1. Impulsive flare of March 16, 1999

1. Fig. 1. 1999/03/16: images in soft X-rays (background), hard X-rays, and microwaves Fig. 5. 1999/02/16: time profiles of three microwave sources (I, 17 GHz) shown in Fig. 4. Italic digits denote time intervals Dashed contours: 17 GHz I difference Color: Yohkoh/SXT Fig. 3. 1999/03/16: Microwave time profiles along two points labeled 1 & 2 in Fig. 2 Blue bars: loops at work Fig. 2. Enlarged microwave images Fig. 4. 1999/02/16: arcade formation as a series of double-loop interactions. Italic digits denote time interval from Fig. 5. Digits 1-7 label microwave footpoint sources. Green inset shows Stokes V & I (17 GHz) in the first interval Fig. 6. From Aschwanden 1999b + arrows and patches that show possible ‘above-the-looptop sources’ due to optical effect A Study of Accelerated Electrons in Solar Flares from Nobeyama, Yohkoh, and Other ObservationsV.V.Grechnev1,2, M.R. Kundu2, A. Nindos3,21Institute of Solar Terrestrial Physics, Irkutsk, Russia; 2University of Maryland, USA 3University of Ioannina, Greece The results presented here have been possible due to the usage of microwave imaging data obtained with the solar dedicated Nobeyama Radio Heliograph operating without interruption for over a decade Abstract.We study manifestations of accelerated electrons in solar flare microwave and hard X-ray emissions. To make our points, we discuss two events—those of March 16 and February 16, 1999. The analysis of the first event leads to the conclusion that: 1) a seemingly single-loop configuration can be actually a double-loop one, and 2) pitch-angle distribution can be beamlike, with practically no non-zero pitch angles. The second event shows seemingly intersecting flaring loops in microwaves—considered as possible radio evidence for magnetic reconnection in flares; it also shows a post-eruptive arcade that can proceed as a series of double-loop interactions. From these and other published results, we conclude that: 1) different types of flares can proceed in closed double-loop configurations, 2) the acceleration site is localized between two or more interacting loops, i.e. close to a footpoint of one of them in impulsive flares, 3) pitch-angle distribution of accelerated electrons is anisotropic, with an excess of small angles, and 4) electron energies responsible for the microwave emission in impulsive flares are 50–200 keV. Introduction.Physical processes in solar flares have been studied over many decades, but many questions remain unanswered. Spatial configurations of flaring sources, sites and mechanisms of acceleration, characteristics of accelerated particles have not yet been well established. One of the reasons for this is that although manifestations of accelerated electrons are easily obtainable from various hard X-ray and radio emission signatures, such information on protons accelerated at the Sun is more difficult. There are reasons to believe that electrons and protons are accelerated simultaneously (Livshits & Belov). The relative rarity of proton events can be due to the difficulties for protons to escape from compact closed configurations typical of impulsive flares, as well as the heliolongitude of the flare site. Because of the deficiency of information on protons, we study accelerated electrons only. We first briefly consider two flare events that can shed some light on the questions mentioned above and then we will discuss known facts related to nonthermal manifestations in solar flares. 1. Impulsive flare of March 16, 1999 A short-duration weak flare (С2.2; Grechnev et al., in preparation) occurred in AR 8485 (N22W27) at about 02:50 UT. The event was observed with the Nobeyama Radioheliograph (NoRH, at 17 & 34 GHz), and with the soft (SXT) and hard X-ray (HXT) telescopes on board Yohkoh. Fig. 1 shows microwave sources (enlarged in Fig. 2) at 17 GHz (Stokes I and V) and 34 GHz (Stokes I) as well as hard X-ray sources superimposed over soft X-ray images. Fig. 3 shows time profiles along two points marked in Fig. 2 (brightness centers of oppositely polarized 17 GHz sources). In the impulsive phase, the soft and hard X-ray sources practically coincide. Softer emission (HXT/L, 13–23 keV) delineates a compact loop, while harder emission (HXT/M1, 23–33 keV) originates in its footpoints. By modeling the NoRH response using its beam pattern, we have found that the real separation of the oppositely polarized sources does not exceed 6; therefore, they are not resolved by X-ray telescopes. The microwave emission reveals a very compact loop, whose position angle differs from that of the loop visible in X-rays. Probably, this is the most compact double-loop configuration ever