Classification and mean behavior of magnetic storms

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 12, NO. A7, PAGES 14,29-14,213, JULY 1, 1997 Classification and mean behavior of magnetic storms C. A. Loewe and G. W. Pr61ss Institut far Astrophysik und Extraterrestrische Forschung, Universit it Bonn, Bonn, Germany Abstract. The D st index is used to identify more than 1 storms which occurred in the time interval 1957 to Using the minimum Dst value as an indicator, we classify the storms as weak (482), moderate (346), strong (26), severe (45), and great (6). For each of these classes the mean time variation is determined. In contrast to the well-known study of Sugiura and Chapman [196], the D st minimum is used as a common reference epoch. This leads to much better agreement between the average and the typical storm behavior We also find that the maximum ap and AE activity precedes the D st minimum by 1 to 2 hours. Finally, we demonstrate that both sudden commencement and gradual commencement storms are associated with a distinct decrease in the B z component of the interplanetary magnetic field. Introduction Storm Classification Large and persistent perturbations of the Earth's magnetic field were first observed by Graham (1722) and have become known as magnetic storms ("magnetische Ungewitter," Humboldt, 1845). These disturbance events remain one of the most challenging topics of space physics, not only because they have a profound influence on the global morphology of the magnetic field but also because they constitute an important link in the complex chain of solar-terrestrial relations, their energy being derived from the solar wind. In addition, this subject is of practical interest since power and communication systems and pipe lines may be severely affected by such perturbations [e.g.,allen et al., 1989; Boteler, 1991]. An important morphological aspect of magnetic storms is their mean D st variation. One of the early, and certainly one of the best known, attempts to establish this average behavior was made by Sugiura and Chapman [196]. Their results have been reproduced in many review articles [e.g., Piddington, 1964, 1967; Cole, 1966; Kamide, 1979; Kamide and Joselyn, 1991] and books [e.g., Chapman and Bartels, 1962; Hines et al., 1965; Rishbeth and Garriott, 1969; Akasofu and Chapman, 1972; Volland, 1984; Hargreaves, 1992] and are shown here in Figure 1. However, when comparing these average Dst curves with individual Dst storms, a number of discrepancies become evident. For example, a magnetic disturbance with a minimum Dst index of -9 nt hardly qualifies as a "great" magnetic storm. Also, the Dst minimum is much wider than is usually observed. Finally, the recovery phase is much too long, especially for weak storms. Considering these deficiencies, it is worthwhile to make a new attempt to establish the average Dst behavior of magnetic storms. Copyfight 1997 by the American Geophysical Union. Paper number 96JA /97/96JA As discussed in the recent literature, it is by no means easy to give a concise definition of what exactly constitutes a magnetic storm [e.g., Akasofu, 197; Kamide, 1979; Feldstein et al., 199; Gonzalez et al., 1994; Tsurutani et al., 1995]. However, for our purposes it is sufficiento assume that one of the principal defining properties of a magnetic storm is the creation of an enhanced ring current which leads to a significant depression of the Dst index. Here we consider a depression as "significant" when it exceeds -3 nt. To qualify as a magnetic storm in our selection procedure we also demand that this depression have a reasonably welldefined onset, minimum, and recovery phase. This implies that we are chiefly interested in isolated disturbances, although in the case of large perturbations more complex variations are also considered, mainly to improve the statistics of these events. Using these (admittedly somewhat "soft") selection criteria, we compiled a list of magnetic storms which is based on the Dst indices published for the years 1957 to 1993 [e.g., Sugiura and Kamei, 1991]. Altogether 185 storms were identified and included in our data set. These storms were selected by visual inspection, not by a computer algorithm. We are well aware that this procedure may introduce some personal biases. However, it is well known that the pattern recognition capability of the human brain is unsurpassed. To classify the intensifies of our storms, the minimum Dst index was used as a criterion. The number of storms with a certain minimum Dst value is shown in Figure 2 with 1 nt resolution. Guided by this distribution and extending previously suggested classification schemes [e.g., Gonzalez et al., 1994], we distinguish between weak (<_-3 nt), moderate (<_ -5 nt), strong (<_ - 1 nt), severe (<_ -2 nt), and great (<_ -35 nt) magnetic storms, see Table 1. As can be seen, "great" is used here in the sense of "exceptional." According to our data set, 3 strong and 2 severe storms are observed on average per year. Great storms occur once every 14,29

