Decay of activity complexes and the formation of coronal holes

Analysis of long-term measurements of solar magnetic fields and the flux of UV radiation from the Sun indicates a cause-effect relationship between activity complexs, their residual magnetic fields, and coronal holes. A comparison of the background magnetic fields of the Sun and the evolution of former activity complexes reveals unipolar magnetic regions that form after the decay of these complexes. The latitude and time evolution of unipolar magnetic regions in solar cycles 21–24 is studied. A North-South asymmetry in solar activity is manifest in the distribution of unipolar regions migrating toward higher latitudes. It is shown that, when residual magnetic fields of the opposite polarity reach the polar regions, this leads to a sign change of the polar magnetic field and a decrease in the area of polar coronal holes, or even their complete disappearance. These interactions can explain the triple sign change of the polar magnetic field of the Sun in cycle 21 and the short-term polarity reversals observed in 2010 and 2011.     

Topological model for the large solar flare of July 14, 2000

Using observational data obtained with the Yohkoh, SOHO, and TRACE satellites, it is shown that the three-dimensional structure of the large solar flare of July 14, 2000 was determined by the topology of the large-scale magnetic field of the active region giving rise to the flare. The locations and shapes of chromospheric ribbons and brightness centers on these ribbons are explained. The observed behavior of the flare is attributed to rapid magnetic reconnection in the corona. The electric field accelerating particles in the reconnecting current sheets is estimated.     

A model of a solar flare: Comparisons with observations of high-energy processes

New data for the energy and location of the hard-emission centers of a solar flare agree with an electrodynamic model of a solar flare based on the idea of the accumulation of free magnetic energy in the field of a current sheet. Three-dimensional MHD simulations are used to show that the energy stored in the preflare magnetic field of the current sheet is sufficient for the development of a flare and a coronal mass ejection. The flare and coronal mass ejection result from the explosive decay of the current sheet. The position of the brightness-temperature maximum of the radio emission during the flare coincides with the maximum of the current in the current sheet. The exponential spectrum of relativistic protons generated during the flare is consistent with acceleration by the electric field during the current-sheet decay.     

Magnetic flux in an active solar region and its correlation with flares

The correlation between the magnetic flux in an active solar region and associated powerful solar flares is studied. The behavior of the active regions AR 10486 and AR 10365 is considered. These regions produced a series of class X flares as they crossed the solar disk. The flares appeared when the magnetic flux exceeded 10²² Mx. The magnetic flux remained constant during all the flares except for one. During this flare, the flux decreased by about 10%; this impulsive decrease of the flux was also recorded in the absence of flares. No energy flux from the photosphere to the corona at the time of the flare was observed. The behavior of the photospheric field in AR 10486 and AR 10365 is consistent with a slow accumulation of energy in the corona and the explosive release of energy stored in the magnetic field of a current sheet above an active region during the flare.     

146-day signal in the geomagnetic field—a probable association with the periodicity of the solar flare index

Results on the spectral analysis using geomagnetic field at three low latitude stations and the planetary magnetic activity index have shown peaks in the power densities in a broad band centred around 146-day period. This periodic behaviour appears to be close to that shown by the solar flare activity index for the same interval. It is suggested that the geoeffectiveness of the flare activity signal in different phases of the solar cycle can be better worked out using long series of ground-based geomagnetic data.     

Relationship between solar-flare hard X-ray sources and magnetic TP singularities

The impulsive phase of the powerful solar flare of September 13, 2005 (～23:18–23:21 UT) observed in hard X-rays (～25–300 keV) by the RHESSI spacecraft is analyzed. The spatial locations of numerous X-ray sources are compared with the locations of Transition Point (TP) singularities in the magnetic field calculated for flare regions. It is shown that the hard X-ray sources are related to TP singularities, i.e., to possible locations of primary flare energy release. Magnetic field lines expelled from the vicnity of these magnetic singularities end in the chromosphere, near hard X-ray sources. The question of how these multiple magnetic singularities are involved virtually simultaneously in the energy-release process requires further study.     

