The comparison between the data set of the pulsations of interest and the entire database aids in understanding the preferred solar wind conditions for their occurrence. The preferred solar wind conditions imply the possible sources of pulsations.
In the following, two sets of variables are defined. The variables X are independent variables such as the physical quantities of the solar wind and the local time of observations. The wave power of the ith frequency band Pi is the dependent variable. Only P2, P3, P4, and P5 are considered in further analysis. The two sets of variables can also be written as
where LT, are local time, IMF cone angle, IMF clock angle, IMF magnitude, and solar wind speed, respectively. Let QPi(Xk) be the occurrence rate of Xk for the Pi events, and let Q(Xk) be the occurrence rate of Xk for the entire database. For both functions, the sums over all possible Xk are 1, that is,
The normalized occurrence rate of Xk for the Pi events is defined as