2 14,21 LOEWE AND PROLSS' CLASSIFICATION AND MEAN BEHAVIOR OF MAGNETIC STORMS zo- ¾ WEAK STORMS I I, I, I storm I, time I o Ds t GREAT STORMS slorm time 12 2/. 36 / NODERA TE STORkfS Y ' I t I, I t t!,, o storm time io ' I -&O -6-2O Figure 1. The storm time variation of D st (H) during the first three days of weak, moderate, and great magnetic storms for mean dipole latitude 3 ø [after Sugiura and Chapman, 196]. 6 years. There are time intervals, however, when no severe recovery proceeds on a reasonable timescale. Also note that storms, let alone great storms, are observed for years (e.g., all curves, although well separated, exhibit very similar be ). havior. The only feature which is not reproduced is the initial phase of sudden commencement storms. Since in the Average Dst Behavior and Associated ap present study this feature is not considered an essential part and AE Variations of a magnetic storm, this deficiency is acceptable (also see the Discussion and Conclusion section). To see how the other commonly used indices ap and AE In order to determine the average storm behavior, D st vary during our storm events, they were superimposed and variations of all events within a storm class were superimaveraged in the same way as the Dst indices, again with the posed. This was done using the time of the D st minimum time of the D st minimum used as the common reference as a common reference epoch. Subsequently, the medians of epoch. As a representativexample, Figure 4 shows the rethese time series were formed and are shown in Figure 3 for sults obtained for the class of strong magnetic storms. As is weak, moderate, strong, and severe storms. Obviously, these evident, both ap and AE indices exhibit a strong and wellcurves differ from those derived by Sugiura and Chapman. defined increase in activity. It is also clear that in both cases The main-phase decrease is sufficiently steep, the minimum the maxima of these increases occur prior to the D st miniwell-defined and, of course, of the right magnitude, and the mum. When all storm classes are considered, time lags of 1 to 2 hours are obtained. To illustrate the typical spread of the 36 values around the median, Figure 4 also includes the upper and lower quartiles of the average data sets. 32 Number of storms / 1 nt interval! weak l 28 Discussion and Conclusions great severe moderate Irong Dst minima [nt] Figure 2. Storm frequency as a function of the D st minimum and storm classification. Why are the results obtained by Sugiura and Chapman [196] so different from those derived in the present study? This has no doubt to do with the different reference times used in both studies. Sugiura and Chapman considered the storm sudden commencement (SSC) as an important, integral part of a magnetic storm. Therefore they chose the time of the SSC as a common reference epoch in their superposition procedure. According to a recent definition, however, a SSC is an impulse-like disturbance of the magnetic field (sudden impulse or SI) which is followed by a magnetic storm within 24 hours [e.g., Joselyn and Tsurutani, 199; Kamide and Joselyn, 1991]. This definition implies that there is no fixed time interval between the SSC and the

