PREFACE The COSPAR Colloquium on Solar-Terrestrial Magnetic Activity and Space Environment (STMASE) was held in the National Astronomy Observatories of Chinese Academy of Sciences (NAOC) in Beijing, China in September 10-12, 2001. The Colloquium was sponsored by: NAOC, the Chinese National Committee for COSPAR (CNCOSPAR), the Committee on Space Research (COSPAR), Bureau and International Solar Cycle Studies (ISCS) of the Scientific Committee on Solar-Terrestrial Physics (SCOSTEP), The Ministry of Science and Technology of China, Chinese Academy of Sciences, and the National Natural Science Foundation of China. The meeting was focused on five areas of the solar-terrestrial magnetic activity and space environment studies, including study on solar surface magnetism; solar magnetic activity, dynamical response of the heliosphere; space weather prediction; and space environment exploration and monitoring. 421 scientists including leading experts in various research areas of Solar Terrestrial Magnetic Activity and Space Environment, from almost all over the world, including Belgium, Canada, China, France, German, Israel, Italy, Japan, Korea, Netherlands, Russia, Spain, Turkey, United Kingdom, and United States, attended this meeting, and presented 421 exciting presentations. Professor Nishda, Vice Chairman of COSPAR, and some officers from Chinese Academy of Sciences and National Natural Science Foundation of China presented opening addresses. During the Plenary Lecture, Professor Shao Liqin, vice director of Basic Research Department, the Ministry of Science and Technology, China; Professor Xifan Hao, Director of Division of System Engineering, China National Space Administration; Professor J. Allen, General Secretary of SCOSTEP; .C E Escoubet, Project Scientist of Cluster-II mission from ESA; and Professor Fang Cheng, Nanjing Univ., presented lectures about China's Space Activities in the Future; Examples of Space weather effects on humans, satellites and Earth; First Results of the Cluster-II Mission; and Magnetic Reconnection in the solar lower atmosphere. 28 invited speakers gave talks on solar surface magnetism, solar magnetic activity, dynamical response of the heliosphere; space weather prediction; and space environment exploration and monitoring. Plans of joint research on the Magnetospheric Imager were also discussed between the Chinese Academy of Sciences and the Canadian Space Agency in the Special Interest Session on September ,21 2001. 421 papers were collected from the colloquium, including 28 invited and contributed oral presentations, and 42 poster presentations. A hot topic of space research, CMEs, which are widely believed to be the most important phenomenon of the space environment, is discussed in many papers. Other papers show results of observational and theoretical studies toward better understanding of the complicated image of the magnetic coupling between the Sun and the Earth, although we still know little about its physical background. Space weather prediction, which is very important for a modem society expanding into out-space, is another hot topic of space research. However, we still have a long way to go to predict exactly when and where a disaster will happen in the space. In that sense, there is much to do for space environment exploration and monitoring. eW organized the manuscripts submitted to this Monograph into: (1) solar surface magnetism, (2) solar magnetic activity, (3) dynamical response of the heliosphere, (4) space environment exploration and monitoring; and (5) space weather prediction. Papers presented in this meeting but not submitted to this Monograph are listed by title as unpublished papers at the end of this book. eW would like to thank the members of the Scientific Organization Committee listed in the first page of this book and conveners of all sessions of this meeting for their contributions and helps. We are grateful to Professor H. N. Wang and other members of the Local Organization Committee for their excellent Professor .G Ai, Director of National Astronomical Observatories, Chinese Academy Sciences (NAOC), giving welcoming address in the opening ceremony of the COSPAR Colloquium on Solar-Terrestrial Magnetic Activity and Space Environment, 10-12 September, 2001, Beijing, China Part of participants visiting Huairou Solar Observing Station of NAO on 41 September 2001. - vi - ecaferP work. During the meeting, the Local Organization Committee arranged special programs: Chinese Opera, visit to the Great wall, Forbidden City and Summer Palace. Our participants were deeply impressed by the great progress and long historical tradition of China, and they had a great time in Beijing. This Monograph was edited by Professors H. N. Wang and R. L. Xu. Professors .W .Y Xu, .Z .Y Pu, .Q .J Fu, .F .Y Zhao and .S D. Bao and anonymous referees made great efforts to review all of submitted papers and improve the quality of the manuscripts submitted to the editors. Thanks a lot to Miss X. L Wang for her hard work to unify the format of all manuscripts in electronic files. We hope that all will enjoy this book. Guoxiang Ai Beijing June 2002 - vii - MAGNETIC RECONNECTION IN THE SOLAR LOWER ATMOSPHERE C. Fang, P. F. Chen, and M. D. Ding tnemtrapeD1 of Astronomy, Nanjing University, Nanjing, ,390012 China ABSTRACT There are many active phenomena, such as Ellerman bombs (EBs), Type II white-light flares (WLFs) etc, appear in the solar lower atmosphere. They have many common features despite of the large energy gap between them. They are considered to result from the local heating in the solar lower atmosphere. This pa- per presents the numerical simulations of magnetic reconnection occurring in such a deep atmosphere, with the aim to account for the common features of some of these active phenomena, especially EBs and Type II WLFs. Numerical results manifest the following two typical characteristics of the assumed reconnection process: )1( magnetic reconnection saturates in ~600-900 s, which is just the lifetime of the phenomena; (2) ionization in the upper chromosphere consumes quite a large part of the energy released through recon- nection, leading to weak heating; On the contrary, in the lower chromosphere, the ionization and radiation have weak effect, resulting a strong heating in the lower chromosphere. The application of the reconnection model to the phenomena is discussed in detail. INTRODUCTION There are many active phenomena, such as Ellerman bombs (EBs), Type II white-light flares (WLFs), surges, spicules and Ha brightening (microflares etc.), which are related to the heating in the solar lower atmosphere and thought to be caused by the magnetic reconnection in the solar lower atmosphere. Ellerman bombs, also known as moustaches, are small brightening events which are observed in Ha wings around sunspots or under arch filament systems (AFS). They have a typical space scale of ~1 arcsec (Kurokawa et al., 1982), and a typical upward flow of 6~-. km s -1 in the chromosphere (Kitai, 1983). EBs are cospatial with bright features in the 3840/~ network, as well as with continuum facular granule (cf. Rust and Keil, 1992), and are pushed away by expanding granules (Denker et al., 1995), where one polarity magnetic features may be driven to meet other opposite polarity features. It was suggested by many authors that the heating originates in the lower atmosphere (e.g., Kitai and Muller, 1984; Dara et al., 1997). Recent observations of EBs show that they are located at the boundaries of magnetic features and associated with heating in lower atmosphere ( Dara et al., 1997; Qiu et al., 2000). Recently, we have proposed that EBs are caused by magnetic reconnection in the solar lower atmosphere, and typical EB line profiles can be reproduced by assuming that they are caused by the nonthermal electron bombardment originated in the lower chromosphere (Ding, H~noux and Fang, 1998; H~noux, Fang and Ding, 1998). Solar white-light flares (WLFs) are among the strongest flaring events, with an increase in the visible continuum. They are of great importance in flare research because they are not only similar in many aspects to stellar flares, but also present a major challenge to the flare atmospheric models and energy transport mechanisms (Neidig. 1989). It was proposed that there are two types of WLFs which show distinctive emission features, i.e., Type I WLFs reveal a Balmer or Paschen jump, while Type II do not (Machado et al., 1986). Such a distinction results from different continuum radiation mechanisms: hydrogen free-bound transitions for Type I while negative hydrogen (H-) radiations for Type II. Mauas et al. (1990) -3- C Fang et .la first investigated the semi-empirical atmospheric models for WLFs, indicating that white-light emission may correspond to the heating of the lower layers in the atmosphere. Further systematic studies on both the spectral characteristics and the atmospheric models for WLFs by Fang and Ding (1995) indicated that the features for Type I WLFs (e.g., a good time correlation between the emission of hard X-ray and the continuum, etc.) can be well explained by the conventional flare picture: energy is initially released in the corona, and then transported into and heats the lower atmosphere. However, for Type II WLFs, since the known mechanisms of energy transport are no longer effective (see Neidig, 1989; Ding et al., 1999 for more references), an in situ heating mechanism deep in the chromosphere or the photosphere is required. Emslie