What caused the large geomagnetic storm of November 19787

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. A8, PAGES 17,445-17,449, AUGUST 1, 1997 What caused the large geomagnetic storm of November H. V. Cane I and I. G. Richardson 2 Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, Greenbelt, Maryland Abstract. We discuss energetic particle and solar wind plasma data for the period around the large geomagnetic storm of November 24-25, The storm was clearly caused by an ejecta interacting with a corotating high-speed stream. We conclude that there are no data to supporthe attribution of this storm to specific solar activity as previously suggested. This case study illustrates the important role of energetic particles in making correct associations between solar and interplanetary phenomena. Even if there had been an identifiable source region for the ejecta, the intensity of the geomagnetic storm resulted from the interplanetary interaction. Thus predictions of the strengths of such storms cannot be based on solar observations alone but also require knowledge of stream interactions. Introduction vember 1978 when an intense geomagnetic storm took place. We believe that the cause of this storm has previously been An importantopic of current research is the study of the reincorrectly identified in several studies [Tang et al., 1989; lationship between phenomena at the Sun and interplanetary Pdrez-Enriquez and Mendoza, 1990; Bravo and Rivera, 1994; disturbances responsible for enhanced geomagnetic activity. In Gonzalez et al., 1996] and point out that the results of studies recent papers [Cane et al., 1996a, b], we have emphasized the attempting to understand the causes of large geomagnetic important role which energetic particles can play in improving storms [e.g., Gonzalez et al., 1996] will be of more significance our understanding of the sources of interplanetary disturbances. if the solar source regions are correctly identified. We note that Prompt particle enhancements exhibiting velocity dispersion are Zhao [ 1992] reached the same conclusion that we have as to the related to solar events which usually involve a flare. The inten- cause of the November 1978 storm, but Zhao's focus was difsity-time profiles of the particle event provide information ferent from our own. about whether the solar event lies east or west of the observer or near central meridian [Cane et al., 1988]. Most large particle events are associated with interplanetary shocks, and if the Solar Wind Disturbances in Late November 1978 event originates farther east than -W30 ø relative to the ob- Figure I shows near-earth solar wind observations from the server, then generally the shock is detected. Thus the particle National Space Science Data Center OMNI database [Couzens profile helps in associating an energetic shock with a specific and King, 1986] along with geomagnetic Dst values for the solar event. The extent of the particle enhancement in energy is period November Also shown in the lower two graphs an indicator of how energetic the associated solar event is and are energetic particle data from the Goddard Space Flight Cenwhether it is likely to have involved a high-energy flare (usually ter (GSFC) experiment on IMP 8 to be described later. The Dst with class M or X soft X ray emission and metric type II and index in the third graph shows the major geomagnetic storm on type IV radio bursts) or a weaker event with only minor flare November There are two solar wind features of interest signatures [Cane et al., 1996a]. We have also shown that the around the time of the geomagnetic storm. The first is the recombination of high (>1 GeV) and low (<100 MeV) energy gion of enhanced magnetic field and plasma density, bounded particle data allows one to determine whether interplanetary by a forward and reverse shock pair (dashed vertical lines), disturbances causing cosmic ray decreases are corotating or which lies at the leading edge of a high-speed stream. Richardtransient and, for a transient disturbance, whether a shock or son and Zwickl [ 1984] suggested that this was a rare example of ejecta (the interplanetary counterpart of the coronal mass ejec- a cotorating interaction region (CIR) with fully developed tion at the Sun), or both, is present. shocks observed at 1 AU. (More typically, CIR shocks develop We have previously pointed out that mistakes have been > 1.5 AU from the Sun [Smith and Wolfe, 1977].) This conclumade in associating interplanetary structures with solar phe- sion was based on the presence of a number of features. These nomena in studies which have not