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Cosmic Research, Vol. 42, No. 5, 2004, pp. 435­488. Translated from Kosmicheskie Issledovaniya, Vol. 42, No. 5, 2004, pp. 453­508. Original Russian Text Copyright © 2004 by Veselovsky, Panasyuk, Avdyushin, Bazilevskaya, Belov, Bogachev, Bogod, Bogomolov, Bothmer, Boyarchuk, Vashenyuk, Vlasov, Gnezdilov, Gorgutsa, Grechnev, Denisov, Dmitriev, Dryer, Yermolaev, Eroshenko, Zherebtsov, Zhitnik, Zhukov, Zastenker, Zelenyi, Zeldovich, Ivanov-Kholodnyi, Ignat'ev, Ishkov, Kolomiytsev, Krasheninnikov, Kudela, Kuzhevsky, Kuzin, V. Kuznetsov, S. Kuznetsov, Kurt, Lazutin, Leshchenko, Litvak, Logachev, Lawrence, Markeev, Makhmutov, A. Mitrofanov, T. Mitrofanov, Morozov, Myagkova, Nusinov, Oparin, Panasenco, Pertsov, Petrukovich, Podorol'sky, Romashets, Svertilov, Svidsky, Svirzhevskaya, Svirzhevsky, Slemzin, Smith, . Sobel'man, Sobolev, Stozhkov, Suvorova, Sukhodrev, Tindo, Tokhchukova, Fomichev, Chashey, Chertok, Shishov, Yushkov, Yakovchouk, Yanke.

Solar and Heliospheric Phenomena in October­November 2003: Causes and Effects
I. S. Veselovsky1, M. I. Panasyuk1, S. I. Avdyushin2, G. A. Bazilevskaya3, A. V. Belov4, S. A. Bogachev3, V. M. Bogod5, A. V. Bogomolov1, V. Bothmer6, K. A. Boyarchuk4, E. V. Vashenyuk7, V. I. Vlasov8, A. A. Gnezdilov4, R. V. Gorgutsa4, V. V. Grechnev9, Yu. I. Denisov1, A. V. Dmitriev1, 10, M. Dryer11, Yu. I. Yermolaev12, E. A. Eroshenko4, G. A. Zherebtsov9, I. A. Zhitnik3, A. N. Zhukov1, 13, G. N. Zastenker12, L. M. Zelenyi12, M. A. Zeldovich1, G. S. Ivanov-Kholodnyi4, A. P. Ignat'ev3, V. N. Ishkov4, O. P. Kolomiytsev1, I. A. Krasheninnikov4, K. Kudela14, B. M. Kuzhevsky1, S. V. Kuzin3, V. D. Kuznetsov4, S. N. Kuznetsov1, V. G. Kurt1, L. L. Lazutin, L. N. Leshchenko4, M. L. Litvak12, Yu. I. Logachev1, G. Lawrence13, A. K. Markeev4, V. S. Makhmutov3, A. V. Mitrofanov3, I. G. Mitrofanov12, O. V. Morozov1, I. N. Myagkova1, A. A. Nusinov2, S. N. Oparin3, O. A. Panasenco1, A. A. Pertsov3, A. A. Petrukovich12, A. N. Podorol'sky1, E. P. Romashets4, S. I. Svertilov1, P. M. Svidsky2, A. K. Svirzhevskaya3, N. S. Svirzhevsky3, V. A. Slemzin3, Z. Smith11, I. I. Sobel'man3, D. E. Sobolev4, Yu. I. Stozhkov3, A. V. Suvorova1, N. K. Sukhodrev3, I. P. Tindo3, S. Kh. Tokhchukova15, V. V. Fomichev4, I. V. Chashey8, I. M. Chertok4, V. I. Shishov8, B. Yu. Yushkov1, O. S. Yakovchouk1, and V. G. Yanke4
Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, Russia 2 Fedorov Institute of Applied Geophysics, Moscow 3 Lebedev Physical Institute, Russian Academy of Sciences, Moscow, Russia 4 Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN), Troitsk, Moscow oblast, Russia 5 Special Astrophysical Observatory, Russian Academy of Sciences, Nizhni Arkhyz, Russia 6 Max-Planck Institut fur Sonnensystemforschung, Katlenburg-Lindau, Germany 7 Polar Geophysical Institute, Kola Science Center, Russian Academy of Sciences, Apatity, Russia 8 Pushchino Observatory, Astro Space Center, Lebedev Physical Institute, Russian Academy of Sciences, Pushchino, Russia 9 Institute of Solar-Terrestrial Physics, Siberian Branch of Russian Academy of Sciences, Irkutsk, Russia 10 Institute of Space Science, Jungli, Taiwan 11 Space Environment Center, Boulder, USA 12 Space Research Institute, Russian Academy of Sciences, Moscow, Russia 13 Observatoire Royal de Belgique, Bruxelles, Belgium 14 Institute of Experimental Physics of the Slovak Academy of Sciences, Ko¡ice, Slovakia 15 Main (Pulkovo) Astronomical Observatory, Russian Academy of Sciences, Pulkovo, Russia
Received May 19, 2004
1

