Two-component photosphere models of a 2N/M2-class solar flare

Physical state of the photosphere during a 2N/M2 solar flare on July 18, 2000, was studied. We used Echelle Zeeman spectrograms obtained by V. G. Lozitsky in orthogonal circular polarizations with a solar spectrograph. Semiempirical photospheric models were constructed for three moments in time in the initial and main phases of the flare using the SIR code applied to Stokes I and V profiles of seven iron and chromium lines. The photospheric model of the flare contains two components: a magnetic-field component and nonmagnetic environment. The height distributions of the temperature, magnetic field, and line-of-sight velocity were derived. The temperature in the nonmagnetic component had a nonmonotonous run with height. The models include layers in the middle and upper photosphere in which temperature is enhanced relative to an unperturbed photosphere model. As the flare developed, the temperature in the lower layers was increasing by 500–800 K. The magnetic field increased by 0.05 T and 0.08–0.1 T in the lower and upper photosphere during the flare, respectively, with the vertical temperature gradient decreasing from 0.0012 to 0.0008 T/km. The model for the onset phase of the flare indicates that there were upflows and downflows of substance in the lower and upper photosphere, respectively. The flow velocities decreased appreciably in the main phase of the flare. The model parameters of the nonmagnetic environment were only slightly different from those of the unperturbed photosphere.     

Preflare changes in the solar photosphere observed using the THEMIS telescope

The physical state of the photosphere 1 h 50 min before a C1 solar flare on May 24, 2012, was studied. The spectropolarimetric data from the French-Italian THEMIS telescope (Tenerife Island, Spain) were used. The modeling was carried out through the inversion method using SIR [B. Ruiz Cobo and J. C. del Toro Iniesta, Astrophys. J. 398, 375–385 (1992)] code. Height distributions of temperature, magnetic field strength, and line-of-sight velocity were obtained. Nine semiempirical models of the photosphere were constructed. Each model has a two-component (a magnetic field component and nonmagnetic surroundings) structure. According to the obtained models, the magnetic field parameters and thermodynamic parameters did change significantly in the course of observations that lasted for 8 min. The models contain layers with increased and decreased temperature values. The magnetic field strength in these models varied, on average, from 0.2 T (lower photospheric layers) to 0.13 T (upper layers). The line-of-sight velocities did not exceed 2 km/s in lower and middle photospheric layers and rose to 5–6 km/s in the upper layers. The differences in the physical state and its changes occurring at different sites within the active region prior to the flare were revealed.     

Spectropolarimetric investigation of an Ellerman bomb: 2. Photospheric models

Semiempirical models of the photosphere of an Ellerman bomb in the NOAA 11024 active region were obtained using profiles of Stokes parameters I, Q, U, and V of photospheric lines. Spectropolarimetric observations were conducted using the French–Italian THEMIS telescope (Tenerife, Spain). The SIR inversion code [28] was used in the modeling. The models have two components: a magnetic flux tube and nonmagnetic surroundings. The dependences of temperature, magnetic field strength, inclination of the magnetic field vector, and line-of-sight velocity in the tube on the optical depth were obtained. The models demonstrate that the thermodynamic parameters of the Ellerman bomb photosphere differ considerably from those of the quiet photosphere. The temperature in the tube model varied nonmonotonically with height and deviated by up to 700–900 K from its values for the quiet photosphere. Downflows were observed in the lower and the upper photospheric layers. The line-of-sight velocity in the upper layers of the photosphere was as high as 17 km/s. The magnetic field strength in the models varied from 0.1–0.13 T in the lower photospheric layers to 0.04–0.07 T in the upper ones. The physical state of the photosphere did change in the course of observations.     

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.     

