Observational evidences for multi-component magnetic field structure in solar flares

Two solar flares of 25 July 1981 and 5 November 2004 of importance 2N and M4.1/1B, respectively, were investigated using observational data obtained with the Echelle spectrograph of the Kyiv University Astronomical Observatory. Stokes I and V profiles of the FeI lines 5233, 5247.1, 5250.2, 5250.6, 5576.1 and of CrI 5247.6 Å have been analyzed. We found several evidences for the existence of spatially unresolved magnetic field structures with kG strengths. In particular, the values of the measured average longitudinal field B _∥ depend on the Lande factors g of the lines: in general, B _∥ increases with increasing factor g. Analogously, the observed line ratio B _∥(5250.2)/B _∥(5247.1) is increasing with increasing distance Δλ from the line center. The observed Stokes V profiles show some deviations from that of an assumed homogeneous field, presented by the Stokes I gradient, dI/dλ. A comparison with the non-split line FeI 5576.1 Å shows that some of these deviations are real and indicate the presence of subtelescopic magnetic elements with discrete field strengths of several kG. The lines with large Lande factors have considerable broadenings of the Stokes I profiles, indicating a strong background magnetic field of mixed polarity. On the basis of all these data we conclude that a four-component magnetic field structure is a possible explanation. The field strengths are about ±1.05 kG in the background field, and 1.3?1.5, 3.9?4.0, and 7.4?7.8 kG at level of middle photosphere (h ≈ 300 km) in the spatially unresolved, small-scale magnetic elements.     

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)     

Changes in the photospheric magnetic field associated with solar flares

The flare-related, persistent and abrupt changes in the photospheric magnetic field have been reported by many authors during recent years. These bewildering observational results pose a challenge to the current flare theories in which the photospheric magnetic field usually remains unchanged in the eruption. In this paper, changes in the photosphere magnetic field during the solar eruption are investigated based on the catastrophe model. The results indicate that the projection effect is an important source that yields the change in the observed photospheric magnetic field in the line-of-sight. Furthermore one may observe the change in the normal component of magnetic field if the spectrum line used to measure the photospheric magnetic field does not exactly come from the photospheric surface. Our results also show that the significance of selecting the correct spectral lines to study the photospheric field becomes more apparent for the magnetic configurations with complex boundary condition (or background field).     

A classification of magnetic field configurations associated with solar flares

On the assumption that solar flares are due to instabilities which occur in current sheets in the Sun's atmosphere, one may classify magnetic-field configurations associated with flares into two types. One is characterized by closed current sheets, magnetic-field lines adjacent to these sheets beginning and ending at the Sun's surface. The other is characterized by open current sheets, magnetic-field lines adjacent to these sheets beginning at the Sun's surface but extending out into interplanetary space. Flares associated with open current sheets can produce Type III radio bursts and high-energy-particle events, but flares associated with closed current sheets cannot. The flare of July 6, 1966 apparently consisted of one flare of each type.     

The magnetic nature of solar flares

The main challenge for the theory of solar eruptions has been to understand two basic aspects of large flares. These are the cause of the flare itself and the nature of the morphological features which form during its evolution. Such features include separating ribbons of H emission joined by a rising arcade of soft x-ray loops, with hard x-ray emission at their summits and at their feet. Two major advances in our understanding of the theory of solar flares have recently occurred. The first is the realisation that a magnetohydrodynamic (MHD) catastrophe is probably responsible for the basic eruption and the second is that the eruption is likely to drive a reconnection process in the field lines stretched out by the eruption. The reconnection is responsible for the ribbons and the set of rising soft x-ray loops, and such a process is well supported by numerical experiments and detailed observations from the Japanese satellite Yohkoh. Magnetic energy conversion by reconnection in two dimensions is relatively well understood, but in three dimensions we are only starting to understand the complexity of the magnetic topology and the MHD dynamics which are involved. How the dynamics lead to particle acceleration is even less well understood. Particle acceleration in flares may in principle occur in a variety of ways, such as stochastic acceleration by MHD turbulence, acceleration by direct electric fields at the reconnection site, or diffusive shock acceleration at the different kinds of MHD shock waves that are produced during the flare. However, which of these processes is most important for producing the energetic particles that strike the solar surface remains a mystery. Received 2 January 2001 / Published online 17 July 2001     

Electric field measurements in solar flares

Meaurements of solar flare spectra have allowed the electric field strengths in two flares to be determined, using the Inglis-Teller formula. Further, an independently estimated value for the electron density has allowed the two components of this field, that is, the interionic component and the external component that arises, for example, through plasma instabilities, to be separately extracted. External electric field strengths 0.5 kV cm^–1 for a limb flare and 1.3 kV cm^–1 for a white-light flare are found. Estimates of electric fields strengths generated by the resistive magnetic tearing instability indicate that this process could account for a significant part of the electric field if pre-existing magnetic field strengths in the flaring regions are characterized by a few kilogauss. Other plasma processes probably contribute measurably as well.Operated by the Association of Universities for Research in Astronomy, Inc., under contract NSF AST84-18716 with the National Science Foundation.     

