Magnetic reconnection and coronal transients

Every two-ribbon flare observed during the Skylab period produced an observable coronal transient, provided the flare occurred close enough to the limb. The model presented here treats these two events as a combined process. Transients that occur without flares are believed to involve magnetic fields that are too weak to produce significant chromospheric emission. Adopting the hypothesis that the rising flare loop systems observed during two-ribbon flares are exhibiting magnetic reconnection, a model of a coronal transient is proposed which incorporates this reconnection process as the driving force. When two oppositely directed field lines reconnect a lower loop is created rooted to the solar surface (the flare loop) and an upper disconnected loop is produced which is free to rise. The magnetic flux of these upper loops is proposed as the driver for the transient. The force is produced by the increase in magnetic pressure under the filament and transient.A quantitative model is developed which treats the transient configuration in terms of four distinct parts- the transient itself with its magnetic field and material, the region just below the transient but above the filament, the filament with its magnetic field, and the reconnected flux beneath the filament. Two cases are considered - one in which all the prominence material rises with the transient and one in which the material is allowed to fall out of the transient. The rate of rise of the neutral line during the reconnection process is taken from the observations of the rising X-ray flare loop system during the 29 July, 1973 flare. The MHD equations for the system are reduced to four non-linear ordinary coupled differential equations which are solved using parameters believed to be realistic for solar conditions. The calculated velocity profiles, widths, etc., agree quite well with the observed properties of coronal transients as seen in white light. Since major flares are usually associated with a filament eruption about 10–15 min before the flare and since this model associates the transient with the filament eruption, we suspect that the transient is actually initiated some time before the actual flare itself.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Evolution of fibrils with special reference to flare activity

We show observational results on the pre-flare evolutions of H structures as well as the developments of H flares. It is shown that the chromospheric features are brought to a sheared state before flares due to motions of footpoints which correspond to particular sunspot motions. Generally in evolutions of the chromospheric features it is found that motions and reconnections of the footpoints play essential roles. The following three stages are found for development of the neutral line filament before flares: (1) formation of a filament as a result of reconnection; (2) increase of the shear of the filament due to the shear motion; and (3) reconnection of fine components of the filament to form an elongated component immediately before flares. We further show developments of two particular flares with and without the filament, and point out basic release processes of flares. The flare that occurred at the filament (July 5, 1974) started with the activation of the elongated component of the filament after the process (3). The main phase of a two-ribbon flare is considered as the rises of short components of the filament triggered by the rising motion of the elongated component. The flare of September 10, 1974 occurred at the region where fibrils connect the sunspots in distorted form. Pre-flare distortion was produced by translational rotation of the sunspot. Development of this two-ribbon flare is interpreted as being due to successive rises of the fibrils with a self-trigger mechanism.On leave from Tokyo Astronomical Observatory (present address).     

Spotless flares and the associated radio continuum emission

We studied 24 spotless flares of Ha importance 1 which occurred during the 21st cycle of solar activity. The spotless flares could be grouped in three categories according to their location and time history of the associated active region. Our association of the flares with radio events was based on relative timing and on the flare importances. Weak microwave gradual rise and fall events were frequently recorded during the occurrence of the spotless flares. A few flares from our sample could be associated with impulsive and complex microwave bursts. Only in one case an association of a spotless flare with a significant metric type II/IV event seems to be justified.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

Determination of force-free factor α and forecast of proton flares

We evaluated the force-free factor α for 18 well-observed proton flare regions during 1967–1972 according to the degree of twist in their neutral line. We found that, on the day of the flare, α ≥ 0.34 for flares of Class 1 or greater and ≥ 0.50 for Class 2 or greater and that α always increased over the one or two days before the flare. We therefore suggest that α can be used in forecasting proton events. We also outline a squeezing force-free field model of large flares which can better explain certain observed facts.     

Rayleigh-taylor instability in solar prominences as a mechanism for dXouble-ribbon flares

In this paper, the energy storage for a spotless two-ribbon flare is discussed with reference to the morphology of the chromospheric fibrils surrounding a filament prior to the flare. Also, on the basis of the Kippenhahn-Schluter model of filaments, we discuss the instability of magnetic structure in these filaments. We found that once the gradient of the magnetic field or the curvature of the magnetic “trough” exceeds certain critical value, the Rayleigh-Taylor instability will be triggered off, leading to the sudden disappearance (Disparition Brusque) of the filament. At the same time, a neutral current sheet will be formed in the field with magnetic flux concentrated on both sides of the filament. Rapid reconnection of the field lines then lead to the onset of a two-ribbon flare.     