Structure of the magnetic field at altitudes of 1–1.15 solar radii

An analysis of the characteristics of unipolar structures detected at latitudes from ?40? to +40?, longitudes of 0??360?, and altitudes of 1–1.15 solar radii during the period from May 1996 (the 23rd solar minimum) to October 2000 (the 23rd solar maximum) has been carried out. Synoptic maps of the solar radial magnetic field calculated in a potential approximation are used. The boundaries between unipolar structures with opposite magnetic polarities (“+/?” and “?/+” polarities) form chains extending along meridians at all the considered latitudes and altitudes. Depending on the latitude, the single-peaked distributions of the number of structures found at the lowest altitudes are replaced by double-peaked distributions at higher altitudes. The time variations of the total number of structures are non-monotonic. The growth in the number of unipolar structures begins before the growth in the Wolf number. This indicates that new unipolar structures already appear together with flocculi, preceding the formation of sunspots. It is found that structures with positive field have larger mean sizes that do structures with negative field. The polar field in the northern hemisphere penetrates to middle latitudes of the southern hemisphere. The existence of sets of structures with typical sizes is shown. The sizes of the smallest structures vary little with latitude, but increase slightly with altitude.     

Open magnetic fields on the Sun and solar wind parameters at the Earth’s orbit

It is shown that the parameters of the solar-wind magnetic field are determined by regions in coronal holes at distances of 1.1–1.4 solar radii, where the field lines are radial at low heights. Expanding further in a narrow nozzle or funnel, the field lines become radial throughout the unipolar region at 2.5 solar radii. Hence, the traditional approach of comparing the characteristics of the interplanetary field at the Earth’s orbit and at the corresponding helio-projection point on the Sun is not quite correct. It gives good results for the signs and sector structure of the field; however, the magnitude of the field is formed in a more extensive area. Taking this into account, we can correlate the field values on the Sun with the interplanetary magnetic field (IMF), and thus explain the absence of weak fields in the vicinity of the IMF neutral line (the two-peaked nature of the distribution).     

Origin of prolonged X-ray flares on active late-type stars

Soft X-ray data for prolonged flares in subgiants in RS CVn binary systems and some other active late-type stars (AB Dor, Algol) are analyzed. During these nonstationary events, a large amount of hot plasma with temperatures exceeding 10⁸ K exists for many hours. Numerical simulations of gas-dynamical processes in the X-ray source—giant loops—can yield reliable estimates of the plasma parameters and flare-source size. This confirms that such phenomena exist while considerable energy is supplied to the top part of a giant loop or system of loops. Refined estimates of the flare energy (up to 10³⁷ erg) and scales contradict the widely accepted idea that prolonged X-ray flares are associated with the evolution of local magnetic fields. The energy of the current component of the large-scale magnetic field arising during the ejection of magnetic field by plasma jets or stellar wind is estimated. Two cases are considered: a global stellar field and fields connecting regions with oppositely directed unipolar magnetic fields. The inferred energy of the current component of the magnetic field associated with distortion of the initial MHD configuration is close to the total flare energy, suggesting that large-scale magnetic fields play an important role in prolonged flares. The flare process encompasses some portion of a streamer belt and may propagate along the entire magnetic equator of the star during the most powerful prolonged events.     

Solar MHD turbulence in regions with various levels of flare activity

The paper continues investigations of MHD turbulence in active solar regions. The statistical distributions of the increments (structure functions) of the turbulent field are studied analytically in the context of a refined Kolmogorov theory of turbulence. Since photospheric transport of the B _z component of the magnetic field is quite similar to that of a scalar field in a turbulent flow, the theory of transport of a passive scalar can be applied. This approach enables us to show that the structure functions are determined by the competition between the dissipation of the magnetic and kinetic energies and to obtain a number of relations between the structure-function parameters and energy characteristics of the MHD turbulence. Taking into account general conclusions that can be drawn on the basis of the refined Kolmogorov turbulence theory, the structure functions of the B _z field are calculated for eight active regions (from measurements of SOHO/MDI and the Huairou Solar Observing Station, China). These calculations show that the behavior of the structure functions is different for the B _z field of each active region. The energy-dissipation index of the fluctuation spectrum (which is uniquely determined by the structure functions) is closely related to the level of flare activity: the more activity, the less steep the dissipation spectrum for a given active region. This provides a means to test and, consequently, forecast the flare activity of active regions.     

