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 suggested that protons with energies ∼5–30 keV play a role in Pg excitation. It is shown that protons with energies ∼5–30 keV, injected into the inner magnetosphere on the nightside, will only drift westward around the Earth on enclosed paths if the E × B drifts due to the magnetospheric convection and corotation electric fields are small. This is the case when the magnetosphere is quiet. If the E × B drifts are large, as is the case for more disturbed times, then their influence may overcome that of the gradient‐curvature drift for these lower energy protons, detrapping them from their enclosed paths and allowing them to follow convective paths to the dayside magnetopause. At these 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 times of Pgs. It can also explain the quashing of Pg activity during substorms and the tendency for Pgs to occur on successive days, 24 hours apart. A similar reasoning can also explain why radially polarized waves with large azimuthal wave numbers, thought to be generated by the bounce resonance mechanism, are frequently observed in the afternoon/evening sector of the magnetosphere but occur infrequently in the morning sector.