The Causes and Uses of Geomagnetic Disturbance Index Kp

Abstract

The planetary indices of magnetic activity Kp and ap are widely regarded as overall measures of solar wind‐magnetosphere interaction and are used in correlation studies of interplanetary and solar phenomena. For three levels of activity, quiet, average and storm, the components of magnetic fluctuation and the current systems responsible are isolated and their contributions to Kp assessed. It is concluded that Kp does not provide an adequate index of overall activity and should be used with care in correlation studies. The main contribution is from ionospheric currents caused by enhanced activity in the geomagnetic tail, so that Kp is a measure of tail activity, perhaps accelerated field annihilation caused by ‘flapping’. There are three important components of magnetic disturbance which make little or no contribution to Kp; there may be substantial solar wind interaction and current systems where Kp= 0. Such current systems are the Chapman‐Ferraro current (measuring wind pressure), a polar cap current system probably measuring frictional interaction, and rapidly fluctuating currents around the auroral oval. The introduction of activity indices additional to Kp seems desirable to provide a full history of overall geomagnetic activity and for use in correlation studies. If a single index is needed then Kp could be extended downward to negative values. The possible significance of some solar wind correlations made using Kp are discussed briefly. Copyright © 1968, Wiley Blackwell. All rights reserved