Abstract--We present new observational data on the phenomena of extremely high activity on the Sun and in the heliosphere that took place in October­November 2003. A large variety of solar and heliospheric parameters give evidence that the interval under consideration is unique over the entire observation time. Based on these data, comparing them with similar situations in the past and using available theoretical concepts, we discuss possible cause-and-effect connections between the processes observed. The paper includes the first results and conclusions derived by the collaboration "Solar Extreme Events-2003" organized in Russia for detailed investigations of these events. As a result of our consideration, it is beyond question that the physical causes of solar and heliospheric phenomena in October­November 2003 are not exclusively local and do not belong only to the active regions and solar atmosphere above them. The energy reservoirs and driving forces of these processes have a more global nature. In general, they are hidden from an observer, since ultimately their sources lie in the subphotospheric layers of the Sun, where changes that are fast and difficult to predict can sometimes take place (and indeed they do). Solar flares can serve as sufficiently good tracers of these sudden changes and reconstructions on the Sun, although one can still find other diagnostic indicators among the parameters of magnetic fields, motions of matter, and emission characteristics.


Deceased. 0010-9525/04/4205-0435 © 2004 MAIK "Nauka / Interperiodica"


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1. INTRODUCTION The goal of this paper is to present and discuss new observational data about various phenomena of extremely high activity on the Sun and in the heliosphere that took place in October­November 2003. Comparing these data with the facts known earlier, one can find certain features of similarity and distinction with other similar periods in several preceding cycles of solar activity. This makes it possible to consider both well established regularities and hypotheses about the nature of these phenomena, as well as about their influence on the processes in the near-Earth space. Below, we present the information about the phenomena observed on the Sun and in the heliosphere at the declining phase of the 23rd solar cycle in October­ November 2003. During this period a quick evolution of the global and local activity occurred and was observed on the entire solar surface and above it. The most significant changes and powerful active regions were concentrated mainly on one side of the Sun which was most fully turned to the Earth on October 26 and November 18. Very strong coronal mass ejections (CME) were detected in October­November, as well as numerous solar flares including those with the highest X-ray importance. Complex plasma and magnetic disturbances were formed after that in the solar wind: some of them reached the Earth and had an effect on the magnetosphere, while others passed far from it. A series of CMEs detected on October 28­30 by the instruments onboard the space observatories SOHO and Coronas-F close to the central solar meridian near the active region no. 10486 was an immediate cause of interplanetary transients and shock waves that reached the Earth in 1­2 days carrying strong electric fields and currents with them. It is precisely this fact that resulted in the development of geomagnetic storms on October 29­31 in three stages. One solar rotation later, on November 20 another strong geomagnetic storm occurred, now a single storm. It was initiated by CME generated at the same regions of the Sun on November 18 and approached the Earth on November 20. Therefore, this geomagnetic disturbance should be classified simultaneously both as recurrent and as a sporadic disturbance. Other events on the Sun in the time interval under consideration turned out to be not so efficient in the sense of generation of geomagnetic storms. This occurred either because their sources on the Sun were located far from the central meridian or because of the geometry of heliospheric magnetic fields that had at this time predominantly positive north­south component. For example, the extremely strong eruptive phenomenon on November 4, with a CME, a record X-ray burst, and a huge post-eruptive arcade on the western limb generated only a relatively weak geomagnetic disturbance. However, acceleration of charged particles to very high energies and the strong electromagnetic and neutron emission accompanying it were reliably detected for this event and some others within this time

