V_r: A new index to represent the variation rate of geomagnetic activity

By calculating the hourly standard deviation of the first-order differences of the horizontal geomagnetic component minute data, a new index V_r to represent the variation rate of the geomagnetic field was introduced. V_r-indices show similar trends in the temporal change at different observatories and have simultaneous peak values at the observatories covering a large span geographically, which reveals that the source of geomagnetic disturbances represented by V_r is in the magnetosphere. Based on the comparison among V_r, K_p and a_p, it is found that generally V_r changes linearly with K_p and a_p, which means that the rapid changes of magnetic field usually exist together with magnetic disturbances. But there are exceptions. As V_r can be easily produced by individual observatory in quasi real time and is more sensitive to the variation rate of geomagnetic field rather than the field itself, it can be expected to serve for monitoring or predicting the geomagnetic-induced event in a quick and intuitive way.