identified in NoRH data. There is no detectable microwave emission from the eastern X-ray source, although the magnetic field strengths are similar in both loop footpoints. The only possible explanation for this situation is that there is a beam-like anisotropy of electrons in the loop visible in X-rays, with practically no particles with non-zero pitch angles (Fleishman & Melnikov 2003). This extreme case suggests the possibility of beam-like anisotropy in other events as well. Thus we conclude that: 1) a seemingly single-loop configuration can be really a double-loop one, and 2) electron pitch-angle distribution can be strongly beam-like. 2. Long-duration post-eruptive flare of February 16, 1999 This event (M3.2/SF, NOAA 8458, S25W17, onset time 02:45 UT) was first presented by Kundu & Grechnev (2001) as a case of possible radio evidence for magnetic reconnection in solar flares, because of the existence of seemingly intersecting/interacting flaring loops in microwaves. In addition, it has been possible (HIDA observatory data along with microwave data) to follow the formation of the post-eruptive arcade. Microwave images (Fig. 4 ) and time profiles (Fig. 5) of the flare are complex, with several sources of different polarization, alternately brightening and fading in various parts of the flare configuration. Fig. 4 shows soft X-ray images as half tone background and Stokes I difference images at 17 GHz as contours. Different symbols denote oppositely polarized microwave sources with similar time profiles. By analyzing loop structures with enhanced brightness within certain time intervals and with similar time profiles of oppositely polarized sources at their ends, we were able to identify flaring loops at certain times (Grechnev 2003) shown schematically as blue bars. The figure show that the formation of the post-eruptive arcade proceeds mainly as a sequence of interactions between two or three loops. The presence of the third loops can be due to insufficient temporal resolution of the method or other technical problems. In the first frame, there is another very compact loop (green inset), detected in a similar way as in the event of March 16, 1999. 3. Discussion 3.1. Spatial configurations of Solar Flares. Currently, the most popular solar flare model is the so-called ‘standard’ flare model, sometimes referred to as ‘CSHKP’ (Kopp & Pneuman 1976; Shibata 1999). This model relates flares with open cusp-like configurations, where magnetic reconnection occurs between magnetic fluxes separated by a vertical current sheet. Several X-ray observations on Yohkoh have been interpreted in support of CSHKP. Yohkoh/SXT does, indeed, show cusp-like structures in some LDE flares (Tsuneta et al. 1992) with hotter outer parts. The energy release in these flares is believed to be due to magnetic reconnection in upper parts of the cusp-like structures. However, long intersecting closed loops are often present above some ‘cusps’, thus providing evidence of quadrupole closed configurations (Uchida et al. 1999). In some flares, above-the-loop top hard X-ray sources were found (Masuda et al. 1994). Tsuneta et al. (1997) believe that the energy release and acceleration site is located above SXR bright loops even in impulsive flares. Alternative models deal with interactions of pairs of loops in closed configurations (e.g., Katsova & Livshits 1988). Heyvaerts, Priest & Rust (1977) consider interaction of an emerging magnetic flux with a pre-existing one. Subsequent 3D modifications of this model consider interaction of closed low-lying loops with not necessarily antiparallel magnetic fields (Mandrini et al. 1996). In the model by Uchida (1980), reconnection occurs in the X-point of a closed quadrupole configuration, and the development of the process is prevented by a filament, which is subsequently ejected. The version of flare model by Longcope (1999) unifies the modifications mentioned above. The class of double-loop flares with remote sources was identified experimentally by Hanaoka (1996, 1997). Subsequent studies of 14 short-duration hard X-ray spectrum events also revealed in them pairs of interacting loops (Nishio et al. 1997). Similar configurations were found in other impulsive flares (Kundu et al. 2001b, Grechnev & Nakajima 2002, Kundu & Garaimov 2003). Grechnev & Nakajima (2002) studied an impulsive flare accompanied by manifestations ascribed to different flare configurations. They showed that the impulsive phase has the best correspondence with double-loop closed configuration, while manifestations of the cusp-like configuration either are of secondary importance, or appear due to an optical effect. Studies of simple