interval both on the ground and onboard various spacecraft. The introduction, discussion, and conclusions of this paper are written by I.S. Veselovsky. The final version of the full text was prepared by him with assistance of other coauthors. The idea of establishing a collaboration to study solar-terrestrial links during the considered strongly disturbed period and to write the joint papers belongs to M.I. Panasyuk, who also convened discussions and workshops on this topic. The contents of all chapters of this paper were prepared either individually by authors and groups of authors or jointly on the basis of materials presented by different institutes. It should be particularly emphasized that the researchers of the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN) made decisive contribution to sections 2, 7, and 14. The scientists from Special Astrophysical Observatory (Nizhni Novgorod) and Main Astronomical Observatory (Pulkovo) endowed basically to section 6. It is pertinent to note that in so large and diverse paper with many coauthors one can find some results, interpretation statements, and conclusions that are not shared by all collaborators and rather represent the point of view of some authors or groups of authors. We did not consider necessary to avoid such debatable issues, moreover, we tried to elaborate a common point of view as far as it was possible. Nevertheless, the first author is responsible for all possible mistakes and inexact formulations made in the text. 2. COMPARATIVE RETROSPECTIVE DESCRIPTION OF SOLAR ACTIVITY FOR THE PERIOD UNDER CONSIDERATION The events somewhat similar to those considered in this paper occurred also on the declining phase of the 20th solar activity cycle in August 1972. At that time four very strong solar flares (of importance 2B and 3B) occurred within a very short period (August 2, 4, and 7), and three strong interplanetary shock waves arrived at the Earth. These events were among the first detected since the space exploration era had began that were comparatively well documented. This was done using direct measurements in the interplanetary medium, including first of all the data of the satellites Prognoz and Prognoz-2, and of the HEOS-2 and Pioneer-9 spacecraft [1]. When the data set obtained in 1972 was analyzed, the emphasis was made on the determination of characteristics of shock waves: their shape, instantaneous and mean velocity, the character of deceleration, and the behavior of solar wind and IMF parameters on the fronts and behind them. Also, a very important achievement of this analysis was the fact that the energy (potential, kinetic, thermal, and magnetic) of each shock wave was estimated. This energy was released during the flare in the form of an ejection of a certain amount of matter into the interplanetary medium. For the flare of August 4, 1972 the record estimates were
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obtained: the energy flux was about 100 erg/cm2, ejected mass was 1017 g, and the released energy was about 1033 erg. Due to a certain malfunction of instruments onboard several spacecraft and some other limitations to make similar estimations for the events in October­November 2003 is more complicated task. The events of August 1972 were also multiple (the entire sequence of four strong flares covered an interval of three days), as the events in the end of October 2003. However, they were certainly much worse studied as far as observations of the Sun are concerned (at that time there were no observations of CMEs using the spacebased coronograph, and so on). The closest analogy can be pointed out between the flare at 06:21 UT on August 4, 1972 and the flare at 10:02 UT on October 28, 2003. In the first event a shock wave arrived at the Earth in 14.5 h, after which the velocity and density of the solar wind increased up to ~2000 km/s and ~30 cm­3, respectively. In the second event the shock wave reached the Earth in 19 h, and the velocity and density of the solar wind increased to 2100­2400 km/s and ~15 cm­3, respectively. Further study will allow one to make this analogy deeper as far as both the solar origin of the interplanetary disturbance and its geoeffectiveness are concerned. The solar activity in all its manifestations is subject to regular and irregular chaotic variations in quite large ranges of amplitudes, durations, and other characteristics that have revealed themselves some way in the time interval under analysis. This general rule does not exclude coronal mass ejections and flares, which represent with respect to each other not the cause and effect (sometimes, such an unjustified assumption is made), but rather two observable manifestations of a single dissipative process related to an increased transport of free energy from the interiors of the Sun outwards into its upper atmosphere and heliosphere (see, for example, a discussion and references in [2]). This free energy is redistributed in thermal, magnetic, kinetic, gravitational, and radiation forms, their relative fractions being changed from event to event depending on the situation determined by the boundary conditions and initial data. It is well known that the periods when flare and eruptive activity of the Sun sharply increases are observed, as a rule, during the years near the solar cycle maximum determined as usual by the sunspot number. Rather frequently, it is in this time that the largest concentration of strong flares and eruptions over the entire cycle is observed. In this case, one or two powerful and quickly developed active regions used to appear, being record in producing flares and CMEs, and providing for the most powerful energy release over the entire cycle. Sometimes, these periods coincide with the first years of decrease after the solar maximum. The well-known events of November 1960 in the 19th cycle, of August 1972 in the 20th cycle, and of June­July 1982 in the 21st cycle can serve as examples. This also took place
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in the current 23rd cycle. But one cannot say so far about some firmly established rules without exceptions (because of small statistic of observations and a strong instability of all cyclic manifestations on the Sun). For example, the strongest flares in the 22nd cycle occurred at the phase of maximum (March 1989 and June 1991), while no flares of X class occurred at the declining phase of the cycle after November 1992. These facts are well known, and they indicate to existence of rather strong and unstable quasi-biennial variations which lead to the non-monotone behavior and so-called Gnevyshev gap, i.e., to double-peak maxima of activity. Below, we present a somewhat more detailed comparative description of the solar flare events over the past few years in order to define the usual and specific conditions in this respect. For the sake of comparison of the flare productivity of active regions, it would be very useful to have long series of absolutely calibrated measurements of energy and power. Since they do not exist, sometimes the index XRI introduced by P. McIntosh is used, which is determined as the total power of all flares of classes X and M in the range of soft X-rays (1­12.5 keV) (see http://www.jps.gov.au/Main.php?CatID=8). The flares of class X are taken with the unity weight (for example, a flare X1.5 gives a value of 1.5), while the flares of class M make a contribution one order of magnitude less (for example, a flare M3.2 gives a value of 0.32). Throughout the entire time of observations of the Sun in this range (from 1970 until October 2003) the largest XRI index was obtained for two active regions of the 22nd cycle of solar activity: active regions AR6659 of June 1991 (> 86.5) and AR5395 of March 1989 (> 55.5). In the time interval of October­November 2003 considered here the XRI index was estimated as 5.73 for AR10484. It was within the limits from > 62.56 (taking the threshold of saturation into account) up to 73.06 (if importance is X28) for AR10486 and reached a value of 8.57 for AR10488. It should be emphasized that the instruments installed onboard various satellites of the GOES series in order to measure the flux of soft X-ray emission in the above-indicated range have different thresholds of saturation. In this case, the X-ray importance is determined for flares in a somewhat conventional manner, proportionally to the time during which the instrument was blocked. Until 1976 the threshold of instrument saturation corresponded to the X-ray importance X5.1; therefore, the famous flares on August 4 and 7, 1972 had formally the importance X > 5.1. Before the beginning of operation of the geosynchronous satellite GEOS-9 (April 1996) the threshold corresponded to the X-ray importance X12.5, and after that, in the 23rd solar cycle, the threshold has increased up to X17.5. Therefore, it would be more objective to characterize the X-ray importance of such flares with saturation not only by the threshold value of an instrument, but by the duration of instrument blocking as well. The extrapola-


