After a brief review of magnetospheric and interplanetary phenomena for intervals with enhanced solar wind-magnetosphere interaction, an attempt is made to define a geomagnetic storm as an interval of time when a sufficiently intense and long-lasting interplanetary convection electric field leads, through a substantial energization in the magnetosphere-ionosphere system, to an intensified ring current sufficiently strong to exceed some key threshold of the quantifying storm time Dst index. The associated storm/substorm relationship problem is also reviewed. Although the physics of this relationship does not seem to be fully understood at this time, basic and fairly well established mechanisms of this relationship are presented and discussed. Finally, toward the advancement of geomagnetic storm research, some recommendations are given concerning future improvements in monitoring existing geomagnetic indices as well as the solar wind near Earth. knowledge of magnetospheric physics using spacecraft, as compared to older epochs when most of that knowledge had to come from ground observations. In addition, past attempts to formulate definitions for storms were restricted only to the near-Earth environment, the ionosphere and magnetosphere. However, with the subsequent accumulation of information obtained in the interplanetary medium, critical aspects of these definitions now involve diverse findings related to the solar wind dynamics [e.g., Burton et al., 1975; Gonzalez and Tsurutani, 1987; Tsurutani and Gonzalez, 1987]. Motivated by an interest in trying to find unifying concepts about the geomagnetic storm and the longstanding problem of storm/substorm relationship, the authors of this review paper met at the National Institute for Space Research of Brazil (INPE), at Sho Jos6 dos Campos, Sho Paulo, during the interval of November 5-8, 1991. The results obtained in this meeting, together with further elaboration, are presented in this paper in the following sequence.Section 2 is devoted to historical aspects of geomagnetic storm research, as based on ionospheric and magnetospheric parameters. In section 3 the interplanetary origin of storms is addressed. A brief review follows on solar wind-magnetosphere coupling, particularly applied to storm intervals. Then, for completeness, the seasonal and solar cycle distribution of storms is briefly considered. Section 4 reviews basic aspects of the storm/substorm relationship problem. In section 5, a discussion about additional mechanisms that contribute to ring current intensification as well as about basic mechanisms for ring current decay is given. A brief review on the relationship of Dst to other geomagnetic indices follows. Section 6 gives summary concepts on geomagnetic storms and on their relationship to substorms. A definition for a geomagnetic storm is suggested. Finally, in section 7 recommendations con-5771 5772 GONZALEZ ET AL.: REVIEW PAPER cerning future improvements in monitoring existing geomagnetic indices as well as the solar wind near the Earth are provided...

[1] Solar wind fast streams emanating from solar coronal holes cause recurrent, moderate intensity geomagnetic activity at Earth. Intense magnetic field regions called Corotating Interaction Regions or CIRs are created by the interaction of fast streams with upstream slow streams. Because of the highly oscillatory nature of the GSM magnetic field z component within CIRs, the resultant magnetic storms are typically only weak to moderate in intensity. CIR-generated magnetic storm main phases of intensity Dst < À100 nT (major storms) are rare. The elongated storm ''recovery'' phases which are characterized by continuous AE activity that can last for up to 27 days (a solar rotation) are caused by nonlinear Alfven waves within the high streams proper. Magnetic reconnection associated with the southward (GSM) components of the Alfvén waves is the solar wind energy transfer mechanism. The acceleration of relativistic electrons occurs during these magnetic storm ''recovery'' phases. The magnetic reconnection associated with the Alfvén waves cause continuous, shallow injections of plasma sheet plasma into the magnetosphere. The asymmetric plasma is unstable to wave (chorus and other modes) growth, a feature central to many theories of electron acceleration. It is noted that the continuous AE activity is not a series of substorm expansion phases. Arguments are also presented why these AE activity intervals are not convection bays. The auroras during these continuous AE activity intervals are less intense than substorm auroras and are global (both dayside and nightside) in nature. Owing to the continuous nature of this activity, it is possible that there is greater average energy input into the magnetosphere/ ionosphere system during far declining phases of the solar cycle compared with those during solar maximum. The discontinuities and magnetic decreases (MDs) associated with interplanetary Alfven waves may be important for geomagnetic activity. In conclusion, it will be shown that geomagnetic storms associated with high-speed streams/CIRs will have the same initial, main, and ''recovery'' phases as those associated with ICME-related magnetic storms but that the interplanetary causes are considerably different.

We present a new technique for mapping high‐latitude electric fields and currents and their associated magnetic variations from sets of localized observational data. The technique generalizes earlier ones that were designed to deduce these electrodynamic features from ground‐based magnetometer data alone. In the new technique, many different types of measurements can potentially be used: electric fields from radars and satellites; electric currents from radars; and magnetic perturbations at the ground and at satellite heights. The technique also makes use of available statistical information about averages and variances of the electrodynamic fields. One of its advantages over earlier techniques is that it quantifies the errors inherent in the mapped fields, taking into account the distribution of available data, their errors, and the statistical variances of the fields. A related application of the procedure is used for estimating the distributions of high‐latitude ionospheric conductances, using available direct and indirect measurements. The new technique is illustrated by application to an example of a substorm that was previously analyzed by Kamide et al. (1982a) with an earlier technique. The new technique tends to yield much simpler patterns of high‐latitude ionospheric convection in regions of low ionospheric conductance. When magnetometer data alone are used, as in this example, the statistical uncertainty in the derived electric fields is largest in regions of low conductance, because the electric fields in these regions have little influence on the magnetic perturbations. A companion paper (Richmond et al., this issue) presents a detailed application of the technique using multiple data sets.

