GEOMAGNETIC STORMS, DST RING-CURRENT MYTH AND LOGNORMAL DISTRIBUTIONS

The definition of geomagnetic storms dates back to the turn of the cen tury when researchers recognized the unique shape of the H-component f ield change upon averaging storms recorded at low latitude observatori es. A generally accepted modeling of the storm field sources as a magn etospheric ring current was settled about 30 years ago at the start of space exploration and the discovery of the Van Allen belt of particle s encircling the Earth. The Dst global 'ring-current' index of geomagn etic disturbances, formulated in that period, is still taken to be the definitive representation for geomagnetic storms. Dst indices,or data from many world observatories processed in a fashion paralleling the index, are used widely by researchers relying on the assumption of suc h a magnetospheric current-ring depiction. Recent in situ measurements by satellites passing through the ring-current region and computation s with disturbed magnetosphere models show that the Dst storm is not s olely a main-phase to decay-phase, growth to disintegration, of a mass ive current encircling the Earth. Although a ring current certainly ex ists during a storm, there are many other field contributions at the m iddle-and low-latitude observatories that are summed to show the 'stor m' characteristic behavior in Dst at these observatories. One characte ristic of the storm field form at middle and low latitudes is that Dst exhibits a lognormal distribution shape when plotted as the hourly va lue amplitude in each time range. Such distributions, common in nature , arise when there are many contributors to a measurement or when the measurement is a result of a connected series of statistical processes . The amplitude-time displays of Dst are thought to occur because the many time-series processes that are added to form Dst all have their o wn characteristic distribution in time. By transforming the Dst time d isplay into the equivalent normal distribution, it is shown that a sto rm recovery can be predicted with remarkable accuracy from measurement s made during the Dst growth phase. In the lognormal formulation, the mean, standard deviation and field count within standard deviation lim its become definitive Dst storm parameters. Published by Elsevier Scie nce Ltd.