The role of photospheric magnetic field in the development of solar flares

A statistical investigation has been made about the flare-process in relation to the photospheric magnetic field and configuration. It is understood from the analysis that the flare energy bears a linear relationship with the rate of change of flux of the longitudinal component of photospheric magnetic field.     

Solar wind disturbances caused by solar flares: Equatorial plane

We study the propagation of solar wind disturbances caused by single, double and six successive flares in the dipolar and quadrupolar patterns of the interplanetary magnetic field (IMF) and the associated solar wind flow. This study is based on a kinematic and empirical method developed by Hakamada and Akasofu (1982). Each flare is characterized by six parameters (such as the highest speed flow, its extent and duration). The successive IMF patterns in the equatorial plane of the heliosphere during a time span of 0.5–60 days after flares are presented for a variety of flares. The solar wind speed and IMF magnitude are also given as a function of distance along a radial line fixed in space and also as a function of time at several points fixed in space (simulating approximately space probe observations). Some of the results are qualitatively compared with recent space probe observations, demonstrating fair similarity with the observed time profiles of solar wind speed variations over a wide range of both distances (0–10 a.u.) and time spans (60 days). Our method provides a first order construction, temporal and spatial, of flare-induced shocks and their multiple interactions with each other, as well as with the corotating interaction regions.     

A magnetic confinement nuclear fusion mechanism for solar flares

We propose a magnetic confinement nuclear fusion mechanism for the evolution of a solar flare in the solar atmosphere.The mechanism agrees with two observed characteristics of explosive flares and coronal mass ejections(CMEs) that have proved to be very difficult to explain with previous mechanisms:the huge enrichments of3 He and the high energy gamma ray radiation.The twisted magnetic flux rope is a typical structure during the solar flares,which is closely related to the solar active region that magnetic fields have almost complete control over the plasma.Consequently,the plasma inside the flux rope is heated to more than 1.0×107 K by an adiabatic compression process,and then the thermonuclear fusion can take place in the flux rope accompanied with high energy gamma rays.We utilize the time-dependent ideal 2.5-dimensional magnetohydrodynamic(MHD) simulation to demonstrate the physical mechanism for producing flares,which reveals three stages of flare development with the process of magnetic energy conversion and intense release during the solar flares and CMEs in the solar atmosphere.Furthermore,we discuss the relationship between magnetic reconnection and solar eruptions.     

Satellite information system malfunctions caused by broadband solar radio emission during solar flares

The paper briefly discusses the impairment of normal functioning of the global positioning system (GPS) and the global navigation satellite system (GLONASS) caused by broadband solar radio emission during large solar flares on October 28, 2003, and December 6 and 13, 2006.     

Evolution of the photospheric magnetic field and coronal null points before solar flares

Based on a topological model for the magnetic field of a solar active region (AR), we suggest a criterion for the existence of magnetic null points on the separators in the corona. With the problem of predicting solar flares in mind, we have revealed a model parameter whose decrease means that the AR evolves toward a major eruptive flare. We analyze the magnetic field evolution for AR 9077 within two days before the Bastille Day flare on July 14, 2000. The coronal conditions are shown to have become more favorable for magnetic reconnection, which led to a 3B/X5.7 eruptive flare.     

Prompt particle acceleration around moving X-point magnetic field during impulsive phase of solar flares

We present a model for high-energy solar flares to explain prompt proton and electron acceleration, which occurs around moving X-point magnetic fields during the implosion phase of the current sheet. We derive the electromagnetic fields during the strong implosion of the current sheet, which is driven by the converging flow toward the center of the magnetic arcade. We investigated a test particle motion in the strong electromagnetic fields derived from the MHD equations. It is shown that both protons and electrons can be promptly (within 1 s) accelerated to 70 and 200 MeV, respectively. This acceleration mechanism can be applicable for the impulsive phase of the gradual gamma-ray and proton flares (gradual GR/P flare), which have been called two-ribbon flares.     