Evidence of Magnetic Helicity in Emerging Flux and Associated Flare

The aim of this paper is to look at the magnetic helicity structure of an emerging active region and show that both emergence and flaring signatures are consistent with a same sign for magnetic helicity. We present a multiwavelength analysis of an M1.6 flare occurring in the NOAA active region 10365 on 27 May 2003, in which a large new bipole emerges in a decaying active region. The diverging flow pattern and the “tongue” shape of the magnetic field in the photosphere with elongated polarities are highly suggestive of the emergence of a twisted flux tube. The orientation of these tongues indicates the emergence of a flux tube with a right-hand twist (i.e., positive magnetic helicity). The flare signatures in the chromosphere are ribbons observed in Hα by the MSDP spectrograph in the Meudon solar tower and in 1600 Å by TRACE. These ribbons have a J shape and are shifted along the inversion line. The pattern of these ribbons suggests that the flare was triggered by magnetic reconnection at coronal heights below a twisted flux tube of positive helicity, corresponding to that of the observed emergence. It is the first time that such a consistency between the signatures of the emerging flux through the photosphere and flare ribbons has been clearly identified in observations. Another type of ribbons observed during the flare at the periphery of the active region by the MSDP and SOHO/EIT is related to the existence of a null point, which is found high in the corona in a potential field extrapolation. We discuss the interpretation of these secondary brightenings in terms of the “breakout” model and in terms of plasma compression/heating within large-scale separatrices.     

Evolution of emerging force-free flux tube to be a flare loop

Evolution of a filamentary magnetic flux tube emerging from the photosphere is investigated in the assumption that the magnetic field is force-free and unchanged during the evolution.If a characteristic radius of the flux tube is 3 km or less setting the field to 1000G, the temperature increases at first due to Joule heating up to about one million degree keeping the plasma density almost constant, and then the density decreases down to a critical value at which a current instability may occur. Thus, a.strong field-aligned electric field of 200 million volts or more is expected to be produced during the following anomalous Ohmic decay of the magnetic field as already shown by a numerical simulation.     

Lorentz Force: A Possible Driving Force for Sunspot Rotation

Zhao and Kosovichev (Astrophys. J. 591, 446, 2003) found two opposite sub-photospheric vortical flows in the depth range of 0 – 12 Mm around a fast rotating sunspot. So far there is no theoretical model explaining such flow motions. In this paper, we try to explain this phenomenon from the point of view of magnetic flux tubes interacting with large-scale vortical motions of plasma. In the deeper zone under the photosphere, the magnetic force may be less than the nonmagnetic force of plasma. The vortical flow located there twists the flux tube and magnetic free energy is built up in the tube. In the shallower zone under the photosphere, the magnetic force may be greater than the nonmagnetic force. Thus, part of the stored magnetic free energy is released to drive the plasma to rotate in two opposite directions, e.g., in the depth ranges of 0 – 3(5) and 9 – 12 Mm. In addition, we also define a vector of nonpotential magnetic stress τ, which can be related to flare occurrence. It is calculated for the active region NOAA 10930 on 11 December 2006. We find that: i) the integral of its line-of-sight (LOS) stress successively increases around the magnetic neutral line (MNL) prior to and during the flare and decreases to a minimum after the flare; ii) the integral of its transverse stress exceeds the integral of its LOS component by one order of magnitude over the whole field of view; iii) the transverse stress first points toward the MNL, then along it, and finally it points away from it. We need other data to verify whether or not the magnetic energy is transported in the horizontal direction to the neutral line, and then partly changes into the energy in LOS direction before and during the flare.     

Flare-associated magnetic changes in an active region

We report an instance of localized chromospheric polarity reversal in a rapidly-formed sunspot which appears to be part of new emerging flux. The chromospheric polarity reversal is preceded by extraordinarily fast growth of the transverse magnetic field and an increase in the line-of-sight magnetic flux of the newly formed sunspot in the photosphere. The strength of this reversal is more than 350 G at maximum, in contrast to approximately - 1300 G for the line-of-sight field and 400 G for the transverse field in the photosphere. Continued flare activity takes place around the site of the reversal with progressively increasing flare size and extent. It is suggested that a kinked or knotted flux loop, or a self-closed flux system developed above the fast-forming sunspot. So far, this phenomenon has been revealed in several active regions.     