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tion of the X-ray flux for very strong flares with long saturation is hardly justifiable. Nevertheless, it is quite probable that in accordance with such an estimate the X-ray flares on June 1 and 6, 1991 were the most intense throughout the entire time of observation. The time of instrument blocking was as long as 26 min, and for three more flares of the same active region it reached 17 min. This was well understood by the researchers who first obtained the data on these flares, and all of them were remained with X > 12.5 but specifying the time of blocking for the X-ray photometer [4]. Unfortunately, in the literature one cannot find such data for every strong flare, and in the majority of cases only the estimated X-ray importance is given for such flares. According to its characteristics, the current 23rd cycle of solar activity belongs to the cycles of moderate intensity. In the course of 7.5 years of its development only four flares with the X-ray importance X 10 have been observed, while only in June 1991 five such flares occurred. Three out of four flares took place in the period of the strongest concentration (October 19­ November 5, 2003) of flare activity of the current cycle, when at once three large flare-active groups of sunspots passed across the visible solar disk: one in the southern hemisphere (the group of sunspots no. 10486 with the largest area in the current cycle of solar activity) and two in the northern hemisphere (nos. 10484 and 10488). The flare-active period in October­November 2003 began with an appearance (from behind the eastern limb on October 17) and rapid development during the first days of a group of sunspots AR10484 (N05L353, the second rotation of AR10464). On the second day (October 19) the first flare of importance X1.1/1N occurred in this region. Until October 26 only moderate flares of importance not higher than M were observed here. After emergence of a new powerful magnetic flux on October 26 that increased the area of AR10484 up to 1700 millionth fractions of solar hemisphere (m.f.s.h.), two more very strong flares (X1.2/2N and M7.6/2N) occurred in this region, after which it went behind the western limb of the Sun in its full development (on October 29). Observations with a sufficiently high spatial and temporal resolution carried out by groundbased and space observatories Coronas-F (orbit height 507 ± 21 km, inclination 82.5°, period of rotation 94.5 min), SOHO, and TRACE, and movies produced on this basis allowed one to trace the development of active regions, brightness variations, and motions in the solar atmosphere at all levels from the photosphere up to the uppermost regions of the solar corona. Powerful CMEs accompanied these phenomena and propagated into the heliosphere. The consequences of some of these events in the sense of arrival at the Earth of solar wind streams and magnetic clouds with a field orientation necessary for generation of strong magnetic storms and accelerated particles turned out to be not always so