Abstract. The semiannual variation in geomagnetic activity is generally attributed to the Russell-McPherron effect. In that picture, enhancements of southward field B.• near the equinoxes account for the observed higher geomagnetic activity in March and September. In a contrary point of view, we argue that the bulk of the semiannual variation results from an equinoctial effect (based on the ½• angle between the solar wind flow direction and Earth's dipole axis) that makes B.• coupling less effective (by ~25% on average) at the solstices. Thus the semiannual variation is not simply due to "mountain building" (creation of B.,) at the equinoxes but results primarily from "valley digging" (loss of coupling efficiency) at the solstices. We estimate that this latter effect, which clearly reveals itself in the diurnal variation of the am index, is responsible for ~65% of the semiannual modulation. The characteristic imprint of the equinoctial hypothesis is also apparent in hourly/monthly averages of the time-differentiated Dst index and the AE index.

For many years, researchers have utilized definitions of the substorm phenomenon that are not consistent among one another, and this has created great difficulties in comparing the results reported in the literature by the various researchers. In August 1978, nine magnetospheric physicists active in the field of substorm research met in Victoria, British Columbia, Canada, to attempt to reach a consensus on an acceptable definition for a magnetospheric substorm. This paper reports the agreements reached at the Victoria workshop and presents an operational definition of the magnetospheric substorm and a critique of the various signatures by which researchers can identify the time sequence and spatial extent of the substorm.

Abstract. This paper attempts to summarize the current understanding of the storm/substorm relationship by clearing up a considerable amount of controversy and by addressing the question of how solar wind energy is deposited into and is dissipated in the constituent elements that are critical to magnetospheric and ionospheric processes during magnetic storms. (1) Four mechanisms are identified and discussed as the primary causes of enhanced electric fields in the interplanetary medium responsible for geomagnetic storms. It is pointed out that in reality, these four mechanisms, which are not mutually exclusive, but interdependent, interact differently from event to event. Interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) are found to be the primary phenomena responsible for the main phase of geomagnetic storms. The other two mechanisms, i.e., HILDCAA (high-intensity, long-duration, continuous auroral electrojet activity) and the so-called RussellMcPherron effect, work to make the ICME and CIR phenomena more geoeffective. The solar cycle dependence of the various sources in creating magnetic storms has yet to be quantitatively understood.

An approximate method of separating the effects of ionospheric currents from those of field‐aligned currents in ground magnetic perturbations observed in high latitudes is developed. The distribution of ionospheric electric fields can also be estimated. The procedure includes the following steps: (1) the calculation of the equivalent ionospheric current function on the basis of magnetic H and D component records on the earth's surface, (2) the computation of the electric potential distribution from the equivalent ionospheric current function by using a simple model of the ionospheric conductivity, (3) the derivation of ionospheric current vectors as well as electric fields and, (4) the derivation of the field‐aligned current intensity by taking the divergence of the ionospheric currents. Several examples for both quiet and disturbed conditions are utilized to demonstrate how our method is successful in estimating the intensities of the electric fields and the ionospheric and field‐aligned currents in the sense that the estimated values are in good agreement with those observed recently by radar and satellite methods. Significant portions of the H component in nightside auroral latitudes appear to result from the east‐west ionospheric currents, called the auroral electrojets, while both the north‐south ionospheric current and field‐aligned current are almost equally important in producing the D component excursions. It is found that the ionospheric and field‐aligned current distributions obtained are not very sensitive to the choice of the ionospheric conductivity model, unless the auroral enhancement is not given in an appropriate place. This indicates that even a simple conductivity distribution inferred from the distribution of the magnetic perturbations can make it possible to estimate the three‐dimensional current system with a reasonable accuracy.

This paper attempts to synthesize the diverse number of observations of electric fields and currents in the high‐latitude ionosphere during substorms. By demonstrating that there are often spatial shifts among regions of high ionospheric conductivity, large electric fields and intense currents in the auroral electrojet, it is shown that substorm time variations of the current patterns over the entire polar region consist of two basic components. The first is related to the two‐cell convection pattern and the second to the westward electrojet in the dark sector, which is in turn related to the three‐dimensional wedge current system. These two components result from the relative strength of electric fields and conductivities in the intensification of the auroral electrojet and are identified as the signatures for directly driven and the unloading components in solar wind‐magnetosphere interactions. We contend that disturbed intervals do not necessitate the presence of substorm expansion‐phase activity and that the vast number of earlier complex results concerning the auroral electrojet can be ascertained from the high degree of variability of the two components, depending on substorm events, substorm phases, and their own spatial/temporal scale sizes. It is demonstrated that several major issues that have remained controversial are now accounted for reasonably well in terms of this two‐component electrojet model. We also predict specific features of the substorm auroral electrojet that have not yet been observed.