Wave heating of the solar chromosphere

The nonmagnetic interior of supergranulation cells has been thought since the 1940s to be heated by the dissipation of acoustic waves. But all attempts to measure the acoustic flux have failed to show sufficient energy for chromospheric heating. Recent space observations with TRACE, for example, have found 10% or less of the necessary flux. To explain the missing energy it has been speculated that the nonmagnetic chromosphere is heated mainly by waves related to the magnetic field. If that were correct, the whole chromosphere, magnetic as well as nonmagnetic, would be heated mainly by waves related to the magnetic field. But contrary to expectation, the radiation emerging from the nonmagnetic chromosphere shows none of the signatures of magnetic waves, only those of acoustic waves. Nearly all the heating of the nonmagnetic chromosphere must therefore be due to acoustic waves. In the magnetic network on the boundary of supergranulation cells, on the other hand, the small filling factor of the magnetic field in the photosphere implies that only a small fraction of the wave flux that travels upward to heat the chromosphere can be channeled by the magnetic field. Hence, while some of the energy that is dissipated in the magnetic network is in the form of magnetic waves, most of it must be in the form of acoustic waves. Thus, the quiet solar chromosphere, instead of being heated mainly by magnetic waves throughout, must be heated mainly by acoustic waves throughout. The full wave flux heating the quiet chromosphere must travel through the photosphere. In the nonmagnetic medium, this flux is essentially all in the form of acoustic waves; TRACE registers at most 10% of it, perhaps because of limited spatial resolution.     

Magnetic reconnection,electric field,and particle acceleration in the July 14, 2000 solar flare

A topological model with magnetic reconnection at two separators in the corona is used to account for the recently discovered changes of the photospheric magnetic field in the active region NOAA 9077 during the July 14, 2000 flare. The model self-consistently explains the following observed effects: (1) the magnetic field strength decreases on the periphery of the active region but increases in its inner part near the neutral line of the photospheric magnetic field; (2) the center-of-mass positions of the fields of opposite (northern and southern) polarities converge; and (3) the magnetic flux of the active region decreases after the flare. The topological model gives not only a qualitative interpretation of the flare phenomena (the structure of the interacting magnetic fluxes in the corona, the location of the energy sources, the shape of the flare ribbons and kernels in the chromosphere and photosphere), but also correct quantitative estimates of the large-scale processes that form the basis for solar flares. The electric field emerging in the flare during large-scale reconnection is calculated. The electric field strength correlates with the observed intensity of the hard X-ray bremsstrahlung, suggesting an electron acceleration as a result of reconnection.     

Magnetic field lines for a flux tube

Equations for the magnetic field components in a two dimensional cylindrically symmetric flux tube equilibrium have been derived and, in a simple case, solved. The resulting magnetic configuration possesses a strong magnetic field in a thin tube below a reference level (solar photosphere). Above this reference level the field lines spread out in all directions.     

灾变理论在太阳物理和天体物理其他研究领域中的应用

叙述和介绍了太阳爆发的磁通量绳灾变理论和模型的发展过程,强调了建立这样的模型所需要的观测基础。讨论了由模型所预言的爆发磁结构的几个重要特征以及观测结果对这种预言的证实。在此模型的基础上,讨论了一个典型的爆发过程中所出现的不同现象及它们之间的相互关系。最后,介绍了作者的一项最新尝试:将太阳爆发的灾变理论和模型应用到对黑洞吸积盘间歇性喷流的理论研究当中,以及研究所取得的初步结果。     

Magnetic fields,loop prominences and the great flares of August, 1972

Lines of magnetic force, computed under the assumption that the solar corona is free of electric currents, have been compared with loop prominence systems associated with three flares in August, 1972. The computed fields closely match the observations of loops at a height of 40000 km at times 3–4 h after onset of the associated flares. Inferred magnetic field intensities in the loops range from 1300 G where the loops converge into a sunspot to 50–80 G at 40 000 km above the photosphere. The first-seen and lowest-lying loops are sheared with respect to the calculated fields. Higher loops conform more closely to the current-free fieldlines. A model of Barnes and Sturrock is used to relate the degree of shear to the excess magnetic energy available during the flare of August 7. On various lines of evidence, it is suggested that magnetic energy was available to accelerate particles not only during the impulsive phase of the flare, but also during the following 2–3 h. The particle acceleration region seems to be in the magnetic fields just above the visible loops. The bright outer edges of the flare ribbons are identified as particle impact regions. The dense knots of loop prominence material fall to the ribbons' inner edges.On leave from Tel Aviv University, Tel Aviv, Israel.     