Solar flares associated with singularities in the differential characteristics of magnetic fields

We analyze the flare events of October 28 and May 27, 2003 to examine the possible connection of solar flares with singularities in the differential characteristics of solar magnetic fields. The relation between flares and the behavior of F indicators (which describe structural singularities of the magnetic fields) introduced earlier is analyzed. It is shown that the generation of flares can be associated with self-intersections (or reconnection) of the F = 0 curves, where magnetic reconnection can occur. Consequently, the current sheets generated in such regions can be sources of energy release.     

Detection of new emerging magnetic flux from the topology of SOHO/MDI magnetograms

A topological method for detecting the new emergence of magnetic flux using SOHO/MDI magnetograms of the full solar disk is proposed. This method uses the number of pixels in the image that can be distinguished from a specified value to within a predetermined threshold (the number of disconnected components). We study more than ten very powerful active regions (ARs) with very high flare activity and show that the number of disconnected components increases directly before the development of a series of M and X flares, or accompanies this process. This behaviour is evident not only when there is an explicit emergence of a new flux and a series of fast flares, such as in AR 9236 (November 2000), but also in groups with many non-stationary processes developing along a neutral line of the large-scale magnetic field. We also discuss the possibility of using the obtained results for flare prediction.     

MHD simulations of current-sheet formation over a bipolar active region

We present the results of numerical simulations of the development of a current sheet in the solar corona over a bipolar region during the emergence of two new sunspots arranged collinearly with older spots. Two fronts of increased plasma density form at the boundary of the rising new magnetic flux. One of these is due to the generation of a current sheet, whose magnetic field accumulates energy for a flare. The other front is a branch of the density perturbation, and separates the old and new magnetic fluxes in a region where the magnetic field lines have the same direction on both sides of the boundary. The development of this perturbation is not associated with the energy accumulation in the corona, and hinders observation of the preflare state and complicates analysis of the results. This second front can be interpreted as the eruption of a filament before the onset of the flare. A scheme conservative with respect to magnetic flux was introduced in the Peresvet code that solves the MHD equations, in order to suppress numerical instabilities in regions of large magnetic-field gradients.     

Integrated characteristics of the radial magnetic field in solar active regions during quiet and flare-productive phases of their evolution

This paper presents a statistical study of various integrated parameters of solar active regions, such as the distance between the polarity centroids, the inclination of the magnetic axis, the flux imbalance between the polarities, and the interosculation parameter of the magnetic fluxes of opposite polarities. The study is based on observations of the longitudinal photospheric magnetic field. We analyze ten active regions for which an appreciable volume of data with good spatial resolution are available. The distributions of the above parameters with field strength are very different for quiet and flare-productive active regions and for quiet and flare-active evolutionary phases of the same active region. Some distributions exhibit substantial and characteristic variations during the development of certain flare processes. The first moments of the distributions reflect specific features in the configuration of the photospheric magnetic fields and are correlated with the level of eruptive processes in the active regions.     

Recent advances in solar radio physics

We review high spatial resolution microwave observations of solar active regions, coronal loops and flares. Observations of preflare active regions are presented; in particular we discuss the interpretations of reversal of polarization at the flare site and the role of newly emerging flux in triggering the onset of flares. We discuss the spatial locations of microwave burst emitting regions; loops or arcades of loops appear to be the sites of flare energy release in microwave bursts. We provide direct observational evidence of magnetic reconnection as the primary cause of acceleration of electrons in microwave bursts.     

Evidence for a relationship between emerging magnetic fields,electric currents,and solar flares observed on May 10, 2012

Multi-wavelength observations and magnetic-field data for the solar flare of May 10, 2012 (04: 18 UT) are analyzed. A sign change in the line-of-sight magnetic field in the umbra of a small spot has been detected. This is at least partly associated with the emergence of a new magnetic field. A hard X-ray flare was recorded at almost the same time, and a “sunquake” was generated by the impact of the disturbance in the range of energy release on the photosphere. A sigmoid flare was recorded at the beginning of the event, but did not spread, as it usually does, along the polarity inversion (neutral) line. SDO/HMI full vectormagnetic-fieldmeasurements are used to extrapolate the magnetic field of AR 11476 into the corona, and to derive the distribution of vertical currents jz in the photosphere. The relationship between the distribution of currents in the active region and the occurrence of flares is quite complex. The expected “ideal” behavior of the current system before and after the flare (e.g., described by Sharykin and Kosovichev) is observed only in the sigmoid region. The results obtained are compared with observations of two other flares recorded in this active region on the same day, one similar to the discussed flare and the other different. The results confirm that the formation and eruption of large-scale magnetic flux ropes in sigmoid flares is associated with shear motions in the photosphere, the emergence of twisted magnetic tubes, and the subsequent development of the torus instability.     