and Machado (1979) and Mauas et al. (1990) suggested that the required in situ heating may be due to the local Joule dissipation of current. Recently, Li et al. (1997) proposed magnetic reconnection in a weakly ionized plasma as the in situ heating mechanism, by which they tried to account for the space scale and the lifetime of Type II WLFs. However, their work is based on a linear analysis. Surges, spicules and Ha brightening (microflares etc.) are also thought to be related to the reconnection in the solar lower atmosphere (e.g., Dere et al., 1991). Recently, some authors have proposed and studied reconnection in the lower atmosphere. Wang and Shi (1991) provide some evidence of photospheric recon- nection as the magnetic cancellation mechanism (see also Litvinenko, 1999). Karpen et al. (1995) made 2.5D simulations and indicated that chromospheric eruptions could be the results of shear-induced reconnection in the chromosphere. Sturrock et a1.(1999) proposed that the reconnection of flux tubes in the chromosphere could contribute to coronal heating. By use of two-component MHD equations, Ji et al. (2001) made 2D numerical simulation and their results support the idea that magnetic cancellation, Ellerman bombs, and type II white-light flares are due to magnetic reconnection in the solar lower atmosphere. In this paper, 2D numerical simulations are performed, with the effects of ionization and radiation included, to study the magnetic reconnection in the lower atmosphere, with the aim to account for some common features of EBs and Type II WLFs. METHOD OF NUMERICAL SIMULATION For the magnetic reconnection in the solar lower atmosphere, ionization and radiation become important, while heat conduction is negligible, contrary to the situation in the corona. For simplicity, in this paper the weakly ionized plasma is approximately described by the one-fluid model. Another difficulty in 2D simulations of the lower atmosphere is the strong density stratification, since the pressure scale height is 100-600 km in the chromosphere and photosphere, resulting in a difference of about 7 orders of magnitude for the density between the top of the chromosphere and the lower photosphere. Incorporating such a stratification needs a very fine numerical mesh which makes the computations impractical. Thus, we further neglect the gravity and assume a uniform atmosphere by considering three cases with different characteristic parameters. The MHD equations we used are as follows: pO + V. (pv) = 0, (1) tO Ov +/--O-P p(v. V)v + VP -j (cid:141) S = 0, (2) 0B Vx(vxB)+Vx (~TVxB)=0, (3) tO 0 P P _~-L~(~--a l + neXn + pv2/2) + V'( V _ 1 + neXH + pv2/2)v -- -V. (Pv) - E.j + R- H = ,0 )4( where 7~ = ~ ,, x + ~_ O_.0vo Y , V_____(Vz, ,yV Vz). The quantities p, v, B, and T have their usual meanings; E is the electric field, while j si the current density; R and H represent the radiative loss and the heating terms, respectively; the gas pressure P = (nil -t-ne)kT, k is the Boltzmann constant, nH and ne are the number -4- citengaM noitcennoceR in eht raloS rewoL erehpsomtA Table .1 Models with Different Characteristic Parameters Case Z (km) oP (10-8kgm -3) oT (K) AV (kms -1) tA (s) 03/ A 0521 0.97 7800 61 851 1 B 625 16.7 5000 9 275 2 C 0 16700 5600 4.3 580 01 density of hydrogen atoms and electrons, respectively; HX is the ionization potential, ne is deduced by a modified Saha and Boltzmann formula for pure hydrogen atmosphere: / (x/r +4nile- r T <_ 501 K, (5) ne ( nil, T > 501 K, where r -- ~ (27r~kT)3/2 e-Xn/kT (cf. Gan and Fang 1990). Radiation is important in the lower atmosphere. Strictly speaking, it should be solved by the non-LTE theory, which is too difficult to deal with in the present 2D simulations. Instead, it is substituted by an empirical formula given by Gan and Fang (1990)" R = nnneo~(Z)f'(T), (6) where a(Z) and f'(T) are functions of Z (the height from 0005T = 1 of the photosphere) and T (the temperature), respectively. Since gravity is neglected, a is set to be uniform accordingly, which si done by fixing the value of Z. The pre-heating rate is given by H = ,/-THn where "k/=(nec~f') 0=t is unchanged during the simulation. In this paper, three cases (A, B, and C) are studied whose characteristic parameters (e.g., Z in Eq. (6), 0P for the density, oT for the temperature, AV for the velocity, At for the time, and/30 for the ratio of gas to magnetic pressure) are shown in Table ,1 where AV is the Alf,~en speed, At = LO/VA, and the length scale 0L equals 2500 km in all cases. In cases A, B, and C, the magnetic field si taken as 21 G, 92 G, and 450 G, respectively. The parameters for cases A, B, and C represent the conditions in the upper chromosphere, lower chromosphere, and photosphere, respectively. Hereafter, model