Radial velocity field in the lower atmosphere of the solar active region during a flare with an ejection: The initial phase of the flare

Horizontal motion has been studied of the matter along the active region at different heights of the photosphere (115–580 km) in the initial phase of the two-ribbon solar flare on September 4, 1990, near the solar limb, accompanied by the ejection. Photospheric velocities varied in the range −3.5 ... 2.5 km/s. The direction of motion in the photosphere and the chromosphere was mainly toward the observer. Kinematic elements have been discovered in the structure of the horizontal velocity field. Their size reduced as they approached the maximum of the flare from 7–12 to 4–5 Mm, and the velocity amplitude decreased. Throughout the whole investigated active region, vortex motions were observed in the photosphere and chromosphere. Temporal changes in the horizontal velocity field in node areas and in their vicinity were oscillatory in nature and occurred almost simultaneously along the entire height of the photosphere.     

On the periodicity of energy release in solar active regions

We investigate the periodic regimes of energy release on the Sun, namely, the recurrence of solar flares in active regions using the Solar Geophysical Data Journal on Hα flares from 1979 until 1981, which corresponds to the maximum of solar cycle 21. We obtained the following series of periods in the manifestation of flare activity bymeans of a correlation periodogram analysis, a self-similarity function, and a wavelet analysis: ～1, 2, 3 h as well as ～0.4, 1, 2, 5 days. We suggest a diffusive model for the quasi-periodic transfer of toroidal magnetic fields from under the photosphere to interpret the retrieved sequence of periods in the enhancement of flare activity. We estimated the typical spatial scales of the magnetic field variations in the solar convection zone: ～17 000 km.     

An essay on sunspots and solar flares

The presently prevailing theories of sunspots and 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 for solar flare energy.In this paper, we pay attention to the fact that there are large-scale magnetic fields which divide the photosphere into positive and negative (line-of-sight) polarity regions and that they are likely to be more fundamental than sunspot fields, as emphasized most recently by McIntosh (1981). A new phenomenological model of the sunspot pair formation is then constructed by considering an amplification process of these largescale fields near their boundaries by shear flows, including localized vortex motions. The amplification results from a dynamo process associated with such vortex flows and the associated convergence flow in the largescale fields.This dynamo process generates also some of the familiar “force-free” fields or the “sheared” magnetic fields in which the magnetic field-aligned currents are essential. Upward field-aligned currents generated by the dynamo process are carried by downward streaming electrons which are expected to be accelerated by an electric potential structure; a similar structure is responsible for accelerating auroral electrons in the magnetosphere. Depending on the magnetic field configuration and the shear flows, the current-carrying electrons precipitate into different geometrical patterns, causing circular flares, umbral flares, two-ribbon flares, etc. Thus, it is suggested that “low temperature flares” are directly driven by the photospheric dynamo process.     

A statistical study on property of spatial magnetic field for solar active region

Magnetic fields dominate most solar activities, there exist direct relations between solar flare and the distributions of magnetic field, and also its corresponding magnetic energy. In this paper, the statistical results about the relationships between the spatial magnetic field and solar flare are given basing on vector magnetic field observed by the Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station (HSOS). The spatial magnetic fields are obtained by extrapolated photosphere vector magnetic field observed by SMFT. There are 23 active regions with flare eruption are chosen as data samples, which were observed from 1997 to 2007. The results are as follows: 1. Magnetic field lines become lower after flare for 16 (69 %) active regions; 2. The free energy are decreased after flare for 17 (74 %) active regions. It can conclude that for most active regions the changes of magnetic field after solar flare re coincident with the previous observations and studies.     

Dynamical processes in flux tabes and their role in chromospheric heating

We model the dynamical interaction between magnetic flux tubes and granules in the solar photosphere which leads to the excitation of transverse (kink) and longitudinal (sausage) tube waves. The investigation is motivated by the interpretation of network oscillations in terms of flux tube waves. The calculations show that for magnetic field strengths typical of the network, the energy flux in transverse waves is higher than in longitudinal waves by an order of magnitude. But for weaker fields, such as those that might be found in internetwork regions, the energy fluxes in the two modes are comparable. Using observations of footpoint motions, the energy flux in transverse waves is calculated and the implications for chromospheric heating are pointed out.