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.     

Force-free magnetic arcades relevant to two-ribbon solar flares

Simple analytic models for the passive evolution of arcade-like magnetic fields through a series of force-free equilibria are presented. At the photospheric boundary, the normal magnetic field component is prescribed together with either the longitudinal field component or the photospheric shear. Analytic progress is made by considering either cylindrically symmetric solutions or using the separation of variables technique. Two distinct cylindrically symmetric force-free fields are obtained that possess the same normal field component and photospheric shear. The scond field contains a magnetic bubble. As the shear increases beyond a critical value, so the magnetic energy of the first configuration exceeds that of the second. The possibility is therefore suggested of an eruption of the first field outwards towards the second. Such an eruptive instability is proposed as the origin of a two-ribbon solar flare.A new analytic solution to the force-free field equations, of separable form, is discovered and it is pointed out that the existence of shear in a magnetic field does not preclude it from being potential.Now at AWRE, Aldermaston, Reading, Berkshire.     

Reconnection,caused by nonexistence of force-free equilibria,as a mechanism for solar flares

The two-dimensional force-free field equations are studied. The solar photosphere is considered as flat and infinitely extended and the magnetic field component perpendicular to the photosphere is prescribed as the field of a submerged line dipole, i.e. with two magnetic polarities divided by a straight infinitely long neutral line. In addition the shear of the field lines along the neutral line, i.e. the difference of the coordinate parallel to the neutral line of the two foot-points of a field line, is prescribed as a function f of the distance to the neutral line times a nonnegative constant . The function f is zero at the neutral line, goes through a maximum and drops to zero at large distances from the neutral line. The case = 0 corresponds to the current-free field. An approximate solution is obtained by a test function method. It is shown that for certain choices of the function f there exists a maximum value of beyond which a steady continuation of the solution is impossible. This forces the field to jump to a state of lower energy. The potential field, for instance, is such a lower energy state. Since the shear was prescribed as a boundary condition, the jump of the magnetic field will always be accompanied by a field line reconnection. Even though the field calculated does not closely resemble the flare geometry it is speculated that discontinuities like this one may also occur in more realistic field configurations and may actually trigger the flare.An extended version of this paper is to be published elsewhere.     

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.     

An evolutionary feature of the magnetic field related to flares in active regions

Wavelength coincidences are noted between laboratory and solar spectral line lists for the twenty-electron ions CaI, TiIII, CrV, MnVI, FeVII, and NiIX, which imply likely identifications for a large number of unidentified lines in the solar lists. These identifications should be useful, e.g., for improving chromospheric/coronal abundance estimates of the less abundant elements titanium, chromium, and manganese.     

An example of radio and optical homologous flares

Fokker (1967) has raised the question of whether the optical and radio homologies of flares are correlated. Two 2b flares occurring nearly 54 h apart (July 9 and 12, 1968) were observed at 29 wavelengths from H+4.00 Å to H–4.26 Å and at 10-cm radio. Adjacent pictures were spaced 0.295 Å and 2 sec apart. The time resolution of the radio traces was about 10 sec. Detailed comparison of the pictures showed near-perfect similarities in the two events. These similarities included flare shape, filament agitation, rising arch formation and surges with line-of-sight velocities nearly 200 km/ sec observed. Comparion of the microwave radio flux traces showed detailed similarities in the shapes and simultaneity of at least eight features.Research supported in part by the Atmospheric Sciences Section, National Science Foundation, NSF Grant GA-4184, while at Harvey Mudd College, Claremont, Calif.     

On the magnetic reconnection of electric currents in solar flares

The role of the electric currents distributed over the volume of an active region on the Sun is considered from the standpoint of solar flare physics. We suggest including the electric currents in a topological model of the magnetic field in an active region. Typical values of the mutual inductance and the interaction energy of the coronal electric currents flowing along magnetic loops have been estimated for the M7/1N flare on April 27, 2006. We show that if these currents actually make a significant contribution to the flare energetics, then they must manifest themselves in the photosphericmagnetic fields. Depending on their orientation, the distributed currents can both help and hinder reconnection in the current layer at the separator during the flare. Asymmetric reconnection of the currents is accompanied by their interruption and an inductive change in energy. The reconnection of currents in flares differs significantly from the ordinary coalescence instability of magnetic islands in current layers. Highly accurate measurements of the magnetic fields in active regions are needed for a quantitative analysis of the role of distributed currents in solar flares.