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...

The ELF (10–1500 Hz) electromagnetic emissions in the midnight sector of the outer magnetosphere have been studied using Ogo 5 search coil magnetometer data. Chorus was detected in conjunction with magnetospheric substorms throughout the region from L = 5 to L = 9 but only during postmidnight hours. No chorus was seen in the 3 hr preceding midnight even when substorms were in progress. The postmidnight chorus occurred only within ±15° of, and most frequently at, the geomagnetic equator. Time‐averaged intensities varied from 10−8 to 10−6 γ²/Hz, which is more than an order below the maximum values reported previously for dayside chorus. The chorus occurred in narrow frequency bands having a bandwidth of 50–200 Hz. Chorus frequencies varied from less than one‐fourth to as high as three‐fourths of the equatorial electron gyrofrequency as determined by the on‐board magnetometer. All frequencies in this range were detected except for a narrow band near one‐half the electron gyrofrequency where the chorus appeared to be strongly attenuated. Chorus was often observed as two distinct bands above and below one‐half the gyrofrequency. The two most common types of chorus were narrow band chorus without structure and falling tones. Rising tones and hooks were also observed. Chorus pulsations were observed often with quasi‐periods of 5–15 s. A correlation was sought, but none was found, between the micropulsations in the ambient magnetic field and the chorus pulsations. Many features of postmidnight chorus can be explained by a cyclotron resonant interaction between the waves and the substorm electrons. The distribution of the chorus as a function of local time and L is strikingly similar to the distribution of enhanced, trapped, and precipitated substorm electrons with energies >40 keV. The postmidnight occurrence of chorus is attributable to the eastward curvature and gradient drift of the injected electrons. Cyclotron resonant interactions should be strongest near the equator, as was observed. The confinement to within 15° of the equator is attributed to Landau damping by low‐energy (1–10 keV) auroral electrons. The attenuation band at one‐half the electron gyrofrequency may result from Landau damping by electrons that have energy corresponding to cyclotron resonance but are traveling in the same direction as the waves. A close correspondence is expected between the occurrence of postmidnight and dayside chorus. The maximum in dayside chorus intensity at approximately 1000 LT, which is correlated with the dayside maximum of energetic electron precipitation, may represent further precipitation of the substorm electrons injected near midnight.

[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 demonstrate extreme ionospheric response to the large interplanetary electric fields during the “Halloween” storms that occurred on October 29 and 30, 2003. Within a few (2–5) hours of the time when the enhanced interplanetary electric field impinged on the magnetopause, dayside total electron content increases of ∼40% and ∼250% are observed for the October 29 and 30 events, respectively. During the Oct 30 event, ∼900% increases in electron content above the CHAMP satellite (∼400 km altitude) were observed at mid‐latitudes (±30 degrees geomagnetic). The geomagnetic storm‐time phenomenon of prompt penetration electric fields is a possible contributing cause of these electron content increases, producing dayside ionospheric uplift combined with equatorial plasma diffusion along magnetic field lines to higher latitudes, creating a “daytime super‐fountain” effect.

[1] The interplanetary shock/electric field event of 5-6 November 2001 is analyzed using ACE interplanetary data. The consequential ionospheric effects are studied using GPS receiver data from the CHAMP and SAC-C satellites and altimeter data from the TOPEX/ Poseidon satellite. Data from $100 ground-based GPS receivers as well as Brazilian Digisonde and Pacific sector magnetometer data are also used. The dawn-to-dusk interplanetary electric field was initially $33 mV/m just after the forward shock (IMF B Z = À48 nT) and later reached a peak value of $54 mV/m 1 hour and 40 min later (B Z = À78 nT). The electric field was $45 mV/m (B Z = À65 nT) 2 hours after the shock. This electric field generated a magnetic storm of intensity D ST = À275 nT. The dayside satellite GPS receiver data plus ground-based GPS data indicate that the entire equatorial and midlatitude (up to ±50°magnetic latitude (MLAT)) dayside ionosphere was uplifted, significantly increasing the electron content (and densities) at altitudes greater than 430 km (CHAMP orbital altitude). This uplift peaked $2 1/2 hours after the shock passage. The effect of the uplift on the ionospheric total electron content (TEC) lasted for 4 to 5 hours. Our hypothesis is that the interplanetary electric field ''promptly penetrated'' to the ionosphere, and the dayside plasma was convected (by E Â B) to higher altitudes. Plasma upward transport/convergence led to a $55-60% increase in equatorial ionospheric TEC to values above $430 km (at 1930 LT). This transport/convergence plus photoionization of atmospheric neutrals at lower altitudes caused a 21% TEC increase in equatorial ionospheric TEC at $1400 LT (from ground-based measurements). During the intense electric field interval, there was a sharp plasma ''shoulder'' detected at midlatitudes by the GPS receiver and altimeter satellites. This shoulder moves equatorward from À54°to À37°MLAT during the development of the main phase of the magnetic storm. We presume this to be an ionospheric signature of the plasmapause and its motion. The total TEC increase of this shoulder is $80%. Part of this increase may be due to a ''superfountain effect.'' The dayside ionospheric TEC above $430 km decreased to values $45% lower than quiet day values 7 to 9 hours after the beginning of the electric field event. The total equatorial ionospheric TEC decrease was $16%. This decrease occurred both at midlatitudes and at the equator. We presume that thermospheric winds and neutral composition changes produced by the storm-time Joule heating, disturbance dynamo electric fields, and electric fields at auroral and subauroral latitudes are responsible for these decreases.