Magnetic field and flares of the large sunspot group of April–May 1978

From magnetograms of the active region 15266 obtained at Yunnan observatory we found that 1) an inverted field configuration and a twisted neutral line are both closely correlated with high-energy flare eruptions. After the flares, the field configuration and the neutral line revert to more stable states. 2) Most of the flares of the present region occurred away from the neutral line; those near the line occurred either in regions of high field gradient or in the vicinity of Severny's “neutral points”.     

Observations of vector magnetic fields in flaring active regions

We present vector magnetograph data of 6 active regions, all of which produced major flares. Of the 20 M-class (or above) flares, 7 satisfy the flare conditions prescribed by Hagyard (high shear and strong transverse fields). Strong photospheric shear, however, is not necessarily a condition for a flare. We find an increase in the shear for two flares, a 6-deg shear increase along the neutral line after a X-2 flare and a 13-deg increase after a M-1.9 flare. For other flares, we did not detect substantial shear changes.Visiting Associate from Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, China.     

The spotless flare of 16 March 1981 – I. Pre-Flare Activations of the Chromospheric Fine Structure

This paper is based on observations in the H line with the aim to carry out a detailed study of the spotless flare of importance 1N that was observed at the Baikal Astrophysical Observatory on 16 March 1981. The study focuses on the evolution of the region of interest from the time of its appearance from behind the limb, and on the pre-flare activation of chromospheric features four hours before the flare. The disturbances that preceded the flare spanned an area of about 120 square degrees. The bulk of activations occurred along and near the path of the polarity inversion line (PIL) of the longitudinal component of the magnetic field. The flare was preceded by an eruptive filament, a disturbance of the fine structure of supergranulation cells, and by the formation of dark vortex structures in regions where flare ribbons form; dark mottles in these regions signaled the operation of an oscillatory process with a period of about 3–4 min, and the region where one of the flare ribbons formed showed a `tunnel' of a system of small-scale dark loops. A close association of the chromospheric activations and flare mottles, with the boundaries of the chromospheric and magnetic networks, is established.     

An interpretation of rapid changes in the magnetic field associated with solar flares

The energy source of a flare is the magnetic field in the corona. A topological model of the magnetic field is used here for interpreting the recently discovered drastic changes in magnetic field associated with solar flares. The following observational results are self‐consistently explained: (1) the transverse field strength decreases at outer part of active regions and increases significantly in their centers; (2) the center‐of‐mass positions of opposite magnetic polarities converge towards the magnetic neutral line just after flares onset; (3) the magnetic flux of active regions decreases steadily during the course of flares. For X‐class flares, almost 50% events show such changes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Properties of flares observed in the Mg i b2 line at 5172 Å

Observations of emission in the Mgi b₂ line at 5172 Å are presented for 13 flares. Also discussed are 3 flares which occurred in regions under observation but which showed no Mg emission. The Mg flare kernels resemble white-light flare kernels in their general morphology and location. Comparison of Mg filtergrams with magnetograms indicates that the Mg kernels occur at the feet of magnetic arches across neutral lines. Time-lapse Mg filtergram films indicate photospheric shearing motions near flare sites for several hours before flare onset. We have compared flare Mg emission with microwave and both hard and soft X-ray flare emissions. Examination at the time development of the 1981, July 27 flare shows that the microwave and X-ray bursts may be clearly related to the appearance of successive Mg flare kernels. We have also compared subjective, relative Mg flare importances with other flare emission measurements. For the full sample of flares, Mg importance is significantly correlated with hard and soft X-ray emission peaks, with X-ray ‘hardness’ (ratio of hard to soft peaks) and with the rise slope of soft X-ray bursts. The Mg importance does not correlate with the microwave peaks when the full sample of flares is used, but for the subset showing Mg emission there is significant correlation. No correlation with Hα importance was found. Our results suggest that Mg emission is associated with an impulsive component which may be absent from some flares. The San Fernando Observatory magnesium etalon filter system is described.     