spectacular as for other similar events. This was the case for purely geometrical reasons, because of specific features in the field structure and its predominantly northern orientation at this time. However, in other cases the conditions were more favorable. Then, powerful fluxes of energetic particles and strong geomagnetic storms were observed on the Earth. The time behavior of the short-wave emission of the Sun in the period of maximum solar activity of October 2003 is presented in Fig. 1 for three spectral ranges: 175 ± 3 å, 8.42 å (the data of Coronas-F/SPIRIT), and 1­8 å (the data of GOES). The plots demonstrate a good similarity, though there are some significant distinctions. For example, the flares on October 22 and 26, detected as rather strong in the range of 175 å, are observed as much weaker in the harder spectral region. After the exit to the visible solar disk of the active region AR10486 (S16 L286), which developed into a large group of sunspots being still in the invisible side of the Sun, the flares X5.4/1B and X1.1/1N occurred in it already on October 23, and then a flare M7.6/1N occurred on October 24. The first observed emergence of a powerful magnetic flux occurred in this region on October 24­25, which increased the area of the sunspot group by 800 m.f.s.h. (Sp = 2200 m.f.s.h.). As a consequence, the flares X1.2/3B (on October 26), and M5.0/1F and M6.7/1F (on October 27) occurred. The emergence of the next new magnetic flux on October 27­28 increased the area of this sunspot group up to Sp =2610 m.f.s.h. (a record value in the current cycle) and allowed a flare X17.2/4B to occur on October 28. The flux of emission in the line 8.42 å of MgXII measured by SPIRIT comprised 1.3 â 10-2 erg cm­2 s­1. Taking into account the width of the spectral band (~5 â 10­3 å) for observation in this channel the spectral density of the flux exceeds the value obtained by GOES (1.7 erg cm­2 s­1) by almost an order of magnitude. Propagating heliospheric disturbances caused by preceding eruptive events, upon reaching the Earth, generated a magnetic storm on October 28. When the disturbance from the last CME arrived, this storm developed into a very strong (G5,|Dst | > 200 nT) storm, one of the strongest in the current magnetic cycle. Energetic solar protons were emitted during the same eruptive flare into the interplanetary space, and they caused there a solar proton event of class S4 with intensity 29500 p.f.u. (the number of protons with energies > 10 MeV on 1 cm2 per one second in one steradian). The next period of strong eruptive and flare energy release in this active region began on November 2 by a flare X8.3/2B and continued on November 4 by a flare which was in the current cycle the most intensive in the flux of soft X-rays. The importance of this flare was X > 17.5/3B ( = 12 min, calculated importance X28). These flares occurred near the western limb of the Sun. Therefore, they had no significant effect on the geomagnetic conditions, though solar proton events of S3
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1.0 â 105 5.0 â 104 Counts/s 0 4 â 106 3 â 106 2 â 106 1 â 106 0 2.0 â 10 1.5 â 10 I, W/m2 1.0 â 10 5.0 â 10
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Fig. 1. Time behavior of short-wave emission in the period of strong flares of October 2003 in the wavebands (a) 175 å ± 3 å (Coronas-F/SPIRIT), (b) 8.42 å (the line Mg XII, Coronas-F/SPIRIT), and (c) 1­8 å (GOES-12).

