An electric-current description of solar flares

The presently prevailing theories of solar flares rely on the hypothetical presence of magnetic flux tubes beneath the photosphere and the two subsequent hypotheses, their emergence above the photosphere and explosive magnetic reconnection, converting magnetic energy carried by the flux tubes to solar flare energy. In this paper, we discuss solar flares from an entirely different point of view, namely in terms of power supply by a dynamo process in the photosphere. By this process, electric currents flowing along the magnetic field lines are generated and the familiar ‘force-free’ fields or the ‘sheared’ magnetic fields are produced. Upward field-aligned currents thus generated are carried by downward streaming electrons; these electrons can excite hydrogen atoms in the chromosphere, causing the optical Hα flares or ‘low temperature flares’. It is thus argued that as the ‘force-free’ fields are being built up for the magnetic energy storage, a flare must already be in progress.     

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.     

On the role of the magnetic configuration of flares for production of type III solar radio bursts

This paper investigates the possibility that the particular location of flare production sites in an active region is intimately connected to the production of type III radio bursts as well as centimetric and hard X-ray events. For the few active regions analysed (viz. McMath 8863, 8905, 8907 and 8921) it is shown that even a crude statistical test is sufficient in revealing significant differences concerning the emissions of these radiations by flares located in various flare production sites. In particular, flares located outside the general bipolar pattern of the active region are characterized by a higher type III flare association rate (? 50 %) than those taking place inside of it (? 20 %). Centimetric and hard X-ray events are more likely to be expected in connection with flares located inside strong magnetic fields arising from well developed sunspots. Such results are pointing out that the concept of ‘flare production sites’ is important not only in relation with the Hα flare activity but also in relation with the non-thermal emissions accompanying the flares. Probably this is due to changing magnetic configuration from one flare site to the other.     

Flares and changing magnetic fields

An observational study of maps of the longitudinal component of the photospheric fields in flaring active regions leads to the following conclusions:

1. The broad-wing Hα kernels characteristic of the impulsive phase of flares occur within 10″ of neutral lines encircling features of isolated magnetic polarity (‘satellite sunspots’).
2. Photospheric field changes intimately associated with several importance 1 flares and one importance 2B flare are confined to satellite sunspots, which are small (10″ diam). They often correspond to spot pores in white-light photographs.
3. The field at these features appears to strengthen in the half hour just before the flares. During the flares the growth is reversed, the field drops and then recovers to its previous level.
4. The magnetic flux through flare-associated features changes by about 4 × 10¹⁹ Mx in a day. The features are the same as the ‘Structures Magnétiques Evolutives’ of Martres et al. (1968a).
5. An upper limit of 10²¹ Mx is set for the total flux change through McMath Regions 10381 and 10385 as the result of the 2B flare of 24 October, 1969.
6. Large spots in the regions investigated did not evince flux changes or large proper motions at flare time.
7. The results are taken to imply that the initial instability of a flare occurs at a neutral point, but the magnetic energy lost cannot yet be related to the total energy of the subsequent flare.
8. No unusual velocities are observed in the photosphere at flare time.

     

Wavelets, Intermittency and Solar Flare Hard X-rays 1. Local Intermittency Measure in Cascade and Avalanche Scenarios

We introduce the local intermittency measure (LIM) as a tool for the investigation of solar flare hard X-ray light curves. Constructed from wavelet amplitudes, the LIM allows us to investigate the extent to which rapid fluctuations reveal an underlying, scale-invariant process, and to identify those episodes during a flare which definitely represent ‘intermittent’ behaviour (e.g. involvement of new spatial structures). We carry out two sets of simple simulations intended to provide generic examples of ‘top-down’ and ‘bottom-up’ scenarios for the development of flares, and show how LIM may discriminate between them.     

A magnetic bald-patch flare in solar active region 11117

With SDO observations and a data-constrained magnetohydrodynamics(MHD)model,we identify a confined multi-ribbon flare that occurred on 2010 October 25 in solar active region 11117 as a magnetic bald patch(BP)flare with strong evidence.From the photospheric magnetic field observed by SDO/HMI,we find there are indeed magnetic BPs on the polarity inversion lines(PILs)which match parts of the flare ribbons.From the 3D coronal magnetic field derived from an MHD relaxation model constrained by the vector magnetograms,we find strikingly good agreement of the BP separatrix surface(BPSS)footpoints with the flare ribbons,and the BPSS itself with the hot flaring loop system.Moreover,the triggering of the BP flare can be attributed to a small flux emergence under the lobe of the BPSS,and the relevant change of coronal magnetic field through the flare is reproduced well by the pre-flare and post-flare MHD solutions,which match the corresponding pre-and post-flare AIA observations,respectively.Our work contributes to the study of non-typical flares that constitute the majority of solar flares but which cannot be explained by the standard flare model.     

