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...

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.

Short‐term (days to weeks) geomagnetic forecasts are valuable for a variety of public and private sector endeavors. However, forecast skill, as measured by the success of predicting geomagnetic indices, is disappointing, especially for disturbed conditions. Possible reasons for this lack of proficiency include an incomplete understanding of the solar origins of interplanetary disturbances, insufficient observations of solar phenomena and interplanetary disturbances, and an underestimation of magnetospheric‐ionospheric control of observed geomagnetic activity. Until more progress can be made on each of these problems, desirable forecasting precision is likely to remain elusive. The best opportunity for improved service to those agencies requiring advance notice of geomagnetic disturbances is “nowcasting” using real‐time, near‐Earth observations of the approaching solar wind.

Disappearing solar filaments have long been suspected as an indicator of terrestrial magnetic disturbances. However, because filament disappearances are a common solar event and because they failed as a candidate source for M region (recurrent) magnetic disturbances, their potential utility as a forecasting aid for geomagnetic storms has largely been neglected. A search for possible solar sources of geomagnetic storms from June 1976 through June 1979 has revealed that a significant number of the storms, including the two largest, can only be associated with filament disappearances. This result is supported by the many recent papers studying SKYLAB and other observations of coronal transients which always find a strong correlation between those transients and eruptive prominences. By analyzing the physical characteristics of those disappearances which precede magnetic storms and those which do not, some tentative guidelines for forecasting geomagnetic disturbances have been developed based on evidence of a significant restructuring of the implied coronal magnetic field which could release solar wind plasma favorably positioned to impact the earth.

The geomagnetic field occasionally exhibits abrupt, worldwide variations that have a morphology similar to that shown in Figure 1. Known as sudden impulses (SIs) or storm sudden commencements (SSCs), these signatures have been successfully explained as a compression of the magnetosphere caused by the passage of a shock or tangential discontinuity in the solar wind [e.g., Nishida, 1978]. Shocks propagate through the solar wind (outward from the Sun for fast forward shocks and inward for reverse shocks, in the solar wind frame of reference) and tangential discontinuities are simply carried with the solar wind. The purpose of this note is to examine the definitions of and distinctions between SSCs and SIs; to modernize present definitions of SIs and SSCs, in which SSCs are a subset of SIs depending on subsequent observed values of Dst or alternate geomagnetic indices; and to recommend quantitative definitions of the two terms for open discussion.

In September 1996, a panel of experts on solar cycle prediction techniques met in Boulder, Colorado, to survey forecasts of solar and geomagnetic activity and to arrive at a consensus on how the solar cycle will develop. After two weeks of deliberation, the panel of 12 scientists (from Australia, Germany, the United Kingdom, and the United States) agreed that a large amplitude solar cycle with a smoothed sunspot maximum of approximately 160 is probable near the turn of the century. The amplitude of the predicted cycle is comparable to that of the previous two solar cycles (see Figure 1). Our ability to predict solar and geomagnetic activity is crucial to many technologies, including the operation of low‐Earth orbiting satellites, electric power transmission grids, geophysical exploration, and highfrequency radio communications and radars. Because the scale height of Earth's upper atmosphere (and thus the drag on satellites in low Earth orbit) depends on the levels of short‐wavelength solar radiation and geomagnetic activity, we need to know the profile and magnitude of the next solar and geomagnetic cycle in order to plan for reboosting the Hubble Space Telescope and assembling the International Space Station.

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A steady three‐fluid model of the solar coronal expansion, in which 4He++ ions (alphas) are treated as a nonminor species, is developed for nonspherically symmetric flow geometries of the general sort thought to be characteristic of coronal holes. It is found that the very high mass fluxes in the low corona, which are associated with rapidly diverging flow geometries, lead to a locally enhanced frictional coupling between protons and alphas and consequently to a significant reduction of the He/H abundance ratio in the lower corona from that normally predicted by multifluid models. In the models considered, the frictional drag on the protons by the alphas (a process neglected in most studies) is found to play an important role near the sun. Heavy ions, other than alphas, are treated as minor species and are seen to exhibit varying responses to the rapidly diverging flow geometries, depending on the ion mass and charge. As for the protons, the frictional effect of the alphas on the heavier ions is found to be significant in the models considered.