[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.

Interplanetary magnetic field (IMF) and plasma data are compared with ground-based geomagnetic Dst and AE indices to determine the causes of magnetic storms, substorms, and quiet during the descending phase of the solar cycle. In this paper we focus primarily on 1974 when the AE index is anomalously high (AE = 283 nT). This year is characterized by the presence of two long-lasting corotating streams associated with coronal holes. The corotating streams interact with the upstream low-velocity (300-350 km s-1), high-density heliospheric current sheet (HCS) plasma sheet, which leads to field compression and ~ 15-to 25-nT hourly average values. Although the B z component in this corotating interaction region (CIR) is often < -10 nT, typically the field directionality is highly variable, and large southward components have durations less than 3 hours. Thus the corotating stream/HCS plasma sheet interaction region can cause recurring moderate (-100 nT < Dst < -50 nT) to weak (-50 nT < Dst < -25 nT) storms, and sometimes no significant ring current activity at all (Dst > -25 nT). Storms of major (Dst < -100 nT) intensities were not associated with CIRs. Solar wind energy is transferred to the magnetosphere via magnetic reconnection during the weak and moderate storms. Because the B z component in the interaction region is typically highly fluctuating, the corresponding storm main phase profile is highly irregular. Reverse shocks are sometimes present at the sunward edge of the CIR. Because these events cause sharp decreases in field magnitude, they can be responsible for storm recovery phase onsets. The initial phases of these corotating stream-related storms are caused by the increased ram pressure associated with the HCS plasma sheet and the further density enhancement from the stream-stream compression. Although the solar wind speed is generally low in this region of space, the densities can be well over an order of magnitude higher than the average value, leading to significant positive Dst values. Since there are typically no forward shocks at 1 AU associated with the stream-stream interactions, the initial phases have gradual onsets. The most dramatic geomagnetic response to the corotating streams are chains of consecutive substorms caused by the southward components of large-amplitude Alfvtn waves within the body of the corotating streams. This auroral activity has been previously named high-intensity long-duration continuous AE activity (HILDCAAs). The substorm activity is generally most intense near the peak speed of the stream where the Alfvtn wave amplitudes are greatest, and it decreases with decreasing wave amplitudes and stream speed. Each of the 27-day recurring HILDCAA events can last 10 days or more, and the presence of two events per solar rotation is the cause of the exceptionally high AE average for 1974 (higher than 1979). HILDCAAs often occur during the recovery phase of magnetic storms, and the fresh (and sporadic) injection of substorm energy leads to unusually long storm recovery phases as noted...

[1] The interplanetary causes of intense geomagnetic storms and their solar dependence occurring during solar cycle 23 (1996-2006) are identified. During this solar cycle, all intense (Dst À100 nT) geomagnetic storms are found to occur when the interplanetary magnetic field was southwardly directed (in GSM coordinates) for long durations of time. This implies that the most likely cause of the geomagnetic storms was magnetic reconnection between the southward IMF and magnetopause fields. Out of 90 storm events, none of them occurred during purely northward IMF, purely intense IMF By fields or during purely high speed streams. We have found that the most important interplanetary structures leading to intense southward Bz (and intense magnetic storms) are magnetic clouds which drove fast shocks (sMC) causing 24% of the storms, sheath fields (Sh) also causing 24% of the storms, combined sheath and MC fields (Sh+MC) causing 16% of the storms, and corotating interaction regions (CIRs), causing 13% of the storms. These four interplanetary structures are responsible for three quarters of the intense magnetic storms studied. The other interplanetary structures causing geomagnetic storms were: magnetic clouds that did not drive a shock (nsMC), non magnetic clouds ICMEs, complex structures resulting from the interaction of ICMEs, and structures resulting from the interaction of shocks, heliospheric current sheets and high speed stream Alfvén waves. During the rising phase of the solar cycle, sMC and sheaths are the dominant structure driving intense storms. At solar maximum, sheath fields, followed by Sh+MCs and then by sMC were responsible for most of the storms. During the declining phase, sMC, Sh and CIR fields are the main interplanetary structures leading to intense storms. We have also observed that around 70% of the storms follow the interplanetary criteria of Ey ! 5 mV/m for at least 3 h. Around 90% of the storms used in the study followed a less stringent set of criteria: Ey ! 3 mV/m for at least 3 h. Finally, we obtain the approximate rate of intense magnetic storms per solar cycle phases: minimum/rising phase 3 storms.year À1 , maximum phase 8.5 storms.year À1 , and declining/minimum phases 6.5 storms.year À1 .

