[1] On 28 February 1998, four quasi-periodic pressure pulses with an amplitude of a few nPa detected by ACE gave rise to periodic compressions of the magnetosphere with period of about 14 min. In concert with periodic compressed and expanded states of the magnetosphere forced directly by the pressure variation, a coherent geomagnetic field fluctuation with the same period appeared on a global scale and was recorded at stations located from polar to equatorial regions. Most ground-level geomagnetic field signatures on the dayside can be interpreted as the result of a global ionospheric current system, like the global Pc5 event examined by Motoba et al. [2002]. In the afternoon polar ionosphere covered with the dense magnetometer stations, a vortical current structure associated with pressure-induced field-aligned currents (FACs) is centered at 72°± 1°and consists of a counterclockwise (clockwise) vortex in response to positive (negative) changes of solar wind pressure oscillation. Although the vortical current signatures are unclear in the morning sector, each afternoon vortex could pair with the morning one with opposite rotation. During this event, the interplanetary magnetic field (IMF) remained steady with a strong southward orientation (À10 nT or less). In addition to the pressureinduced FAC system, the steady southward IMF drives the dayside Region 1 (R1) current system, resulting in the familiar large-scale two-cell convection pattern in the ionosphere observed by SuperDARN radars. The SuperDARN convection patterns indicated that the ionospheric convection reversal boundary (CRB) in the afternoon was located in the range of 73°$ 77°N around 15 MLT. The ionospheric footprint of the pressure-induced FAC in the afternoon was found to be 1.5°± 1.1°$ 4.0°± 1.4°e quatorward of the CRB. This suggests that the pressure-induced FAC is started inside the R1 current system originating from the outer magnetospheric boundary layer. We argue that the paired FAC system responsible for the global geomagnetic fluctuations on the ground arises from the oscillatory large-scale dynamical convection originating well inside the closed field lines in direct response to the quasi-periodic pressure variations, not from the localized undulations on the magnetopause nor from global eigenmode oscillations of the magnetospheric cavity.

[1] A global Pc5 geomagnetic pulsation (period of $6 min) was observed coherently from the auroral to equatorial latitudes in the local time sector of 0700 -2100 LT during 1834 -1900 UT on 21 April 1993. The Pc5 at the dayside dip equator (Sao Luiz, Brazil, and Ancon, Peru) was characterized by an equatorial enhancement with an enhancement ratio of $4. In the afternoon sector (1200 -1600 magnetic local time) the Pc5 at the auroral latitude was coherent with that at the dip equator within a time resolution of 3 s. The Pc5 amplitudes decreased sharply away from the auroral zone but were enhanced in the dayside dip equator, in a manner that resembled the latitudinal profile of a DP 2 magnetic fluctuation (period of $40 min) studied by Kikuchi et al. [1996] except for the enhanced amplitude in the auroral zone. In the dayside auroral oval the Pc5 fluctuations in the magnetic X/H component were reversed in phase between the morning and afternoon sectors. Additionally, the magnetic Y/D component fluctuations around noon were correlated with the dayside equatorial Pc5. At most dayside middle-/low-latitude stations the Pc5 fluctuations in the H component, which are regarded as evidence for magnetospheric compressions, were strong. However, near the dawn terminator the D component was dominant rather than the H component, suggesting the influences of ionospheric currents originating in the polar region. There was no significant azimuthal phase propagation away from noon for the auroral to equatorial Pc5 signals. The global observational results suggest that the dawn-dusk electric field in the polar ionosphere accompanying a pair of field-aligned currents extends instantaneously to the equatorial ionosphere and completes a DP 2-type ionospheric current system responsible for the global coherent Pc5.

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In order to enhance the reproduction of the recovery phase D st index of a geomagnetic storm which has been shown by previous studies to be poorly reproduced when compared with the initial and main phases, an artificial neural network with one hidden layer and error back-propagation learning has been developed. Three hourly D st values before the minimum D st in the main phase in addition to solar wind data of IMF southward-component B s , the total strength B t and the square root of the dynamic pressure, √ nV 2 , for the minimum D st , i.e., information on the main phase was used to train the network. Twenty carefully selected storms from 1972-1982 were used for the training, and the performance of the trained network was then tested with three storms of different D st strengths outside the training data set. Extremely good agreement between the measured D st and the modeled D st has been obtained for the recovery phase. The correlation coefficient between the predicted and observed D st is more than 0.95. The average relative variance is 0.1 or less, which means that more than 90% of the observed D st variance is predictable in our model. Our neural network model suggests that the minimum D st of a storm is significant in the storm recovery process.

Abstract. During the recovery phase of geomagnetic storms, the flux of relativistic (>2 MeV) electrons at geosynchronous orbits is enhanced. This enhancement reaches a level that can cause devastating damage to instruments on satellites. To predict these temporal variations, we have developed neural network models that predict the flux for the period 1-12 h ahead. The electron-flux data obtained during storms, from the Space Environment Monitor on board a Geostationary Meteorological Satellite, were used to construct the model. Various combinations of the input parameters AL, AL, D st and D st were tested (where denotes the summation from the time of the minimum D st ). It was found that the model, including AL as one of the input parameters, can provide some measure of relativistic electron-flux prediction at geosynchronous orbit during the recovery phase. We suggest from this result that the relativistic electron-flux enhancement during the recovery phase is associated with recurring substorms after D st minimum and their accumulation effect.