A global analysis of the 1991 geomagnetic jerk

A recent examination of the geomagnetic annual mean values for the European magnetic observatories has shown the existence of a sudden change in the s ecular acceleration in about 1991 (Cafarella & Meloni 1995; Macmillan 1996) . Using first differences of the Y (east geomagnetic field component) mean values from 74 observatories, the worldwide character of the 1991 impulse h as been determined (De Michelis ct al. 1998). Using data from 109 observatories widely distributed all over the world, th e structure of the secular variation for the X (north) and Z (vertical) mag netic field intensities around 1990 was investigated, and evidence of this most recent jerk was found. External effects were removed from the annual m ean data by comparing the long-term variations of the geomagnetic field com ponents at individual observatories with the long-term variations of two ge omagnetic indices, aa and D-st, and of a solar index, the Wolf number R. A careful analysis has been carried out on the amplitude of the external dist urbance, on its dependence on latitude, and on the weights of the geomagnet ic indices in the evaluation of the resulting external field. The secular v ariation has been evaluated from the corrected annual means. Around 1990, t he secular variation can be fitted at many observatories by two straight li nes with a sudden and marked change in slope. In this manner the jerk occur rence time and the intensity of the step in the second time derivative (Del taX'', DeltaY" and DeltaZ") were computed. Maps of DeltaX", DeltaY" and Del taZ" provide information on the worldwide intensity distribution of the exa mined event. Mars of the jerk occurrence-time distributions are also given. The mean jerk occurrence time is 1990.1 +/- 0.6. Finally, a spherical harm onic analysis was used to complete the quantitative description of this phe nomenon in order to study the trend of the energy density spectrum as a fun ction of the harmonic degree n.