We present a review on the interplanetary causes of intense geomagnetic storms (Dst ≤ −100 nT), that occurred during solar cycle 23 (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005). It was reported that the most common interplanetary structures leading to the development of intense storms were: magnetic clouds, sheath fields, sheath fields followed by a magnetic cloud and corotating interaction regions at the leading fronts of high speed streams. However, the relative importance of each of those driving structures has been shown to vary with the solar cycle phase. Superintense storms (Dst ≤ −250 nT) have been also studied in more detail for solar cycle 23, confirming initial studies done about their main interplanetary causes. The storms are associated with magnetic clouds and sheath fields following interplanetary shocks, although they frequently involve consecutive and complex ICME structures. Concerning extreme storms (Dst ≤ −400 nT), due to the poor statistics of their occurrence during the space era, only some indications about their main interplanetary causes are known. For the most extreme events, we review the Carrington event and also discuss the distribution of historical and space era extreme events in the context of the sunspot and Gleissberg solar activity cycles, highlighting a discussion about the eventual occurrence of more Carringtontype storms.

The solar wind‐magnetosphere coupling problem is investigated for the ten intense magnetic storms (Dst<−100 nT) that occurred during the 500 days (August 16, 1978 to December 28, 1979) studied by Gonzalez and Tsurutani [1987]. This investigation concentrates on the ring current energization in terms of solar wind parameters, in order to explain the |−Dst| growth observed during these storms. Thus several coupling functions are tested as energy input and several sets of the ring current decay time‐constant τ are searched to find best correlations with the Dst response. From the fairly large correlation coefficients found in this study, there is strong evidence that large scale magnetopause reconnection operates during such intense storm events and that the solar wind ram pressure plays an important role in the ring current energization. Thus a ram pressure correction factor is suggested for expressions concerning the reconnection power during time intervals with large ram pressure variations. The best set of values found from the present study is in accord with recent similar and independent suggestions. With respect to the ring current energy injection rates during intense storm events, typical values of 150±50 nT/hour are obtained. These are considerably larger than values extrapolated from previous studies restricted to moderate storms. Such rates of energy injection are observed to get transmitted from the magnetopause to the inner magnetosphere with an average time delay of about 1 hour, although this delay can become shorter as the storm events get more intense. It is also found that AE does not respond as well as Dst to the coupling functions, except for the events that have Dst values only >−100 nT (less intense energization). This point suggests that there is a decoupling between the ring current and auroral processes during very intense storms with respect to their dependence on solar wind energization. Finally, a discussion is presented on associations of the coupling functions with the solar wind features described by Tsurutani et al. [1988] for the causes of these storm events.

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Abstract. For the sets of magnetic clo•ds studied in this workwe have shown the existence of a relationship between their peak magnetic field strength and peak velocity values, with a clear tendency that, clouds which move at, higher speeds also possess higher core magnetic field strengths. This result suggests a possible intrinsic property of magnetic clouds and also implies a geophysical consequence. The relatively low field strengths at low velocities is pres•mably the cause of the lack of intense storms during low speed e. jecta. There is also an indication that, this type of behavior is peculiar for magnetic clouds, whereas other types of non cloud-driver gas events do not, seem to show a similar relationship, at least, for the data studied in this paper. We suggest that, a field/speed relationship for magnetic clouds, as that obtained in our present study, could be associated with the cloud release and acceleration mechanism a.t the sun.Since for magnetic clouds the total field tyically has a substantial southward component, B•, our results in,ply that the interplanetary dawn-dusk electric field, given by v x Bs (where v is the cloud's velocity), is enhanced by both factors. Therefore, the consequent magnetospheric energization (that is governed by this electric field) becomes more efficient for the occurrence of magnetic storms.