The rate of occurrence of dayside Pc 3,4 pulsations - The L-value dependence of the IMF cone angle effect

The normalized rate of occurrence of dayside Pc 3,4 pulsations from L=2.4 to 4.3 has a strong enhancement for low cone angles of the interplanetary magnetic field. When the angle of the IMF to the earth sun line, θBX, is 15° or less the occurrence rate is 7-8 times the average rate at L=2.4 to 2.8 and 2.2 to 3.5 times the average rate at L=4 to 4.3. These waves disappear when the IMF is nearly at right angles to the sun-earth-line. This absence of pulsations occurs over the widest range of angles at lowest L-values. These observations are consistent with a source originating in the waves upstream of the subsolar bow shock which are transported by convection to the magnetopause where they couple to oscillations of magnetospheric field lines. Since the index of refraction of the magnetospheric plasma decreases with decreasing radial distance, except at the plasmapause, inward propagating waves should be refracted away from the radial direction. Thus, to reach low L-values the waves should couple near the stagnation point and propagate nearly radially inwards. Upstream waves should be convected to the stagnation point for only a limited range of θBX. However, to reach higher L-values the coupling may be at later local times where cross streamline propagation can bring waves from a larger range of θBX. The streamline geometry and its connection to the foreshock region is illustrated for a variety of IMF orientations using a simple approximation to the magnetosheath flow field.