and S2 classes took place (with maximums on November 2 and 4, respectively). The active region AR10488 (N09 L292) was formed on October 27 in the central part of the solar disk. In spite of its rapid development, it generated only modest flares, and on November 3 two flares X2.7/2B and X3.9/2F occurred in it. In total, for 16 days in 3 active regions 16 strong flares occurred, 11 of them had X-ray importance X. The flare period in the middle of November was associated with the active region AR10501 (N05 L012, the next rotation of AR10484). It began on November 17 and continued for 94 h. In this period two strong flares and five flares of middle importance M occurred in the active region. The eruptive flare event of optical importance 2N on November 18 had the strongest impact on
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the near-Earth space. In the course of its development two X-ray bursts M3.2 and M3.9 with powerful CMEs took place. Reaching the Earth's vicinity the disturbances from these eruptive events generated one of the strongest magnetic storms (intensity G5) of the current solar cycle. When considering eruptive events and solar flares, one cannot lose sight of the following circumstance. The origination and development of these strongest processes of energy release in the solar atmosphere are in the final analysis related to arrival of free energy from the subphotosphere layers and its redistribution. Observations of pre-flare situations on the Sun unequivocally indicate that frequently used conceptions of eruptive and flare events as initiated exclusively


440 Jan. 1, 2003

VESELOVSKY et al. (a) March 17 March 2 Sept. 13 Sept. 28 Aug. 14 Aug. 29 May 16 May 31 June 15 June 30 Apr. 16 Feb. 15 Oct. 13 Nov. 27
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50 0 ­50 Dst ­ 100 ­ 150 ­ 200 ­ 250 March 17 (b) Sept. 13 Sept. 28 Aug. 14 Aug. 29 Nov. 12 Dec. 12 May 16 May 31 June 15 June 30 Apr. 16 Oct. 13 Oct. 28 July 15 July 30 May 1 Apr. 1 Dec. 27, 2003
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250

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Fig. 2. Mean daily values of the Dst index (a) and Ap index (b) of geomagnetic activity in 2003.

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by some instability and release of free energy pre-accumulated in the solar corona are insufficient. Not denying the complexity and importance of consideration of these coronal processes, one should take into account that the strongest CMEs and solar flares often accompany each other and without exceptions take place because of and after the appearance of quite

specific changes in plasma and magnetic fields at the photospheric level. This unequivocally testifies an important role of energy "signals from below." In this case the arrival of additional free energy from interior of the Sun plays a key physical role. It is quite understandable that for the most powerful phenomena both plasma and electromagnetic channels of energy release turn out to be open to one extent or another: we see both
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Fig. 3. The sequence of strong disturbances on the Sun and on the Earth in October (three upper panels) and November (three lower panels) 2003. From top to bottom in three panels are presented the Kp and Dst indices of geomagnetic activity, and X-ray emission measured by GOES-12 (letters with figures designate the class of flares). The X-ray importance and coordinates on the sphere are given for some flares. COSMIC RESEARCH Vol. 42 No. 5 2004


