Investigation of magnetic fields in solar active regions

The magnetic-field structure in solar active regions outside spots is studied. The line-of-sight fields were measured using the new Crimean digital magnetograph in three spectral lines—Fe I 5253 Å, Fe II 5234 Å, and Ti I 5193 Å. Observations in the Fe II 5234 Å line indicate systematically higher field strengths than those in the Fe I 5253 Å line. The magnetic fluxes in 2″ elements are ～4.3×10¹⁸ Mx, ～4.6×10¹⁸ Mx, and ～6.2×10¹⁸ Mx according to the Fe I 5253 Å, Ti I 5193 Å, and FeII 5234 Å observations, respectively. Elements 2″–8″ in size make the largest contribution to the magnetic fluxes of active regions outside spots.     

Long-term oscillations in solar active regions based on magnetic fields and radio emission

A comparative analysis of sunspot oscillations and related radio sources in the active regions AR 8949, AR 8951, and AR 8953 is carried out using SOHO MDI data and simultaneous observations with the Nobeyama Radioheliograph, with a one-minute time resolution on scales of tens to hundreds of minutes. The radio sources in the selected active regions are ～40 000–60 000 km away from the corresponding spots, with the periods of long-term oscillations of the radio sources being ～12% longer.     

Determining the spatial configurations of the coronal magnetic fields of solar active regions

The fundamental possibility of reliably removing the π ambiguity from the transverse magnetic field detected in solar vector magnetographic measurements, independent of the location of the vector magnetograms on the solar disk is demonstrated. The corrected magnetograms are then used as boundary conditions for the reconstruction of the three-dimensional magnetic field. The calculated field lines agree well with observed non-potential magnetic loops. The π ambiguity is removed using a modified Metropolis algorithm adapted to a spherical geometry. The spatial configuration of the magnetic field is calculated in a nonlinear force-free approximation using an optimization method. Tests of the new algorithm for resolving the π ambiguity are demonstrated for various model cases and comparisons with results of the NPFC method.     

Evolution of the magnetic fields of solar flare active regions from the geometry and topology of HMI/SDO magnetograms

The testing and development of topological approaches to the analysis of solar magnetic fields are considered. A technique based on the geometry of random fields, mathematical morphology and topology, and scale-space analyses are applied to describe and diagnose the pre-flare dynamics of the magnetic fields of solar active regions using HMI/SDO magnetograms. The results show that this formalism can be used to diagnose pre-flare dynamics over time intervals that are of practical interest.     

Emergence of magnetic flux at the solar surface and the origin of active regions

The evolution of photospheric velocities from the first minutes after the emergence of fresh magnetic flux and the formation of the first pores in active region NOAA 10488 is studied with a time resolution of 1 min and spatial resolution of 4″. The emerging magnetic flux of a major active region is initially a bundle of magnetic-flux loops. Some of these loops erupt through the system of supergranular cells with speeds of up to 1 km/s within 15–25 min and form pores and small spots. It is suggested that the development of a pore represents the emergence of a horizontal magnetic field, which is converted into elements with a strong vertical magnetic field. The region of ascending plasma initially coincides with the zero line of a bipolar magnetic pair. Downflow and upflow regions are related to and appear with the development of pores. During the first hours of their evolution, the trailing-polarity pores exhibit downflows with mean speeds of ∼500 m/s, while upflows with speeds of ∼250 m/s dominate near the leading-polarity pores. It is concluded that a matter flow from the leading to the trailing end is present in the rising loop of a magnetic flux tube, in agreement with well-known numerical-simulation results. The flow that develops in the magnetic-flux tube erupting through the convection zone persists when pores and small spots emerge in the photosphere, at least during the first hours of their evolution.     

Horizontal magnetic fields in the solar photosphere

Two-dimensional simulations of time-dependent solar magnetogranulation are used to analyze the horizontal magnetic fields and the response of the synthesized Stokes profiles of the IR FeI λ1564.85 nm line to the magnetic fields. The 1.5-h series of MHD models used for the analyses reproduces a region of the magnetic network in the photosphere with an unsigned magnetic flux density of 192 G at the solar surface. According to the magnetic-field distribution obtained, the most probable absolute strength of the horizontal magnetic field at an optical depth of τ ₅ = 1(τ ₅ denotes τ at λ = 500 nm) is 50 G, while the mean value is 244 G. On average, the horizontal magnetic fields are stronger than the vertical fields to heights of about 400 km in the photosphere due to their higher density and the larger area they occupy. The maximum factor by which the horizontal fields are greater is 1.5. Strong horizontal magnetic flux tubes emerge at the surface as spots with field strengths of more than 500 G. These are smaller than granules in size, and have lifetimes of 3–6 min. They form in the photosphere due to the expulsion of magnetic fields by convective flows coming from deep subphotospheric layers. The data obtained qualitatively agree with observations with the Hinode space observatory.     