short-duration microwave bursts (Kundu et al. 2001a) revealed in these events manifestations of a single loop. However, Kundu, Nindos & Grechnev (2004) showed that double-loop configurations can be responsible for such events also. The event of March 16, 1999 (Section 1) shows that the second closed loop can not be detectable due to unfavorable brightness or orientation. Finally, the event of February 16, 1999 (Section 2) shows that double-loop interactions can be responsible not only for short-duration impulsive flares, but also for long-duration post-eruptive flares. Time-of flight analyses of hard X-ray emissions of several X class flares showed that the sources of energetic electrons in the events were located above closed loops (Aschwanden et al. 1996). This result seems to support the CSHKP model. However, the authors subsequently explained it by the interactions of two closed loops, with the overlying one being not bright enough to be detected (Aschwanden 1999a, b). So, the source of accelerated electrons is probably localized in the interaction region between two closed loops in those flares also. This idea can be also applied for alternative interpretation of above-the-loop top nonthermal HXR sources (Masuda et al. 1994), which are sometimes observed on the limb above soft X-ray loops. In reality, such a source can be due to thin-target emission from the top part of a closed loop visible above the limb with a small angle to the line of sight (Fig. 6). When observed on the solar disk, such emission is not detectable against much stronger thick-target emissions from the loop footpoints, but they can be occulted when observed on the limb. Another origin of such sources is also implied by the results by Grechnev & Nakajima (2002). Nonthermal sources as well as thermal sources in the impulsive phase are always compact both in microwaves (Bastian 1999, Grechnev & Nakajima 2002), where their intensity strongly increases with the magnetic field, and in hard X-rays, strong magnetic fields prevent electron precipitation, and therefore the generation of HXR emission (Kundu et al. 1995). So, strong nonthermal manifestations are associated with sufficiently strong magnetic fields, and therefore are compact (cf. Asai et al. 2004). In summary, the double-loop configuration inherently turns out to be not only a separate class of flares, but a configuration responsible for most intense phenomena in the impulsive phase of different flares. Electrons are probably injected into a loop close to its footpoint in a weak impulsive flare, and close to its top in powerful, large-scale flares. Since no manifestations are observed of electron transport from the acceleration site to the injection site, it is reasonable to suppose that they are accelerated somewhere very close to the injection site, likely in the interaction region between two closed loops. 3.2. Pitch angle distribution and energies of accelerated electrons The trapping effect is present in many flare events (e.g., Holman, Kundu & Papadopoulos 1982, Melnikov & Magun 1998). The effect is observed even in simple short-duration events (Kundu et al. 2001a). Hence, electrons with large pitch angles are present in most cases. At the same time, the March 16, 1999 event shows that this is not always the case. Hanaoka (1999) also argues that gyrosynchrotron emission in double-loop flares is generated predominantly by electrons with small pitch angles. It would seem that in remote sources far from the injection site, the pitch-angle distribution becomes isotropic. However, Livshits & Tomozov (1974) showed that the lifetime of beams is comparable with the lifetime of a single electron due to nonlinear stabilization. From the analysis of the similarity of microwave and hard X-ray light curves for about 400 events, Kosugi, Dennis & Kai (1988) showed that for the microwave impulsive bursts at 17 GHz, electrons with energies < 200 keV in magnetic fields of order 1000 G are responsible for the emission, and for long-duration bursts, MeV-electrons in magnetic fields of order 100 G are responsible for the emission. Hanaoka (1999) from time-of-flight analysis of several double-loop flares obtained electron energies of some 100 keV (up to 1 MeV) under the assumption that they are accelerated in the interaction region of two closed loops, close to a footpoint of one of them. Similar estimates were also obtained by Grechnev & Nakajima (2002); Kundu et al. (2001b); Grechnev, White & Kundu (2003). Moreover, Grechnev & Nakajima (2002) showed that the assumption about injection region to be localized high in the corona, above loop tops, results in electron energy of order 1.5 keV. This energy is too low