The 3-4 Nov 1993 storm in the context of recurrent geomagnetic activity during the decline of solar cycle 22 N. U. Crooker Center for Space Physics, Boston University Continuous solar wind data and the x-ray and white light views of the sun available during the decline of solar cycle 20 in 1973-1975 led to major advances in solar-terrestrial physics. Coronal holes were discovered as the source of recurrent high- speed streams, and coronal mass ejections (CMEs) were discovered as the source of transient interplanetary disturbances. Thereafter coronal holes and CMEs became the accepted sources of recurrent and nonrecurrent geomagnetic storms, respectively. Recent research on the 1974 data, however, has resulted in a broader view of the source of recurrent storms. Crooker and Cliver [1994] re-emphasized the importance of an older association between recurrent storms and corotating interaction regions (CIRs), raised the possibility that CMEs may play a central role in recurrent as well as nonrecurrent storms, and demonstrated that the Russell-McPherron effect exerts strong control over recurrent activity [Russell and McPherron, 1973]. Although continuous solar wind data are not available for most of the decline of cycle 22, some of these effects are apparent in 27-day recurrence plots of the (provisional) Dst index of geomagnetic activity in Figure 1 and Figure 2, upon which the inferred interplanetary sector structure has been superposed. Figure 1 shows the period from June solstice through September equinox to December solstice in 1993, ordered by Bartels rotation. The superposed shading indicating IMF polarity was obtained from the OMNI spacecraft data set, when available, and supplemented with polarities from the Stanford mean solar magnetic field recurrence maps published in Solar-Geophysical Data. The resulting sector structure agrees well with the polarity maps provided by S.Watari for this period, based on interpolated spacecraft measurements alone. In particular, it shows the evolution from a four-sector to a two-sector pattern pointed out by Watari. The 3-4 Nov 1993 storm began at the end of Bartels rotation 2188. As pointed out by D. Knipp, it was the fourth in a series of five recurrent storms, with Dst maxima of -78, -169, -82, -116, and -111 nT, respectively. (Unfortunately, these storms do not stand out in the figure because they span days from the end of one rotation to the beginning of the next.) This was the only series of recurrent storms in the interval. The storms were typical of the 1974-5 recurrent storms. They occurred in the sector with IMF polarity favorable for the Russell-McPherron effect for that season--away polarity, in this case, for the September equinoctial period. The peak of each storm occurred shortly after sector boundary passage, where the combined effects of CIRs and CMEs can create strong southward magnetic fields. As pointed out in the background information for the 3-4 Nov event, the available Yohkoh and Mauna Loa data show activity on the sun consistent with the view that CMEs (and/or less spectacular but, nevertheless, transient features) were responsible for peak activity in these recurrent storms. Each of the five storm peaks was followed by sustained, low-level activity that extended across the sector passage. Sustained activity has been ascribed to the combination of the high speed of coronal hole flow and southward IMF created by its intrinsic Alfvenic fluctuations [Tsurutani and Gonzalez, 1987]. It is clear from the 1974 observations, however, that sustained activity is strongly controlled by the Russell-McPherron effect, which provides an added southward IMF from the projection of the predominantly ecliptic magnetic fields onto Earth's dipole- ordered magnetospheric (GSM) coordinate system. This effect is also apparent in Figure 1. For example, the storm that followed the 3-4 Nov storm, on 18-19 Nov, occurred in a toward sector and had no sustained activity. In general, Dst hugs the baseline in the shaded regions of toward polarity in the figure and hovers below the baseline in the open regions of away polarity. Figure 2 shows the period from December, 1993, solstice through March equinox to June, 1994, solstice. Although this period goes beyond the storm interval of interest, it is included to illustrate similar recurrence characteristics under a stable, two-sector configuration, as in 1974. In the solstitial rotations 2191 and 2197, at the top and bottom, there was no difference between activity in the two sectors; but in the five intervening rotations, Dst was clearly more depressed in the toward compared to the away sector, opposite to Figure 1, consistent with the Russell-McPherron effect for this March equinoctial season. That there was any sustained activity at all in the away sector suggests that a baseline adjustment is needed for these provisional Dst values. Peak activity at the leading edge of the toward sectors was relatively weak compared to the sustained activity, suggesting weak speed gradients in the associated CIRs and/or no CME activity associated with that sector boundary. The only two pronounced storms, peaking at the beginnings of rotations 2193 and 2195, occurred at the other sector boundary, where they fell into the same recurrence pattern as the five storms in Figure 1. These form the subject of two coordinated studies undertaken by STEP and clearly were produced by CMEs. In contrast to Figure 1, however, the active streamer belt region coincided with passage from favored to unfavored polarity, which places the storms at the beginning of the unfavored sector. Thus no following sustained activity would be predicted. What appears to be sustained activity may be the result of the baseline problem. In summary, the 3-4 Nov 1993 event was a typical recurrent storm. Peak activity occurred shortly after passage into the sector favored by the Russell-McPherron effect for that season, and it was followed by sustained activity throughout most of the remaining sector passage. The peak activity most likely was caused by southward IMF from streamer belt transients, possibly a CME, which may have been enhanced both by compression in a CIR and by the Russell-McPherron effect. The sustained activity can be more directly ascribed to high speed flow from a coronal hole, consistent with the 1970's view of recurrent activity; but its amplitude was strongly controlled by the Russell-McPherron effect. References Crooker, N. U., and E. W. Cliver, Postmodern view of M-regions, J. Geophys. Res., 99, 23,383-23,390, 1994. Russell, C. T., and R. L. McPherron, Semiannual variation of geomagnetic activity, J. Geophys. Res., 78, 92-108, 1973. Tsurutani, B. T., and W. D. Gonzalez, The cause of high-intensity long-duration continuous AE activity (HILDCAAS): Interplanetary Alfven wave trains, Planet. Space Sci., 35, 405-412, 1987. Acknowledgments. The provisional Dst index values were obtained from NOAA, World Data Center A for Solar-Terrestrial Physics, with the special help of H. Coffey, and the OMNI spacecraft data were obtained from the National Space Science Data Center, both through the World Wide Web. This research was supported by the National Science Foundation under grant ATM94- 21814.