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IMF clock angle $\phi$ (Figure 7)

The conditions for different IMF clock angles $\phi$ are better discussed together with the IMF cone angle $\theta_{BX}$.For instance, when $\theta_{BX}$ is very small, the magnetic field is basically along the Sun-Earth line regardless of the value of $\phi$.Conversely, $\phi$ becomes important in determining the IMF orientation when $\theta_{BX}$ is large. Previously, it was shown in Figure 6 that low $\theta_{BX}$ angles are favorable for the occurrence of the P4 (16-32 mHz) and P5 (32-64 mHz) events. This trend is also clearly found in Figure 7. In contrast to the P2 events, the P5 (32-64 mHz) events occur predominantly when $\theta_{BX} < 50\deg$ and when the IMF is northward
($\vert \phi \vert < 90\deg$). Moving toward lower frequencies, the waves tend to occur more under high $\theta_{BX}$ conditions. The conditions for the P3 and P4 events seem to be mixtures of the two extremes.

It should be noted here that the comparison between the results of two neighboring frequency bands needs to be made with some caution. If a wave event has the peak frequency located at the boundary between two frequency bands, it will be counted as an event in only one of the two bands because of the nature of the selection criterion. As a result, the occurrence patterns for the two neighboring frequency bands may look complementary to some degree. For example, P4 has a high occurrence rate when $\theta_{BX} \simeq 40\deg$ and $\phi \simeq 120\deg$,but the occurrence rate of P5 shows a deficit at the same IMF orientation (Figure 7). However, this effect should not be significant since the number of events for any frequency band is less than 30% of the total entries of the database. In addition, for the two frequency bands that are not adjacent to each other, their events are selected independently. For example, the difference between the occurrence patterns of P2 and P5 reflects reality rather than an artifact of the selection criterion.

Figure 7. Normalized occurrence rates for $P_2, \ldots , P_5$ events at different IMF orientations.


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Next: IMF magnitude Bt (Figure Up: Preferred IMF/Solar Wind Conditions Previous: IMF cone angle (Figure