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Table 1. The most significant and geoeffective solar flares and phenomena associated with them in the near-Earth space Date Oct. 19 Oct. 22 Oct. 23 Oct. 23 Oct. 24 Oct. 26 Oct. 26 Oct. 26 Oct. 27 Oct. 27 Oct. 28 Oct. 29 Nov. 2 Nov. 3 Nov. 3 Nov. 4 Nov. 5 Nov. 18 Nov. 20 Nov. 20 Onset, UT 16:29 19:45 08:17 19:50 02:22 05:17 17:17 21:26 09:21 12:27 09:51 20:37 17:03 01:06 09:43 19:29 10:46 07:16 07:35 23:42 Duration, min 79 > 41 64 38 66 213 179 60 23 37 >269 136 171 91 >36 80 >12 159 61 .16 Coordinates N06 S20 S21 S17 S19 S17 N02 N01 S16 S17 S16 S15 S14 N10 N08 S19 S16 N00 N01 N00 E58 E90 E88 E84 E72 E38 W38 W38 E26 E25 E08 W02 W56 W83 W77 W83 W90 E18 W08 W17 Importance X1.1/1N M9.9 X5.4/1B X1.1/1N M7.6/1N X1.2/3B X1.2/1N M7.6/2N M5.0/1F M6.7/1F X17.2/4B X10.0/2B X8.3/2B X2.7/2B X3.9/2F X28/3B M5.3/SF 2N/M3.2/M3.9 M9.6/2B M5.8/2N IRB R3 R2 R3 R3 R2 R3 R3 R2 R2 R2 R5 R4 R3 R3 R3 R5 R2 R1 R2 R2 Pr pfu IPr Magnetic storms Ims

323 466

S2 S2

W Oct. 28 W Oct. 28

G1 G1

29 500 330 1540

S4 S2 S3

VS Oct. 29­30 VS Oct. 31 W Nov. 4

G5 G4 G2

353

S2 VS Nov. 20­21 Nov. 22 G5 G1

10

S1

mass ejection and bright flare. The sensitivity and thresholds of their detection by corresponding instruments are such that CMEs usually look as longer events, especially if we turn our attention to images in the field of view of the LASCO/C3 coronograph within 30 solar radii. As for the flares which sometimes are seen in white light and in harder chromosphere and coronal emissions, as a rule, they look as shorter phenomena, though a somewhat subjective partition into impulsive (minutes) and long (hours) events is also used for them. Impulsive flares are more compact in space and take place usually at lower altitudes. Therefore, they are frequently not accompanied by significant mass ejections into the interplanetary space, and all motions are basically held in the gravity field of the Sun. Long events contain more energy within themselves. They are larger in occupied volume, area, and height which facilitates to overcome the gravity forces and to eject mass from the Sun. We emphasize once more that the phenomena of flares and CMEs to some extent accompany each other, but there is no physical cause-and-effect relations between them. When they are considered, it is usually difficult (and sometimes even impossible) to isolate and localize in space and time the most substantial details and to establish genetic relationships between them. This situation is generally typical for nonisolated and open physical systems which are crossed by large fluxes of energy,

momentum, and mass, as is the case for the solar atmosphere at all its altitudes. Characteristics of the most interesting flares and their geoeffectiveness are presented in Table 1. Here, speaking about the "geoeffectiveness of flares" we follow the prevalent tradition. In general, this term is true for the strongest geomagnetic storms, as was explained above. Of course, one should keenly aware that geomagnetic storms are by no means caused by optical phenomena. They result from arrival to the Earth of plasma with magnetic fields of appropriate strength and orientation, while an optical phenomenon is but a good diagnostic indicator of strong events. So called "problem storms" when rather strong geomagnetic disturbances are sometimes observed without detect