A means the case A without considering ionization and radiation, model AI means the case A with ionization only, and model AIR means the case A with both ionization and radiation considered. The similar notations are used for cases B and C. The domain of simulation is -1 <_ x/Lo _< ,1 0 < y/Lo <_ ,1 where x- and y-axes are horizontal and vertical, respectively; the initial static atmosphere is isothermal and uniform, i.e., P/Po = 1 and T/To = ;1 the initial magnetic configuration is the same as in Chen et al. (1999a), i.e., a force-free current sheet. In this paper, an assumed anomalous resistivity ~/(#VALo) = 0.01 cos(57rx/Lo)coslO(y/Lo - 0.5)~r is imposed in a local region x/Lo _< 0.1, y/Lo -0.51 < 0.05 for cases A and B 77( = 0 elsewhere). For case C, the resistivity region is shifted to x/Lo < 0.1, ly/Lo -0.11 <_ 0.05. In the partially ionized atmosphere, the anomalous resistivity can be due to either the molecular resistivity, or the interaction between the ionized plasma and the neutral gas or other microscopic effects (e.g., the current driven instability), but a detailed discussion of which si beyond the scope of this paper. Due to the symmetry about the y-axis, calculation is performed only in the right half region. The numerical mesh consists of 16 (cid:141) 19 grid points, with 81 points lying within the half current sheet. Line-tying conditions are applied to the bottom boundary, and symmetry conditions to the left-hand side; other boundaries are free ones. The numerical simulations are performed with a multistep implicit scheme (Hu, 1989). Besides, an important numerical technique is applied to avoid possible pseudo-reconnection (cf., Chen et al., 2000). -5- C Fang et La Fig. .1 Distributions of the temperature ( grey scale ), projected magnetic field (solid lines) and velocity field (vector arrows) in BIR model at two times Fig. .2 Temporal evolution of the magnetic reconnection rates (R) in the three cases RESULTS As the anomalous resistivity sets in, two symmetrical convergent inflows move towards the diffusion region. Meanwhile, two narrow jets are ejected vertically. As indicated in our previous papers (e.g., Chen et al., 1999a), the reconnected field lines above the reconnection point (X-point) are ejected along with the upward jet; their counterparts below the X-point, however, pile up due to the line-tying effect of the bottom boundary, so that the closed magnetic loop system rises. Figure 1 shows the evolution of the temperature, the velocity and the magnetic configuration for model BIR. When the loop system becomes close to the resistivity region, it hinders the reconnection inflow, and magnetic reconnection is slowed down. The saturation of magnetic reconnection was discovered by Chen et al. (1999b), who showed that the saturation time-scale is approximately proportional to the height of the X-point. Although the reconnection nearly stops after t ~6 ,A-~ a global upward flow is seen, which results from the melon-seed effect of the magnetic configuration after the reconnection. The magnetic pressure gradient accelerates the plasma upward to a speed of ~ .AV2.0 Similar evolution is found in other cases, only that in case A, the reconnection process is more violent due to its smaller ~0, whereas in case C, the reconnection is much slower. Since the magnetic energy in case C is much smaller than the thermal energy, the resulting heating is weak even in the model without ionization ~nd radiation. gure 2 shows the temporal evolution of the magnetic reconnection rate, R, defined as the closing rate of the field lines (Chen et al., 1999a). It can be seen that in any case the reconnection slows down self-consistently, with an e-folding decay time of ~600-900 s, which is shown to be independent of the ionization and the radiation. It is also found that both ionization and radiation have very weak effect on the reconnection rate. Compared to the energy release rate, the thermodynamic quantities (e.g., p, T, etc.) are much more sensitive to the ionization and the radiation. The distributions of p and T in three cases along the y-axis at t = 400 s are plotted in Figure 3. It can be seen that in case A, which characterizes the middle and the upper chromosphere, the ionization process consumes a large part of the energy released through reconnection, -5- citengaM noitcennoceR ni eht raloS rewoL erehpsomtA P/Po T/T o A 1.5 1 1.0 o . 