3 _ LOEWE AND PROLSS: CLASSIFICATION AND MEAN BEHAVIOR OF MAGNETIC STORMS 14,211 Table 1. Storm classification Storm class Fraction Dstmin-Range Dstmin Ig max Kpmax AEmax weak % -3 to -5 fir -36 fir fir moderate % -5 to -1 fir -68 fir fir strong % -1 to -2 fir -!31 fir 111 7_ 849 fir severe 45 4 % -2 to -3õ fir -2õ4 fir fir great 6 1% < -35 fir -427 fir 3 9_ 1335 fir The bars above the magnetic indices indicate median values. onset of the magnetic storm main phase. Accordingly, the Dst minimum may occur a few hours after the SSC or up to this finding would make the initial phase an essential part of a magnetic storm, we checked on their results using our own 24 hours after the SSC, and any averaging procedure based data set, The solar wind pressure and the Bz component of on the ssc as a common reference epoch will necessarily the IMF were superimposed and averaged in the same way smear out these Dst minima. To check on this conjecture, we have attempted to recover the results of Sugiura and Chapman using our own as the Dst indices, again with the time of the Dst minimum used as the common referenc epoch. The resul ts obtained for moderate storms (-5 nt ) Dstmin > - 1 nt) are data set. With the help of a list of SSCs compiled by Mayaud shown in Figure 6.. As is evident, both SC and GC storms ("Geomagnetic storm sudden commencements (SSC) present (NGDC)"), 469 (or 43 %) of our 185 storms could be identified as SSC storms. This subset was divided into 16 weak (-3 nt > Dstmin > -- 5 nt), moderate (-5 nt > Dstmin > -1nT), and "great"(-1nt > Dstmin) 12 storms and arithmetically averaged using the SSC as a common referenc epoch. The results are shown in Figure 5 8O together with the average storm variations derived by Sugiura and Chapman. Considering that the two sets of curves are based on different data sets and different stor TM biassift- 4O cation schemes, the agreement between them is surprisingly, I I, I I.I, good; especially with respect to the shallow and ill-define d -2 -! o depression minima. This supports our claim thathe choice 12 of referencepoch is crucial for the results obtaine& ' ' '/]!,...' ' ' '"' ''A ' ß Recently, Taylor et al. [1994] have suggested that sud- 8 den commencement (SC) and gradual commencement (GC) storms have different origins. They found that SC storms 6 of moderate intensity (-5 nt ) Dstmin > -1 nt for ) 4 hours) e controlled by the solar wind pressure, whereas 4 GC storms of the same intensity are correlated with the Bz 2 compo nent of the interplanetary magnetic field (IMF). Since !, i i 7',. i... ß... :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: '.,... i/'...!, i J i/!] Figure 3. Mean Dst storm variations. weak... moderate... strong......, I, i, I, I., I, I, !i ' ' "-.' -8o il/"... ' ,,,,,,,,,,,,, Figure 4. Comparison of the mean variations of the ap, AE, and D st indices during strong magnetic storms. The time of the Dst minimum serves as a common reference epoch. The spread of the data is indicated by the upper and lower quartiles.

4 ... ß 2,,, 2 nt.....,... Weak t nt ß Great, 2 nt Stir time [ ys -4 Moderate -6-8 Dst ! 2 3 i i i Figure 5. Comparison between the mean storm time variation of the D st as derived by Sugiura and Chapman [ 196] (solid lines) and our attempt to reproduce their results using the SSC as a common reference epoch (dashed lines) ' 18 Moderate SC Storms ' 115 Moderate GC Storms 4 4 = 2 1 ß œ:.:....'.'.z'2... i I,... /¾.%/. /'""'"V\.,.."'/ ß i.. ""-' \;.,...; /' '. i..':'/... d'--.',.\,,/'"/"'"'... P"'" i i!,../'",. œ'./ Figure 6. Comparison of the mean variations of the Bz component of the interplanetary magnetic field (GSM coordinates), the solar wind kinetic pressure, Psw, and the D st index during moderate magnetic storms (-5 nt _> Dstmin > -1 nt). Only storms for which interplanetary data were available have been considered. (left) SC storms, (right) GC storms. The spread of the data is indicated by the upper and lower quartiles. 14,212

5 LOEWE AND PROLSS: CLASSIFICATION AND MEAN BEHAVIOR OF MAGNETIC STORMS 14,213 are associated with the same distinct decrease in the Bz com- Boteler, D. H., Predictingeomagnetic disturbances on power sysponent of the interplanetary field. tems, Eos Trans. AGU, 72(14), , Chapman, S., and J. Bartels, Geomagnetism, Oxford Univ. Press, Again, it is the different reference times used in the study New York, of Taylor et al. [1994] and in our own study that appear to Cole, K. D., Magnetic storms and associated phenomena, Space be responsible for the divergent results. Thus studying the Sci. Rev., 5, , average behavior of SC storms, Taylor et al. use the SSC as Feldstein, Y. I., A. Grafe, V. Y. Pisarsky, A. Prigansova, and P. V. a common referenc epoch. Accordingly, the initial phase of Sumaruk, Magnetic field of the magnetospheric ring current and its dynamics during magnetic storms, J. Atmos. Terr. Phys., 52, these storms is well described (as was the case in the Sug , 199. iura and Chapman study) but the main phase is smeared out. Gonzalez, W. D., A. L. Clfia de Gonzalez, O. Mendes Jr., and B. T. In fact, the D st curve shown in Figure 2j of Taylor et