Resolving Geomagnetic Disturbances Using "K-Like" Geomagnetic Indices with Variable Time Intervals
Journal of Atmospheric and Solar-Terrestrial Physics
One of the efforts of the space weather community is to find better near real-time indices of geomagnetic activity. Many models developed for, and used by, space weather forecasters rely on the Kp/ap indices. However, the 3-h time interval between index computations is now considered a shortfall with regard to specifying and forecasting phenomena known to have faster time constants, e.g., auroral electrojet variations during a substorm. Therefore these indices have been targeted for high-time resolution development. Our study addresses this question by attempting to develop a “K-like” station index with a variable time interval. By comparing the distributions of these different time interval indices with the traditional 3-h K index an immediate difficulty is uncovered. Namely, the new distribution has a markedly different average index value. Hence no straightforward use of these indices in lieu of the traditional K index is possible. However, we demonstrate that, through a simple re-normalization process, the frequency distribution of our variable-time indices can be made to approximate the 3-h K index distribution. Further, when the temporal variability of the 3-h K index is compared against both a “sliding” 3-h index and a 15-min index, it is found that the 15-min index is significantly better at reproducing the station geomagnetic variability. The question not addressed here, but one that needs to be raised, concerns the possible difference in physical processes reflected by the 3-h index vs the shorter time interval indices. If there is no difference, then after a suitable range-to-index conversion these higher time resolution indices could potentially be used in place of the traditional 3-h station K index.
Della-Rose, D. J., J. J. Sojka, and L. Zhu, Resolving geomagnetic disturbances using "K-like" geomagnetic indices with variable time intervals, J. Atmos. Solar-Terr. Phys., 61, 1179-1194, 1999.