Wave Amplitude as a Function of Interplanetary
Parameters

Although studying the occurrence rate helps us to understand the sources of pulsations, the relationships between the wave power and solar wind parameters need to be studied further since it is possible that some solar wind parameters do not control the occurrence of pulsations but instead affect the magnitude of their wave power. Below we examine the most important parameters that affect the wave power, namely, the solar wind velocity and the IMF orientation.

Figure 10 shows the scatterplots and linear least squares fits of wave power data versus solar wind speeds for the $P_2, \ldots , P_5$ events. For all the frequency bands, the wave power in the logarithmic scale increases approximately linearly with the solar wind speed although the scatter in wave power is quite large. There is no apparent threshold for the solar wind velocity that enhances the power of pulsations.

Figure 10. Scatterplots and linear least squares fits of wave power data versus solar wind speeds for the $P_2, \ldots , P_5$ events.

The relationships between the wave power and the IMF orientation is presented in Figure 11, where we plot the mean values of P₂ and P₅. The vertical bars in Figure 11 span between +1 and -1 standard deviation of the means. The data are divided into three subsets, corresponding to the conditions of low IMF cone angles ($0\deg \leq \theta_{BX} < 30\deg$), intermediate IMF cone angles ($30\deg \leq \theta_{BX} < 60\deg$), and high IMF cone angles ($60\deg \leq \theta_{BX} < 90\deg$). The error of estimation for P₂ as $\theta_{BX} \leq 30\deg$ is greater than those of other cases since P₂ events occur less frequently under such a condition. On the average, P₂ is greater when the IMF is southward ($\vert \phi \vert \gt 90\deg$). However, for P₅ events, the wave power does not seem to be affected by the IMF clock angle $\phi$ except for low IMF cone angle conditions. The only condition when P₅ is larger than the average level is when the IMF is roughly aligned with the Sun-Earth line and also close to the equatorial plane.

Figure 11. (left) Mean and standard error of mean for P2 under three different ranges of IMF cone angles $\theta_{BX}$. (right) Same plot for P5.