgs. It can also explain the quashing of Pg activity during subs torms and the tendency for Pgs to occur on successive days, 24 hours a part. A similar reasoning can also explain why radially polarized wave s with large azimuthal wave numbers, thought to be generated by the bo unce resonance mechanism, are frequently observed in the afternoon/eve ning sector of the magnetosphere but occur infrequently in the morning sector.