considered energetic particle include a clear stream interface [Burlaga, 1974] midway bedata [Cane et al., 1996b]. In this paper, we use energetic parti- tween the shocks (at 1 UT on November 25, indicated by cle data to help elucidate the nature of the events of late No- an increase in proton temperature and solar wind speed, decrease in plasma density, and deflection of the solar wind flow Also at Physics Department, University of Tasmania, Hobart, angle toward the west) and a double-peaked energetic particle Tasmania, Australia. enhancement similar to those observed at CIRs in the outer 2Also at Department of Astronomy, University of Maryland, heliosphere [e.g., Barnes and Simpson, 1976], though extending to hundreds of kev rather than to MeV energies. There was also a small coronal hole with the correct underlying photospheric magnetic field orientation (outward) consistent with the solar connection longitude of the high-speed stream inferred from mapping the observed solar wind speed back to the Sun. College Park. Copyright 1997 by the American Geophysical Union. Paper number 97JA /97/97JA ,445

2 17,446 CANE AND RICHARDSON: NOVEMBER 1978 GEOMAGNETIC STORM 3o 15 ' 0,, $0 a ]. Here the dates correspond to the time of central meridian passage (cmp) and cover a period commencing 3.5 days before the IMP 8 solar wind data shown in Figure 2 thereby allowing for Sun-Earth transit time for solar wind at 500 km/s. This strea may have been associated with the coronal hole at cmp November 22-23, ~N35 ø, as noted by Richardson and Zwickl HELZOS I (0.4 AU, E90) 10 $ ; O o lo 1øo_1 lo lo -2 DOY Nov 1978 Figure 1. Near-Earth (OMNI database) solar wind magnetic field and plasma parameters, the Dst geomagnetic activity index, and >60 MeV and ~ 1 MeV ion intensities, for November 18-28, The major geomagnetic storm on November is evident in the Dst data. The dashed vertical lines indicate a corotating forward-reverse shock pair bounding a corotating interaction region (CIR). The solid region indicates abnormally low-temperature plasmassociated with an ejecta. The southward magnetic field in the ejecta, which extends to the stream interface in the CIR, is enhanced by plasma compression in the CIR and causes the stron geomagnetic storm. As discussed in the text, the shocks or ejecta have no plausible association with the solar event at ~W65 ø on November 20 which produced the energetic particlevent evident in the bottom graph. 6O ' 30 u 0 < -30 o ß,i... i... i... i... i... i I,,,,.I..,,,I,..,I.,,,,I,...,I,,,.1.,,,,I,..,. I,,, HELZOS 2 (0.5 AU, E22) ß... i... i... i' I...I...I...I...I...I...I...I...I... OMNI... I,,,,,I... I... I... I... I...I... I... I... I CORONAL HOLES Solar wind data from the Helios 1 and 2 spacecraft substanti- '" i... I... i" ate that the stream was corotating. Figure 2 compares the solar wind speed profiles observed by the Helios spacecraft and by NOVEMBER 1978 IMP 8. The data for Helios I and 2 have been displaced relative Figure 2. The top three graphshow solar wind speed profiles to the IMP 8 data by intervals consistent with the expected observed by the Helios I and 2 spacecraft and by IMP 8. The corotation delays of ~9 and ~3.5 days, respectively, from these Helios data are offset by the corotation delay from the spacespacecraft (at the locations shown in Figure 2) to the Earth. The calculation of these delays assumes a solar wind speed of 500 craft to Earth (for a solar wind speed of 500 kin/s) so that corotating structures will be aligned vertically. The magnetic field is km/s which is intermediate between the slow and fast solar outwarduring all the intervalsøshown. The bottom panel wind speeds in Figures I and 2 and hence likely to characterize shows boundaries of He 10,830 A coronal holes (dashed lines the spiral configuration of a CIR formed at the stream leading indicate a questionable or variable boundary), all associated with outward magnetic field. The region shown commences at edge. The high-speed stream commencing on November 25 at the position of central meridian 3.5 days before the start of the Earth was clearly observed first at Helios 1 off the east limb of IMP 8 solar wind data thus allowing for the Sun-Earth transit the Sun from November 16 and then encountered Helios 2 time at 500 km/s. Two high-speed streams were seen at all three (where the solar wind was more structured) on November 22. spacecraft and associated with the coronal holes indicated by The bottom panel shows He 10,830 coronal hole data arrows. The right-hand stream is related to the shock pair in [adapted from Solar-Geophysical Data, Number 412, Part l, p. Figure 1.