to produce gyrosynchrotron emission detectable at 17 GHz. Note that these estimates are sensitive to electron pitch-angle distribution: if it is anisotropic, then the apparent velocity would be increased by a factor of 1.23 (Hanaoka 1999). The electron energies estimated by means of different methods agree if pitch angles are small, and in this case they are 50–200 keV for impulsive flares, which corresponds to the velocities of 0.4–0.70 of the speed of light. Electrons of lower velocities do not contribute significantly to the gyrosynchrotron emission (Dulk 1985). This limitation together with the fact that the electrons are subrelativistic account for the small velocity spread, less than 30%. Higher electron energies in LDE flares are probably due to: 1) rapid relaxation of low-energy electrons (Melnikov & Magun, 1998), and 2) additional acceleration in shrinking loops or collapsing magnetic traps (Somov & Bogachev 2003). • References • Asai A., Masuda S., Yokoyama T. et al. ApJL, 2004, 578, L91. • Aschwanden M.J., Hudson H.S., Kosugi T., Schwartz R.A. ApJ, 1996, 464, 985. • Aschwanden M.J., Kosugi T., Hanaoka Y., Nishio M., Melrose D.B. ApJ, 1999a, 526, 1026. • Aschwanden M.J. In: Solar Physics with Radio Observations. Proc. Nobeyama Symp., Kiyosato, Japan, NRO Rep. No. 479, 1999b, p. 307. • Bastian T.S. In: Solar Physics with Radio Observations... 1999, 211. • Dulk G.A. ARA&A, 1985, 23, 169. • Fleishman G.D., Melnikov V.F. ApJ, 2003, 587, 823. • Grechnev V.V. Sol. Phys., 2003, 213, 103 • Grechnev V.V., Nakajima H. ApJ, 2002, 566, 539. • Grechnev V.V., White S.M., Kundu M.R. ApJ, 2003, 588, 1163. • Hanaoka Y. Sol. Phys., 1996, 165, 275. • Hanaoka Y. Sol. Phys., 1997, 173, 319. • Hanaoka Y. PASJ, 1999, 51, 483. • Heyvaerts J., Priest E.R., Rust D.M. ApJ, 1977, 216, 123. • Heyvaerts J., Priest E.R., Rust D.M. ApJ, 1977, 216, 123. • Holman G.D., Kundu M.R., Papadopoulos K. 1982, ApJ, 257, 354. • Katsova M.M., Livshits M.A. In: Activity in Cool Star Envelopes. Astrophys. and Space Sci. Library, vol. 143. D. Reidel Publishing Company, Kluwer Academic Publishers, Dordrecht, Holland, 1988, p. 143. • Kopp R.A., Pneuman G.W. Sol. Phys., 1976, 50, 85. • Kosugi T., Dennis B.R., Kai K. 1988, ApJ, 324, 1118. • Kundu M.R., Garaimov V.I. 2003, Adv. Space. Res, 32, 2497. • Kundu M.R., Nitta N., White S.M. et al. ApJ, 1995, 454, 522. • Kundu M.R., Grechnev V.V. Earth, Planets and Space, 2001, 53(6), 585. • Kundu M.R., White S.M., Shibasaki K., Sakurai T., Grechnev V.V. ApJ, 2001a, 547, 1090. • Kundu M.R., Grechnev V.V., Garaimov V.I., White S.M. ApJ, 2001b, 563, 389. • Kundu M.R., Nindos A., Grechnev V.V. Astron. Astrophys., 2004, 420, 351. • Livshits M.A., Belov A.V. Astron. Rep., 2004, 48(8), 665. • Livshits M.A., Tomozov V.M. 1974, Soviet Astronomy, 18, 331. • Longcope D. In: "Explosive Phenomena in Solar and Space Plasmas", Proc. of the Yohkoh 8th Anniversary Symp. Eds. T. Kosugi, T. Watanabe and M. Shimojo (Sagamihara: ISAS), 1999, p. 102. • Mandrini C.H., Demoulin P., van Driel-Gesztelyi L. et al. Sol. Phys., 1996, 168, 115. • Masuda S., Kosugi T., Hara H. et al. Nature, 1994, 371, 495. • Melnikov V.F., Magun A. Sol. Phys., 1998, 178, 591. • Nishio M., Yaji K., Kosugi T., Nakajima H., Sakurai T. 1997, ApJ, 489, 976. • Shibata K. In: Solar Physics with Radio Observations... 1999, p. 381. • Somov B.V., Bogachev S.A. 2003, Ast. Let., 29, 621. • Tsuneta S., Hara H., Shimizu T. et al. PASJ, 1992, 44, L63. • Tsuneta S., Masuda S., Kosugi T., Sato J. 1997, ApJ, 478, 787 • Uchida Y. 1980, Skylab Workshop "Solar Flares", P.A. Sturrock, Univ. Colorado Press, 67, 110. • Uchida Y., Morita S., Torii M. et al. In: Solar Physics with Radio Observations... 1999, p. 397. • 4. Conclusions. • Closed double-loop configurations can be responsible for various types of flares. • The site of electron acceleration is at the region between two or more interacting loops, close to a footpoint of one loop in impulsive flares, and probably close to the top of one loop in powerful flares. • Pitch angle distribution is anisotropic, with excess of small angles. • Energies of electrons responsible for microwave emission in impulsive flares are 50–200 keV. • The authors thank A.Altyntsev, A.Uralov, M.Livshits, S.Bogachev, V.Melnikov, G.Fleishman, and V.Tomozov for useful discussions, and S.White for his efforts in preparation of NoRH images with AIPS package. This study is supported by the Russian Foundation of Basic Research under grants 03-02-16591, 03-02-16229, 02-02-39030-GFEN, and the Ministry of Education and Science under grant NSh-477.2003.2. The research at the University of Maryland is supported by NASA grant NAG 5-12860, and NSF grant ATM 0233907.