行星摄动力对太阳活动的调制

本文采用天体力学方法，考虑太阳系九大行星对太阳表面局部区域的摄动力，建立了太阳表面受行星起潮力的数值计算模型．利用此模型，针对历史上发生的100个大太阳耀斑事件，计算各耀斑区耀斑发生前后所受行星起潮力的变化．从耀斑发生的时间分布统计得到：在100个耀斑中，有75个耀斑发生在行星综合起潮力合力极大前后三天内．证明行星摄动对太阳活动有调制作用．最后，本文还对太阳活动起源、活动周期等问题进行了简要的讨论．     

Flares,magnetic configurations,and magnetic energy release

Using a newly developed Aerospace digital videomagnetograph, three solar active regions are studied as to their magnetic configurations and their flare productivity. These three regions have very different types of magnetic configurations and different types of flare productivity. We review previous theoretical and experimental research on flares and magnetic energy storage, and discuss various ways to observe magnetic energy release due to flares. Results for six subflares are presented. Five showed no measurable magnetic energy change and one result is questionable.We show three counterexamples to Zirin's (1972) contention that as a rule H plage brightness is proportional to magnetic field strength. Each of these three cases involved two plage regions of the same polarity and equal field strengths with one of the plages adjacent to a neutral line. In all three cases the plage region nearer the neutral line was much brighter.     

Flare positions relative to photospheric magnetic fields

Of 21 flares of importance 1 or greater, observed on 15 days, all were found to lie adjacent to a neutral line in the longitudinal component of photospheric magnetic fields. In most of these cases, the flare consisted of two or more segments separated by the neutral line and located in areas of strong field and high-longitudinal field gradient. In some cases, the flare segments extended into areas of weak-magnetic field and low-field gradient, but maintained an orientation adjacent to a neutral line.Optical and magnetic field records of higher resolution were obtained on 6 July 1965. These observations reveal an excellent correlation between the size, shape, and intensity of the H fine structures and the longitudinal component of the photospheric magnetic fields, except in the vicinity of the neutral line. Sections of the neutral line are marked by long fibrils lying perpendicular to the neutral line or by small filaments lying along the neutral line.The development of a flare of importance 1 in this region appeared to be more precisely related to the neutral line than was found for the flares and magnetic fields observed with lower resolution. The two major segments of this flare lengthened in directions approximately parallel to the neutral line, while simultaneously drifting perpendicularly away from the neutral line. The initial rate of drift systematically varied from 1 to 12 km/sec at a series of positions approximately parallel to the neutral line and corresponding to increasing distance from strong fields. The rate of drift was also observed to decelerate throughout the life of the flare.     

The Relationship between Magnetic Gradient and Magnetic Shear in Five Super Active Regions Producing Great Flares

We study the magnetic structure of five well-known active regions that produced great flares (X5 or larger). The six flares under investigation are the X12 flare on 1991 June 9 in AR 6659, the X5.7 flare on 2000 July 14 in AR 9077, the X5.6 flare on 2001 April 6 in AR 9415, the X5.3 flare on 2001 August 25 in AR 9591, the X17 flare on 2003 October 28 and the X10 flare on 2003 October 29, both in AR 10486. The last five events had corresponding LASCO observations and were all associated with Halo CMEs. We analyzed vector magne-tograms from Big Bear Solar Observatory, Huairou Solar Observing Station, Marshall Space Right Center and Mees Solar Observatory. In particular, we studied the magnetic gradient derived from line-of-sight magnetograms and magnetic shear derived from vector magne-tograms, and found an apparent correlation between these two parameters at a level of about 90%. We found that the magnetic gradient could be a better proxy than the shear for predicting where a major flare might occur: all six flares occurred in neutral lines with maximum gradient. The mean gradient of the flaring neutral lines ranges from 0.14 to 0.50 G km-1, 2.3 to 8 times the average value for all the neutral lines in the active regions. If we use magnetic shear as the proxy, the flaring neutral line in at least one, possibly two, of the six events would be mis-identified.     