Flares before and after coronal mass ejections

Flare characteristics such as the flare occurrence number density and the distribution of peak flux as well as duration of flares occurring on either side of a coronal mass ejection(CME) onset time are studied. While the flares are rather evenly distributed statistically on either side of the CME onset time,the flare peak flux and duration tend to decrease depending upon their occurrence either before or after the CME onset. This is consistent with the earlier findings that flares emit higher energy before a CME whereas the energy is less in flares occurring after a CME.     

Some studies on solar microwave bursts in relation to the phases of the associated Hα-flares and their spectral nature

Occurrences of the flare-associated microwave bursts as well as their peak flux and energy excess spectra have been examined in relation to the pre- and post-maximum phases of the respective flares during the period 1969–72. Results obtained are: (i) about 76% of the flare-associated bursts occur in the pre-maximum phase and the remaining 24% occurs in the post-maximum phase irrespective of the flare classification, intensity-wise or area-wise; (ii) ‘impulsive’ and ‘gradual rise and fall’ bursts are relatively more important in the pre-maximum phase while ‘post burst increase’ bursts show comparatively higher occurrences in the post-maximum phase; (iii) peak flux and energy excess spectra of the concurrent microwave bursts in the pre-maximum phase of the flare are mostly of ‘inverted U’ and ‘increasing with frequency’ spectral types. Of these, ‘impulsive’ bursts are predominantly of the ‘inverted U’ and the ‘grf’ bursts are of the ‘increasing with frequency’ spectral type.     

Flares on opposite sides of the star: Observational aspects

The problem of the flare taking place on opposite sides of a star is considered. Such a screened flare, diffused through the star's atmosphere (chromosphere), may also be registered. The theoretical light curve for diffused flare event is derived, which differs strongly from the usual flare light curves. The light curve of diffused flare is characterized first of all by its very slow rise of brightness. This result opens quite a new direction to understand the nature of the so-called slow flares, observed often among the UV Cet-type stars as well as flare stars in aggregates. All slow flares can be interpreted as quite ordinary flares of quite ordinary flare stars — taking place, however, on the opposite sides of the star. The results of interpretation of some slow flare events of YY Gem and three flare stars in Orion are presented. An attempt is made for the determination of actual amplitudes of screened flares taking place on the opposite sides of a star.     

Multi-Wavelength Observations of Quasi-Periodic Pulsations in a Solar Flare

We have performed a search for flares and quasi-periodic pulsations (QPPs) from low-mass M-dwarf stars using Transient Exoplanet Survey Satellite (TESS) two-minute cadence data. We find seven stars that show evidence of QPPs. Using Fourier and empirical mode decomposition techniques, we confirm the presence of 11 QPPs in these seven stars with a period between 10.2 and 71.9 minutes, including an oscillation with strong drift in the period and a double-mode oscillation. The fraction of flares that showed QPPs (7%) is higher than other studies of stellar flares, but it is very similar to the fraction of solar C-class flares. Based on the stellar parameters taken from the TESS Input Catalog, we determine the lengths and magnetic-field strengths of the flare coronal loops using the period of the QPPs and various assumptions about the origin of the QPPs. We also use a scaling relationship based on flares from the Sun and solar-type stars and the observed energy, plus the duration of the flares, finding that the different approaches predict loop lengths that are consistent to within a factor of about two. We also discuss the flare frequency of the seven stars determining whether this could result in ozone depletion or abiogenesis in any orbiting exoplanet. Three of our stars have a sufficiently high rate of energetic flares, which are likely to cause abiogenesis. However, two of these stars are also in the range where ozone depletion is likely to occur. We speculate on the implications of the flare rates, loop lengths, and QPPs for life on potential exoplanets orbiting in their host star’s habitable zone.

     

Hydrodynamic models of solar and stellar flares

This paper discusses the hydrodynamic modeling of flaring plasma confined in magnetic loops and its objectives within the broader scope of flare physics. In particular, the Palermo-Harvard model is discussed along with its applications to the detailed fitting of X-ray light curves of solar flares and to the simulation of high-resolution Ca xix spectra in the impulsive phase. These two approaches provide complementary constraints on the relevant features of solar flares. The extension to the stellar case, with the fitting of the light curve of an X-ray flare which occurred on Proxima Centauri, demonstrates the feasibility of using this kind of model for stars too. Although the stellar observations do not provide the wealth of details available for the Sun, and, therefore, constrain the model more loosely, there are strong motivations to pursue this line of research: the wider range of physical parameters in stellar flares and the possibility of studying further the solar-stellar connection.     

Acceleration of electrons and solar flares due to quasi-static electric field

The storage of flare energy, efficient acceleration of electrons and the trigger of the flares are suggested to be attributed to a quasi-static electric field caused by a gas motion near the photosphere without satisfying the frozen condition. The primary cause of the onset of flares would be the acceleration of electrons due to the electric field above a critical strength. The electrons excite plasma waves which make the conductivity lower by several orders in the lower corona, so that the electro-magnetic energy I ² L stored before the onset of the flare would be rapidly converted into the heat due to the ohmic loss in about 10 s.     

