Three-hour activity index based on power spectra estimation

SUMMARY We propose a new index of magnetic activity during three-hour intervals. The new index E is based on the energy spectra. As a result of the fact that energy is additive, we can use information from both horizontal components simultaneously. After calibration of the new index in relation to the commonly used K index, we made some numerical tests comparing both indices. The tests showed that the values of indices are similar (differences greater than ±1 occurred only in 0.1 per cent of cases). We also made calculation of correlation with some magnetospheric parameters. The new index seems to correlate slightly better than the old one. It is still an open question whether the new index can be treated as an improved version of the K index and create a prolongation of old series.