Meridional drift of large-scale solar magnetic fields

It is shown that the meridional drift of large-scale fields starts in the equatorial zone and continues over 15–16 yrs (16–17 according to another estimate), i.e., during three fourths of the 22-year cycle. There is an abrupt retardation of the drift at latitudes of 30°–50°, and a stagnation region where the drift rate does not exceed several meters per second arises. The drift becomes rapid again at higher latitudes. The stagnation region coincides with the area in which the radial gradient of the rotational velocity is close to zero in the convective zone. This drift is compared with helio-seismological data on the rotation in the convective zone. A model taking into account some elements of dynamo theory is proposed.     

Peculiarities of polarized radio emission of solar active regions

Spectra of solar-flare active regions displaying peculiarities in their polarized radio emission observed on the RATAN-600 radio telescope at 2–16 GHz are considered. An appreciable dip of the circularly polarized emission (Stokes parameter V) in the middle of the microwave range (6–12 GHz), sometimes with a reversal of the sign of the polarization, is unusual. In some cases, the ordinary emission also dominates at long microwave wavelengths. Expected peculiarities of the frequency structure of microwave sources are calculated in simple models with loops in the form of hot and cool tori. Numerical calculations of these spectra show that the above features of the polarized emission can be explained by the presence of a hot region in the solar corona. It is shown that the parameters of the spectrum of the polarized emission can be used to determine the magnetic field in this hot region and the product of the relative magnetic-field gradient and the loop thickness.     

A generalized polarity rule for solar magnetic fields

It is shown that, when all components of the large-scale solar magnetic field are longitudinally averaged, the N polarity and the eastward transverse component of the B _φ field associated with both local and large-scale fields over the Northern hemisphere are somewhat stronger and occupy a smaller area during odd cycles than does the field of opposite polarity. This behavior is reversed for even cycles or the Southern hemisphere. The regular Hale law is a particular form of the above rule. The nature of this asymmetry seems to be rooted in the dynamo mechanism itself, and should be important for fields on any scale.     

Geometry of solar prominences and magnetic fields in the corona

The spatial location of the surface at which most of the prominence mass is concentrated is compared with the location of the “neutral surface” where B _r = 0 (B _r is the magnetic field) calculated in a potential approximation using photospheric data. More than fifty prominences (filaments) observed in 1999–2003 are studied. The vertical deviations of the prominences (predominantly toward the west) correspond well to the inclination of the neutral surface. The results provide evidence for the magnetic support of filaments of opposite polarities (the magnetic-rope model).     

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.     

Stokes-meter observations of the solar mean magnetic field: Probable manifestations of strong small-scale magnetic fields

For many years, information on the solar mean magnetic field (SMMF) of the Sun—an important heliophysical and astrophysical parameter—was restricted to magnetographic measurements in only one spectral line, FeI λ525.02 nm. More informative observations of the Stokes-meter parameters of the SMMF were first initiated on a regular basis at the Sayan Solar Observatory. The availability of I and V data obtained simultaneously in several spectral lines has made it possible to study fundamentally new physical problems. In this paper, based on a comparison of SMMF observations in several spectral lines, we find high correlations in the data and important systematic differences in the magnetic-field strength B, which we interpret as a manifestation of kilogauss magnetic fields in fine-structure magnetic elements. Results of theoretical modeling of the SMMF strength ratios for the FeI λ525.02 nm-FeI λ524.70 nm and FeI λ630.15 nm-FeI λ630.25 nm lines are presented. The asymmetries of the V profiles of four lines near the FeI λ525.02 nm line are examined; these lines are important diagnostics for studies of small-scale dynamical processes. The Sayan Solar Observatory SMMF measurements are in good consistency with the Wilcox Solar Observatory data for 2003: for a comparison of N = 137 pairs of points in the two data sets, the correlation coefficient ρ is 0.92 for the linear regression between the datasets B_WSO = 0.03(±0.05) + 0.93(±0.03)B_SSO.     

