Preferred IMF/Solar Wind Conditions

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 P_i 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, $\theta_{BX}, \phi, B_t, V_{p}$ are local time, IMF cone angle, IMF clock angle, IMF magnitude, and solar wind speed, respectively. Let Q_Pi(X_k) be the occurrence rate of X_k for the P_i events, and let Q(X_k) be the occurrence rate of X_k for the entire database. For both functions, the sums over all possible X_k are 1, that is,

The normalized occurrence rate of X_k for the P_i events is defined as

$$q_{P_i} \equiv \frac{Q_{P_i}(X_k)}{Q(X_k)}.$$ (3)

If q_Pi(X_k) > 1, P_i events are more likely to occur at X_k than the average condition. On the contrary, if q_Pi(X_k) < 1, P_i events are less likely to occur than average. The normalized occurrence rates for pulsations for different parameters are described below:

 

• Local time LT (Figure 5)
• IMF cone angle $\theta_{BX}$(Figure 6)
• IMF clock angle $\phi$ (Figure 7)
• IMF magnitude B_t (Figure 8)
• Solar wind speed V_p (Figure 9)