al. Tsurutani, Difficulties defining storm sudden commencements, looks very similar to the curves for moderate storms shown Eos Trans. AGU, 73(16), , in Figure 5. Since the initial phase of a storm is presum- Gonzalez, W. D., J. A. Joselyn, Y. Kamide, H. W. Kroehl, G. Rostoker, B. T. Tsurutani, and V. M. Vasyliunas, What is a geomagably produced by an increase in the solar wind pressure, it netic storm?, J. Geophys. Res., 99, , comes as no surprise that this correlation is well reproduced Hargreaves, J. K., The Solar-Terrestrial Environment, Cambridge in the data set presented by Taylor et al. In contrast, the Bz Univ. Press, New York, decrease associated with the storm main phase evidently is Hines, C. O., I. Paghis, T. R. Hartz, and J. A. Fejer, Physics of the smeared out to such an extent that it appears to be unimpor- Earth's Upper Atmosphere, Prentice-Hall, Englewood Cliffs, N.J., tant as a controlling parameter. Since the suggestion of Tay- Joselyn, J. A., and B. T. Tsurutani, Geomagnetic sudden impulses lor et al. that SC storms are caused by coronal mass ejections and storm sudden commencements: A note on terminology, Eos and GC storms are caused by high-speed/low-speed interac- Trans. AGU, 71(47), , 199. tions is based not only on the B component but also on so- Kamide, Y., Relationship between substorms and storms, in Dylar wind parameters like density, velocity, and temperature, namics of the Magnetosphere, edited by S.-I. Akasofu, pp , D. Reidel, Norwell, Mass., it may still be correct. Kamide, Y., and J. A. Joselyn, Toward a standardizedefinition In conclusion, we believe that sudden impulses, identified of geomagnetic sudden impulses and storm sudden commenceas SSCs, are not an essential part of magnetic storms and ments, Eos Trans. AGU, 72(28), 3, this has been previously suggested, for example, by Akasofu McPherron, R. L., Magnetospheric dynamics, in Introduction to [197], Gonzalez et al. [1992], and McPherron [1995]. In Space Physics, edited by M. G. Kivelson, and C. T. Russell, pp , Cambridge Univ. Press, New York, support of this view we demonstrate that both GC and SC Piddington, J. H., Geomagnetic storms, auroras and associated efstorms are mainly controlled by the B component of the fects, Space Sci. Rev., 3, , nterplanetary magnetic field. Accordingly, when it comes Piddington, J. H., A hydromagnetic model of geomagnetic storms to describing the mean behavior of magnetic storms, the use and auroras, in Physics of Geomagnetic Phenomena, edited by of the SSC as a reference epoch should be discontinued. S. Matsushita and W. H. Campbell, pp , Academic, San Diego, Calif., Rishbeth, H., and O.K. Garriott, Introduction to Ionospheric Acknowledgments. We are very grateful to M. Sugiura and Physics, Academic, San Diego, Calif., T. Kamei for their helpful comments on the derivation of the D st Sugiura, M., and S. Chapman, The Average Morphology of Geoindex. We also would like to thank Y. Kamide for stimulating dis- magnetic Storms With Sudden Commencement, Abhandl. Akad. cussions and continuing encouragement. The magnetic indices and Wiss. GOttingen, Math.-Phys. KI., Sonderheft 4, GOttingen, the SSC data used in this study were provided by World Data Cen ter A in Boulder, ColOrado. The interplanetary data (solar wind Sugiura, M., and T. Kamei, Equatorial Dst index , and IMF) were obtained directly from NSSDC/WDC-A in Green- IAGA Bull., 4, belt, Maryland. Taylor, J. R., M. Lester, and T. K. Yeoman, A superposed epoch The Editor thanks W. Gonzalez and the othereferee for their analysis of geomagnetic storms, Ann. Geophys., 12, , assistance in evaluating this paper Tsurutani, B. T., W. D. Gonzalez, A. L. C. Gonzalez, E Tang, J. K. Arballo, and M. Okada, Interplanetary origin of geomagnetic ac- References tivity in the declining phase of the solor cycle, J. Geophys. Res., 1, , Akasofu, S.-I., Diagnostic of the magnetosphere using geomagnetic, auroral and airglow phenomena, Ann. Geophys., 26, , 197. Akasofu, S.-I., and S. Chapman, Solar-Terrestrial Physics, Oxford Univ. Press, New York, Allen, J., L. Frank, H. Sauer, and P. Reiff, Effects of the March 1989 solar activity, Eos Trans. AGU, 7(46), 1479, , Volland, H., Atmospheric Electrodynamics, Springer Verlag, New York, C. A. Loewe and G.W. PrOlss, Institut fiir Astrophysik und Ex- traterrestrische Forschung, Universit it Bonn, Auf dem Hagel 71, Bonn, Germany (Received May 1, 1996; revised November 11, 1996; accepted December 11, 1996.)