3 CANE AND RICHARDSON: NOVEMBER 1978 GEOMAGNETIC STORM 17,447 [1984]. Figure 2 also shows a second, complex, corotating production of geomagnetic storms. Since "a compound stream stream which was observed earlier at each spacecraft and apparently associated with a group of coronal holes lying at cmp November Although these high-speed streams can be identified at each spacecraft, the solar wind profiles are not identical. This can probably be attributed to the different spacecraft heliocentric distances and latitudes, as well as small changes in the coronal hole and stream configuration during the interval in which the stream corotares from one spacecrafto another [Schwenn, 1991 ]. The second feature of interest in Figure I is the solid region in the proton temperature (fourth graph) which denotes an interval of abnormally low-temperature plasma. Specifically, the solid area indicates the difference between the observed is more than a linear superposition of its parts" [Burlaga et al., 1987], detailed studies of the interplanetary interactions between solar wind flows are required if reliable predictions of these geomagnetic storms are to be achieved. We also note that Bothmer and Schwenn [1995] discussed a similar example of a strong geomagnetic storm (April 1, 1973) caused by the amplification of the southward field in an ejecta by interaction with a CIR. The solar wind and energetic particle observations for this event are similar to those in Figure 1, including a -4% decrease in the counting rate of the anticoincidence guard of the GSFC instrument on IMP 7 inside the ejecta. Although it is clear that the shocks on November were formed at the leading edge of a corotating high-speed stream, proton temperature (Tp, solid line) and the temperature expected other authors have searched for a solar event origin. Tsurutani for normal solar wind expansion (Tcx, dotted line) [Richardson et al. [1988] first considered the shocks to be CIR-associated, and Cane, 1995] when Tp<0.5 T x. This region has been previ- but in their subsequent paper [Tang et al., 1989], the same ously identified as an ejecta based on the presence of bidirec- authors decided that the shocks of November were tional solar wind electron heat fluxes [Gosling et al., 1987; Kahler et al., 1996], and this conclusion is consistent with the low proton temperatures which are typical of ejecta [Richardson and Cane, 1995]. Further evidence for an ejecta is provided by the -6% depression in the counting rate of the anticoincidence guard of the GSFC instrument on IMP 8 (shown in the seventh graph) which indicates the density of-2 GV cosmic rays. Such short-term cosmic ray depressions provide a robust signature of ejecta [Cane et al., 1997]. The Dst index indicates that the strongest part of the geomagnetic storm followed passage of the forward shock of the caused by a prominence eruption near N35øW55 ø and long duration flare at N40øW65 ø commencing before 1955 UT on November 20. Joselyn and Mclntosh [ 1981 ] proposed the same solar event as being one of three possible prominenceruptions associated with the geomagnetic storm. This association was also assumed in a recent paper [Gonzalez et al., 1996] which concluded that the solar origins of large geomagnetic storms (including this event) are associated with active regions occurring close to the streamer belt and growing low-latitude coronal holes. We note that there is a major flaw in the argument against corotating shocks advanced by Tang et al. [1989]. They CIR when the magnetic field had a large southward component associated the high-speed stream with the near-equatorial (0<<0ø). We note that the magnetic field was already southward coronal hole with central meridian passage on November in the ejecta preceding the shock and that the geomagnetic in Figure 2 and then claimed that the -7 day propagation delay activity which culminated in the major storm actually commenced around the time of passage of the front boundary of the ejecta. The interval of southward field evidently extended into the region behind the forward shock and (as also noted by Tsurutani et al. [1988] and Zhao [1992]) terminated at the stream interface. A stream interface typically separates plasma in the slow solar wind from the high-speed coronal hole plasma. Thus we suggest that the high-speed stream is interacting with the slower ejecta, compressing and heating the trailing edge of the from the Sun (implying a propagation speed of 250 km/s) was too long based on the observed solar wind speed in the high-speed stream (- km/s). (However, this coronal hole was associated with the separate corotating stream in Figure 2 discussed