Analogue model,relating Kp index to solar wind parameters

In a previous work the authors have developed a model, providing K_p as a function of the interplanetary magnetic field B_z component. They introduced a modified B_z function (denoted as B_zm), exhibiting a delayed reaction to B_z changes. The modified function B_zm was defined by using the analogy with a damping RC-circuit output voltage. The delaying reaction of B_zm to B_z was characterized by two time constants, one for rising and one for decreasing parts of B_z. The cross-correlation between K_p and B_zm has increased to 0.7, compared with −0.4 between K_p and B_z. In this paper, new dependences of K_p on solar wind velocity and dynamic pressure are included in the model to improve its accuracy. These solar wind parameters are found to correlate best with K_p. The hourly interpolated values are also added to the 3-h K_p values to increase the statistics. The new K_p data set is denoted as K_p1. The mean dependence of K_p on B_zm and dynamic pressure are approximated with parabolas, while the dependence on the velocity is linear. The constants in the model expression are obtained by using ACE data (1998–2000). The overall model error is estimated at 0.63 units K_p. The improvement over the previous simpler dependence in terms of the model error is about 30%.