0.5 o.o 0.5 i .o o.o 0.5 I .o 3 1.5 1 "" --- ":- -:-- 1.o o 0.5 o.o 0.5 i _o o.o 0.5 i .o C o.o 0.5 I .o o.o 0.5 i .o y/Lo Y/Lo Fig. .3 Distributions of Density and Temperature in the Three Cases at t=4OOs. Dotted line (B), Dashed line (BI) Solid line(BIR) and radiation further cools the plasma down so that the actual temperature increase AT is only 300 K at most, and ,-,150 K in general with strong plasma condensation in the reconnection upflow; in case B, which characterizes the lower chromosphere, both the ionization and the radiation have weak effect on T and p, resulting in a strong heating with AT up to ,,~2000 K; in case C, which represents the photospheric level, a weak heating is produced even in the model without ionization and radiation with AT of 041~-, K, which is high enough to account for the negative hydrogen radiation in Type II WLFs (Fang et al., 1993). CONCLUSIONS Due to the saturation caused by the line-tying effects at the bottom boundary, magnetic reconnection in the lower atmosphere has a short life time which si independent to the ionization and the radiation. However, both ionization and radiation may alter much the temperature and density distributions: )1( In the upper chromosphere, the ionization consumes a lot of the energy released by reconnection, leading to weak heating; (2) In the lower chromosphere, the ionization and radiation have weak effect, resulting a strong heating; )3( In the photosphere, the magnetic energy is much smaller than the thermal energy, resulting in weak heating; (4) Magnetic reconnection in the lower atmosphere can account for Ellerman Bombs and Type II WLFs in many aspects, such as the lifetime (600-900 s), temperature increase (i.e., high in the lower chromosphere or photosphere, while low in the upper chromosphere), and so on. A CKN OWLED G EMENTS This work was supported by a fund from the National Natural Science of Foundation (No. 4990451), a fund from the Doctoral Program of the Ministry of Education of China and a National Basic Research Priorities Project(No.G2000078402). REFERENCES Chen, P. F., C. 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Martinez Pillet 1 l lnstituto ed Astroflsica ed Canarias, E-38200, aL Laguna, Tenerife (Spain) ABSTRACT Processes like magnetic flux emergence, shear build up in active regions and magnetic flux cancellation are known to have a direct effect on space weather related phenomena. The role played by them is considered in the light of recent observations from the ground and spacecrafts. The evolution of the O-loop observed by Lites et al. (1995) is contrasted with the cancellation processes observed in filament channels. The physical origin of the cancellation events is studied in some detail. INTRODUCTION The photosphere is the portion of the atmosphere where we are able to measure with some confidence the magnetic field that pervades the Sun outer layers. Our understanding of the structuring of the magnetic field in the photosphere (Solanki, 1999) has provided the background to explain some of the energetic processes that take place in higher layers. It is of interest to see the large number of works that have appeared in recent years where a relation between interacting bipoles and energy release events is presented. They are described to some extend in this work. The flux losses (cancellation) associated with these interacting bipoles is believe to play a major role in filament formation, eruption and, in general, CME initiation. We study several examples where this mechanism is seen to operate and discuss the alternatives to explain it. PHOTOSPHERIC PROCESSES RELEVANT TO CME EVENTS The successful combination of instruments on board SOHO (in particular EIT, LASCO and MDI magne- tograms) are helping us to increase our knowledge of CME events and their observed signatures from the photosphere to the outer parts of the corona (see, e.g., Subramanian & Dere, 2001). Before this SOHO era, Yohkoh observations and white light coronograph were used to investigate CMEs, mainly as seen in the solar limb in contrast to on-disk events. In this section, we want to compare part of the knowledge acquired using data from these spacecrafts and one case observed in detailed with high resolution vector magnetic field in the photosphere (and Yohkoh observations). During the interval 41 to 12 June 1992 (before SOHO), the Advanced Stokes Polarimeter (ASP) observed a short-lived ~-spot that appeared in active region NOAA 7201. A full description of the emergence of this small spot was presented in Lites et al. (1995, LLM hereafter). The idea expressed in this paper was that we observed the bodily transport of an almost closed loop system (termed O-loop) from the photosphere to the corona. From June 41 to ,61 the emergence took place as a classical process of ~t-loop appearance (see Martinez Pillet, 2000). The bipolar footpoints were connected by transverse fields that pointed perpendicular to the polarity inversion line (PIL), a configu- ration usually referred to as potential. In about only 5 days, it developed a very compact (15(cid:141) Mm )2 -9-