Fractal properties of active regions

The dynamics of active regions have been investigated using multi-fractal analysis methods, based on magnetograms of the full solar disk in the 630.2 nm line obtained with the SOLIS vector spectromagnetograph of Kitt Peak Observatory (USA) during 2006?C2007 and January 1, 2009?CApril 12, 2010. The applied method of multi-fractal segmentation reveals the appearance of new magnetic fluxes on the Sun disk. A comparison of these fluxes with flare activity shows that the flares are generated in areas of interaction of emerging fluxes with existing structures.     

A Green’s function method for calculating the potential magnetic field in solar active regions in a spherical geometry

A Green’s function solution of Laplace’s equation for the potential magnetic field in an external spherical region is found using the derivative of the potential along a selected direction as a boundary condition. A set of programs applying this solution to construct the potential magnetic-field lines in solar active regions based on the photospheric line-of-sight field component has been developed. The method is tested using some model fields, and the optimal step size is found for realistic conditions. The developed software is applied to four real solar active regions, adopting HMI/SDO magnetograms as the boundary conditions. The potential magnetic field in the chromosphere and corona have been reconstructed for the selected regions. The calculated field lines are compared with flux tubes observed by AIA/SDO in the EUV. This comparison is used as a basis to discuss the applicability of a potential field approximation to the magnetic fields in solar active regions.     

Studying coronal holes through observations of an HeI infrared line and the Hα line

New results from electrophotometric scanning of the solar disk in the HeI λ 10830 Å and H_α lines are presented. The intensity at the center of the HeI λ 10830.30 Å line is 1–3% higher in the regions of coronal holes than in quiescent regions; this is accompanied by a decrease in the size and contrast of the chromospheric network compared to the network in quiescent regions. Our observations in the HeI line revealed chains of “dark points” surrounding coronal holes. The H_α±0.5 observations show increased velocities of ascent near the dark points compared to the velocities inside coronal holes and in quiescent regions. It is proposed that the intensification and acceleration of the flows of solar plasma from the dark points are due to reconnection of the magnetic fields of the bipolar chromospheric network and the predominantly unipolar magnetic field inside the coronal holes. Our observations suggest that the same reconnection process takes place near the temperature minimum, in the presence of certain conditions at the boundary between coronal holes and bipolar active regions. The reconnection process produces plasma flows from the chromosphere to the corona, which are sufficient to form prominences.     

Magnetic-field variations and solar flare activity

Solar filtergrams obtained at the Crimean Astrophysical Observatory at the center and wings of the H?? line are used to study variations in filaments, in particular, in arch filament systems (AFSs). These are considered as an indicator of emerging new magnetic flux, providing information about the spatial locations of magnetic-field elements. Magnetic-field maps for the active region NOAA 10030 are analyzed as an example. A method developed earlier for detecting elements of emerging flux using SOHO/MDI magnetograms indicates a close link between the increase in flare activity in theNOAA 10030 group during July 14?C18, 2002 and variations in the topological disconnectedness of the magnetograms. Moreover, variations in the flare activity one day before a flare event are correlated with variations in the topological complexity of the field (the Euler characteristic) in regions with high field strengths (more than 700 G). Analysis of multi-wavelength polarization observations on the RATAN-600 radio telescope during July 13?C17, 2002 indicate dominance of the radio emission above the central spot associated with the increase in flare activity. In addition to the flare site near the large spot in the group, numerous weak flares developed along an extended local neutral line, far from the central line of the large-scale field. The statistical characteristics of the magnetic-field maps analyzed were determined, and show flare activity of both types, i.e., localized in spot penumbras and above the neutral line of the field.