above.) Also, the coronal hole we have associated with the shocks of November and subsequent corotating stream was not evident on the previous solar rotation, explaining the absence of any related corotating structure during earlier solar rotations noted by Tang et al. [1989]. The error in the ejecta and leading to the tbrmation of a CIR. The strong south- coronal hole association was also pointed out by Pdward magnetic field between the forward shock and the interface, associated with high geomagnetic activity levels, then arises from compression of the southward magnetic field of the rez-enriquez and Mendoza [1990], though, for the reasons discussed above, we do not agree with their conclusion that the coronal hole alone was responsible for generating the geomagejecta by interaction with the corotating high-speed stream. netic activity. Pdrez-Enriquez and Mendoza [1990] based their Zhao [1992] reached the same conclusion and suggested that this is a previously unrecognized type of interaction leading to shock formation. He suggested that the unusual conditions in the ejecta and the interaction with the high-speed stream might be the reason for the unusually small heliocentric distance at which the CIR shocks formed. Thus the strong geomagnetic storm is not simply caused by the high-speed stream but also by the presence of an ejecta with southward field which is intensified because of the interaction with a high-speed stream. This interpretation is consistent with the conclusion of Tsurutani et al. [ 1988] that "the field responconclusions on the work of Hewish and Bravo [1986], who associated high-speed streams detected via the interplanetary scattering technique with coronal holes. In response to Pdrez-Enriquez and Mendoza [1990], Tang and Tsurutani [1990] objected that the proposed responsible coronal hole was too far from the ecliptic to have produced a corotating stream. However, on the basis of the association inferred from Figure 2, it appears that the coronal hole flow did extend in latitude to the ecliptic. In addition, we note that other studies [Sheeley et al., 1976; Burlaga et al., 1978] have shown that corotating streams extend in longitude and latitude beyond sible for the magnetic storm is simply shocked interplanetary coronal hole boundaries and that coronal holes located up to magnetic field", though they did not recognize that the shocked -40 ø latitude can give rise to streams observed near the ecliptic. plasma was associated with an ejecta. Thus the November 1978 event is an example of a "compound" stream, consisting in this event of an interacting ejecta and corotating stream, which Burlaga et al. [1987] have pointed out are important for the Bravo and Rivera [ 1994] have suggested that the November 25 shock arose from a sudden increase in the speed of the flow from a coronal hole caused by an increase in the size of the coronal hole resulting from a nearby filament disappearance.

4 17,448 CANE AND RICHARDSON: NOVEMBER 1978 GEOMAGNETIC STORM However, from Figure 2, the coronal hole they discuss (at cmp November 18-19, S30 ø in Figure 2) is not the source of the high-speed stream following the November 25 shock. Also, the multispacecraft observations in Figure 2 suggesthat the peak flow speeds in the stream were similar over a period of at least 9 days before the shocks were observed, suggesting that the shocks were not formed by a sudden eruption of the coronal hole flow. Although it is not possible to associate the November 25 shock with the solar event on November 20, we examine instead whether the ejecta we have identified as a major contributor to the generation of the geomagnetic storm might have originated in this solar event. We believe this is extremely unlikely based on energetic particle observations (from the GSFC instrument on IMP 8) which show that the solar event of November 20 was responsible for a particle enhancement observable to about 50 MeV. The particle energies attained imply that this was a relatively energetic solar event. Such an event is likely to have been associated with a fast ejecta and shock, not with the slow ejecta (speed of-420 km/s) observed at Earth on November 24. The bottom graph of Figure I shows the MeV proton intensity-time profile. The particle event commences promptly at about 2100 UT on November 20 (a data gap precludes precise timing). Note that there is no evidence of a shock at Earth observed during the duration of the November particle event. This is not unexpected since a shock must be very energetic to reach the Earth from an event as far west as W65 ø [Cane, 1988]. If a shock is observed from a western event, it tends to pass