Quasi-periodicity of MgXII X-ray bursts revealed by CORONAS-F SPIRIT data for solar active regions

A spectral analysis of a series of integrated MgXII 8.42 Å X-ray intensities recorded by the CORONAS-F SPIRIT spectroheliometer is presented. Statistically significant peaks for periods in the intervals 12–30 min and 40–200 min were found in the power spectra. The power spectrum for these periods changed after the emergence of new photospheric magnetic flux in the active region NOAA 9840.     

Pre-flare changes in the turbulence regime for the photospheric magnetic field in a solar active region

Observations of the total magnetic field in the active region NOAA 6757 have been used to study the turbulence regime from 2.5 h before the onset of a 2B/X1.5 flare until two minutes after its maximum. The curvature of the exponent ζ(q) for the structure functions of the B _z field increases monotonically before the flare (i.e., the multifractal character of the B _z field becomes more complex) but straightens at the flare maximum and coincides with a linear Kolmogorov dependence (implying a monofractal structure for the B _z field). The observed deviations of ζ(q) from a Kolmogorov line can be used for short-term forecasting of strong flares. Analysis of the power spectra of the B _z field and the dissipation of magnetic-energy fluctuations shows that the beginning of the flare is associated with the onset of a new turbulence regime, which is closer to a classical Kolmogorov regime. The scaling parameter (cancellation index) of the current helicity of the magnetic field, k _h , remains at a high level right up until the last recording of the field just before the flare but decreases considerably at the flare maximum. The variations detected in the statistical characteristics of the turbulence can be explained by the formation and amplification of small-scale flux tubes with strong fields before the flare. The dissipation of magnetic energy before the flare is primarily due to reconnection at tangential discontinuities of the field, while the dissipation after the flare maximum is due to the anomalous plasma resistance. Thus, the flare represents an avalanche dissipation of tangential discontinuities.     

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.     

Positional evolution of magnetic foci in the polar regions of the Sun with the phase of the solar activity cycle

The behavior of magnetic foci—intersection points of tangents to ray structures in the polar corona of the Sun—is studied. This behavior reflects the evolution of the large-scale magnetic field near the poles, and is one of only a few sources of information on the polar magnetic field of the Sun. For the first time, the positions of the magnetic foci are plotted as a function of the solar-cycle phase for a full cycle, based not only on total-eclipse images but also on daily coronal observations in the FeIX and FeX (171 Å) lines carried out with the EIT telescope on the SOHO satellite. The temporal behavior of the foci over a cycle differs appreciably from that inferred from isolated observations during eclipses. The curve agrees fairly well with the model of the global-field evolution suggested by us previously.     

Effect of magnetic fields on leveling

Normally, the establishment of national leveling networks was for the purpose of supporting national mapping programs of engineering projects. They are also used as a base on which to establish national gravity networks. During recent years, much study and investigation have been done concerning the characteristics and capabilities of these networks to provide data for determining height velocity changes due to crustal movements.

In the early 1960's the use of automatic (compensator) leveling instruments became common for establishing precise geodetic leveling networks. In 1981, at FIG, Montreux, Professor W.E. Rumpt and Mr. H. Meurisch reported the influence of DC-AC magnetic fields on the line of sight of compensator instruments. As a result, several manufacturers modified their automatic leveling instruments to minimize the AC-DC magnetic effect. Wild Heerbrugg, whose unmodified NA-2 level was least affected by magnetic fields, further reduced the effect by shielding it's compensator.

At the very least, the impact of this discovery will cause national geodetic agencies to modify their existing automatic leveling equipment and possibly embark on a releveling or reanalysis of their existing network. When studying micro/macro earth crustal movements, the geodesist must take this new factor into account in analysing data.     

The electric fields of radio pulsars with asymmetric nondipolar magnetic fields

The effect of the curvature of open magnetic field lines on the generation of electric fields in radio pulsars is considered in the framework of a Goldreich-Julian model, for both a regime with a free outflow of electrons from the neutron-star surface and the case of a small thermoemission current. An expression for the electron thermoemission current in a strong magnetic field is derived. The electric field associated with the curvature of the magnetic flux tubes is comparable to the field generated by the relativistic dragging of the inertial frames.     

Localization of magnetic reconnection from the differential characteristics of magnetic fields

Magnetic reconnection for arbitrary magnetic fields without null points is analyzed in the absence of plasma. A potential model field for four sources is considered. A method for the localization of probable reconnection regions using computable differential characteristics of magnetic fields is proposed. Some separator properties are examined, and separator regions are found for the best-known reconnection conditions.