Solar and stellar flares

Short-lived increases in the brightness of many red dwarfs have been observed for the last 30 yr, and a variety of more or less exotic models have been proposed to account for such flares. Information about flares in the Sun has progressed greatly in recent years as a result of spacecraft experiments, and properties of coronal flare plasma are becoming increasingly better known. In this paper, after briefly reviewing optical, radio and X-ray observations of stellar flares, we show how a simplified model which describes conductive plus radiative cooling of the coronal flare plasma in solar flares has been modified to apply to optical and X-ray stellar flare phenomena. This model reproduces many characteristic features of stellar flares, including the mean UBV colors of flare light, the direction of flare decay in the two-color diagram, precursors, Stillstands, secondary maxima, lack of sensitivity of flare color to flare amplitude, low flux of flare X-rays, distinction between so-called spike flares and slow flares, Balmer jumps of as much as 6–8, and emission line redshifts up to 3000 km s^–1. In all probability, therefore, stellar flares involve physical processes which are no more exotic (and no less!) than those in solar flares. Advantages of observing stellar flares include the possibilities of (i) applying optical diagnostics to coronal flare plasma, whereas this is almost impossible in the Sun, and (ii) testing solar flare models in environments which are not generally accessible in the solar atmosphere.     

耦合热传导的磁重联的数值模拟

磁重联被认为是太阳耀斑的产生机制,本文数值模拟在日冕中发生在磁重联过程,结果表明耀斑环的表观运动是磁重联的自洽结果;由重联点发出的慢激波对耀斑环的加热有贡献;耀斑环的上升并不意味着重联点的上升。     

Photospheric Magnetic Evolution in the WHI Active Regions

Sequences of line-of-sight (LOS) magnetograms recorded by the Michelson Doppler Imager are used to quantitatively characterize photospheric magnetic structure and evolution in three active regions that rotated across the Sun??s disk during the Whole Heliosphere Interval (WHI), in an attempt to relate the photospheric magnetic properties of these active regions to flares and coronal mass ejections (CMEs). Several approaches are used in our analysis, on scales ranging from whole active regions, to magnetic features, to supergranular scales, and, finally, to individual pixels. We calculated several parameterizations of magnetic structure and evolution that have previously been associated with flare and CME activity, including total unsigned magnetic flux, magnetic flux near polarity-inversion lines, amount of canceled flux, the ??proxy Poynting flux,?? and helicity flux. To catalog flare events, we used flare lists derived from both GOES and RHESSI observations. By most such measures, AR 10988 should have been the most flare- and CME-productive active region, and AR 10989 the least. Observations, however, were not consistent with this expectation: ARs 10988 and 10989 produced similar numbers of flares, and AR 10989 also produced a few CMEs. These results highlight present limitations of statistics-based flare and CME forecasting tools that rely upon line-of-sight photospheric magnetic data alone.     

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).     

The statistical analyses of flares detected in B band photometry of UV Ceti type stars

In this study, we present the unpublished flare data collected from 222 flares detected in the B band observations of five stars and the results derived by statistical analysis and modeling of these data. Six basic properties have been found with a statistical analysis method applied to all models and analyses for the flares detected in the B band observation of UV Ceti type stars. We have also compared the U and B bands with the analysis results. This comparison allowed us to evaluate the methods used in the analyses. The analyses provided the following results. (1) The flares were separated into two types, fast and slow flares. (2) The mean values of the equivalent durations of the slow and the fast flares differ by a factor of 16.2 ± 3.7. (3) Regardless of the total flare duration, the maximum flare energy can reach a different Plateau level for each star. (4) The Plateau values of EV Lac and EQ Peg are higher than the others. (5) The minimum values of the total flare duration increase toward the later spectral types. This value is called the Half-Life value in models. (6) Both the maximum flare rise times and the total flare duration obtained from the observed flares decrease toward the later spectral types.     

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)     

High abnormal rotation rates of sunspots and flare productivity

Solar activity, such as flares and CMEs, affect the interplanetary medium, and Earth’s atmosphere. Therefore, to understand the Space Weather, we need to understand the mechanisms of solar activity. Towards this end, we use 1135 events of solar H_α flares and the positional data of sunspots from the archive of Solar Geophysical Data (SGD) for the period January–April, 2000 and compute the abnormal rotation rates that lead to high flare productivity. We report that the occurrence of 5 or more flares in a day in association with a given sunspot group can be defined as high flare productivity and the sunspots that have an abnormal rotation rates of ～4–10 deg day^?1 trigger high flare productivity. Further, in order to compare the flare productivity expressed as the strength of the flux emitted, especially the soft X-ray (SXR) flares in the frequency range of 1–8 Å, we compute the flare index of SXR flares and find that 8 out of 28 active regions used in this study satisfy the requirement for being flare productive. This enables us to conclude that the high rotation rates of sunspots are an important mechanism to understand the flare productivity, especially numerical flare productivity that includes flares of all class.