by just after the start of the decay phase of the associated particle event [Cane et al., 1988]. A further reason for rejecting an association between the ejecta at Earth on November 24 and the prominenceruption/flare of November 20 is that we have previously shown that ejecta are detected at Earth only when the solar event occurs within 50 ø of central meridian [Richardson and Cane, 1993; Cane et al., 1996a]. This conclusion is for major flareassociated events. Less energetic events are likely to have less extended ejecta and hence would be expected to originat even closer to central meridian if the ejecta is to be detected at Earth events at the Sun to consider all available evidence, in particular, that provided by energetic particles and multispacecraft observations. In the case discussed here, we argue that the shocks of November 25-26, 1978 and the major geomagnetic storm on November 25 were not related to the prominence eruption/flare of November 20 as proposed by several studies or with a sudden increase in the flow speed from a coronal hole. Rather, the cause for the storm is an enhancement of southward magnetic field associated with an ejecta caused by a corotating stream compressing the ejecta. The solar origin of the ejecta cannot be determined, but on the basis of other work [e.g., Cane et al., 1986; Sanahuja et al., 1991 ] it probably originated within about 30 ø of central meridian. However, the geomagnetic storm results from an interplanetary interaction, and so its strength could not have been predicted based on the solar origin of the ejecta, even if it could be established. We finally note that interplanetary interactions, besides being important for producing geomagnetic effects, may also be a source of radio emission in the interplanetary medium. During several days prior to the arrival of the ejecta and CIR, the ISEE- 3 radio astronomy experiment detected narrow banded, fast drift radio emission [Cane et al., 1982] similar to that detected by radio experiments on WIND and Ulysses in January 1997 [Bougeret et al., 1997] also in association with an ejecta and CIR. Acknowledgments. We acknowledge the use of data from the tape of Helios merged hourly averaged plasma data (principal investigator, H. Rosenbauer) and from the OMNI database, both provided by the National Space Science Data Center. I. G. R. was supported by NASA grant , and H. V. C. was supported at GSFC by a contract with the Universities Space Research Association. The referees are acknowledged for useful suggestions. The Editor thanks N. U. Crooker and another referee for their assistance in evaluating this paper. References Barnes, C. W., and J. A. Simpson, Evidence for interplanetary acceleration of nucleons in corotating interaction regions, Astrophys. J. 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F., Interplanetary stream interfaces, J. Geophys. Res., 79, 3717, mass ejections, the counterparts of ejecta near the Sun, have no Burlaga, L. F., K. W. Behannon, S. F. Hansen, G. W. Pneuman, and W. good associations with "traditional" indicators of solar activity C. Feldman, Sources of magnetic fields in recurrent interplanetary such as He and X ray flares and radio emissions. This point streams, J. Geophys. Res., 83, 4177,! 978. should be emphasized since it implies that one should not ex- Burlaga, L. F., K. W. Behannon, and L. W. Klein, Compound streams, pect to be able to associate every ejecta or geomagnetic storm magneti clouds, and major geomagnetic storms, J. Geophys. Res., 92, 5725, with an event identifiable with these signatures. However, the Cane, H. V., The large-scale structure of flare-associated interplanetary situation may improve in the light of recent Yohkoh observa- shocks, J. Geophys. Res. 93, 1, tions [Hudson et al., 1996] which suggest that soft X ray imag- Cane, H. V., R. G. Stone, J. Fainberg, J.-L. Steinberg, and S. Hoang, ing may provide a means of detecting coronal mass loss and the Type li Solar Radio Events Observed in the Interplanetary Medium: origins of all coronal mass ejections. In summary, we conclude Part I - General Characteristics, Sol. Phys. 78, 187, Cane, H. V., S. W. Kahler, and N. R. Sheeley Jr., Interplanetary shocks that the source region for the eject associated with large No- preceded by solar filament eruptions, J. Geophys. Res., 91, 13,321, vember 1978 geomagnetic storm remains unknown Cane, H. V., D. V. Reames, and T. T. yon Rosenvinge, The role of interplanetary shocks in the longitude distribution of solar energetic Conclusions particle events, J. Geophys. Res., 93, 9555, Cane, H. V., K. Harvey, and I. G. 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