Some comments on flares after many years of observation

Ground based observations of flares are reviewed to seek implications for a flare build-up on either a long or a short time scale. Plots of flare frequency and importance for certain individual centers of activity suggest a possible crescendo in flare occurrence days and hours before the development of large and significant flares. The X-ray records follow the same pattern of apparent build-up. A possible dependence between successive major flares, as phases one and two of a single complex flare event, suggests that the time scale in which the total flare event takes place may show extreme variation.Since all flares start as small features, there is a short term build-up in the optical records. The characteristics of this build up are not clear. The initial brightenings in a flare may or may not show a flash phase, and the rise to maximum may or may not be accompanied by filament activity. Flares rise to maximum H intensity at markedly different rates. Although most flares occur in centers of activity with well defined and often complex magnetic fields, certain large and relatively energetic flares have developed in centers of activity with apparently very simple circumstances.     

Magnetic shear in flaring regions

We have evaluated the shear angle of the neutral line of the non-potential magnetic field for one or two days prior to and after the flare event for 10 cases. We have used the H filament positions to evaluate the shear in the neutral line. We find from the samples we have studied that it is the change in the shear that occurs a day prior to the flare that can lead to the event. This change can be in either direction, i.e., it can be a large increase from a small value or a decrease from a large initial value. Thus it is the change in the shear angle that seems to be a deciding criterion for a flare to occur and not a large value for the shear angle itself. We have one instance where there was no significant change in the shear angle over a period of a few days and this region, although similar to other active regions studied, did not produce any flare activity.     

The association of flares to cancelling magnetic features on the sun

Previous work relating flares to evolutionary changes of photospheric solar magnetic fields are reviewed and reinterpreted in the light of recent observations of cancelling magnetic fields. In line-of-sight magnetograms and H-alpha filtergrams from Big Bear Solar Observatory, we confirm the following 3 associations: (a) the occurrence of many flares in the vicinity of emerging magnetic flux regions (Rust, 1974), but only at locations where cancellation has been observed or inferred; (b) the occurrence of flares at sites where the magnetic flux is increasing on one side of a polarity inversion line and concurrently decreasing on the other (Martres et al., 1968; Ribes, 1969); and (c) the occurrence of flares at sites where cancellation is the only observed change in the magnetograms for at least several hours before a flare (Martin, Livi, and Wang, 1985). Because cancellation (or the localized decrease in the line-of-sight component of magnetic flux) is the only common factor in all of these circumstances, suggest that cancellation is the more general association that includes the other associations as special cases. We propose the hypothesis that cancellation is a necessary, evolutionary precondition for flares. We also confirm the observation of Martin, Livi, and Wang (1985) that the initial parts of flares occur in close proximity to cancellation sites but that during later phases, the flare emission can spread to other parts of the magnetic field that are weak, strong, or not cancelling.     

On the triggering of a spotless double-ribbon flare

We have studied the evolution of the double-ribbon, spotless flare of 21 February, 1992, using Kodaikanal H and Kf1 observations. The analysis of the data shows that the H filament underwent a large change in shear prior to the day of the onset of the flare. We find considerable rotation of the plage region before the emergence of a small magnetic pore. It is concluded that shear plays an important role in the triggering of a spotless flare.     

1986年2月4日太阳耀斑的演化研究

本文根据乌鲁木齐天文站的H_α耀斑及3.2cm射电流量观侧资料、云南天文台的黑子精细结构照相和Marshall Space Flight Center的向量磁场图,对1986年2月4日的六个耀斑的形态相关及演化联系,特别是0736UT 4B/3X大耀斑的发展过程进行了综合分析。主要结果是: 1.4日大耀斑的初始亮点和闪光相的主要形态演化,与活动区中沿中性线新浮现的强大电流/磁环系密切相关。后者的主要标志是沿中性线的长的剪切半影纤维及它两端的偶极旋涡黑子群(1_3F_3)。 2.上述大耀斑与1972年8月4日0624 UT大耀斑爆发的磁场背景及主要形态特征相似,表明两者的储能和触发机制可能相同。 3.大耀斑爆发的H_α初始亮点,双带出现,环系形成,亮物质抛射和吸收冕珥等现象同3.2cm射电流量的变化在时间上有较好的对应关系。 4.重复性的前期小耀斑爆发位置和发展趋势与大耀斑的主要形态及演化特征相似。它们相对于剪切的纵场中性线两侧的位置相近或相同。因而,可以看作上述强大电流/磁环系不稳性发展过程中的前置小爆发。