Active longitudes and north-south asymmetry of the activity the Sun as manifestations of its relic magnetic field

The distributions of dominant magnetic polarities in synoptic maps of photospheric magnetic fields and their extrapolations to the corona based on Stanford Observatory data are studied. Both dipolar and quadrupolar magnetic patterns are detected in the distributions of dominant polarities in the near-equatorial region of the photosphere for activity cycles 21, 22, and 23. The field in these patterns often has opposite signs on opposite sides of the equator, with this sign changing from cycle to cycle. A longitude-time analysis of variations of the mean solar magnetic field shows that the contribution of the large-scale magnetic patterns to the total field does not exceed 20 µT. The most stable magnetic structures at a quasi-source surface in the solar corona are separated by approximately 180° in heliographic longitude and are close to dipolar. The nature and behavior of these large-scale magnetic patterns are interpreted as a superposition of cyclic dynamo modes and the nonaxially symmetric relic field of the Sun. The contribution of the relic field to the mean solar magnetic field appears as a weak but stable rotational modulation whose amplitude does not exceed 8 µT.     

Parameters of the turbulent magnetic field in the solar photosphere: Power spectrum of the line-of-sight field

Ground-based (Big Bear Solar Observatory) and extra-atmospheric (SOHO/MDI) measurements of the photospheric line-of-sight magnetic field of one active and two quiet regions are used to calculate power spectra of the field, taking into account the characteristic function for the diffraction limit of the telescope resolution. At high frequencies, the physically meaningful linear interval in the spectrum extends to a wave number of k=4.6 Mm^?1 (spatial scale l=1.4 Mm) for the quiet regions and k=3.35 Mm^?1 (l=1.9 Mm) for the active region. A high-frequency spectral break at k≥3 Mm^?1 is associated with the characteristic telescope function; the position of the break and the spectral slope beyond the break do not reflect the turbulent state of the field. As the field recording improves, the break shifts toward higher frequencies. The spectral indices in the physically meaningful linear interval are substantially different for the active and quiet regions: in the active region (NOAA 8375), the spectrum behaves as E(k)≈k ^?1.7 (very close to the Kolmogorov index, ?5/3) in the interval 0.78≤k≤3.35 Mm^?1, while in the quiet regions E(k)≈k ^?1.3 for 0.77≤k≤4.57 Mm^?1. This difference can be explained by the additional effect of a small-scale turbulent dynamo in the unperturbed photosphere. In this case, this mechanism can generate at least 6% of the magnetic energy of the photospheric line-of-sight field in quiet regions.     

Magnetic-field variations in the active region NOAA 10486 and their relationship to X-ray flares and coronal mass ejections

SOHO/MDI magnetograms are used to analyze the time variations in the magnetic parameters of the active region (AR) NOAA 10486, which was part of a large activity complex that passed over the solar disk from October 26 to 31, 2003, during solar cycle 23. The results are compared with X-ray flares in the AR and the parameters of coronal mass ejections associated with the AR. The time variations in the distributions of themagnetic-field strengths associated with the total magnetic flux (Fa), the flux imbalance between the northern and southern polarities (Im), the complexity of the field, as a measure of the mutual overlapping of the opposite polarities (Co), and the tilt angle of the magnetic axis (An) are considered. The time variations in the free energy accumulated in current sheets of ARs were traced using a parameter introduced for this purpose (Sh). The following results were obtained. First, the parameters Fa, Im, Co, An, and Sh quantitatively describe the current state of the AR and can be used to trace and analyze the dynamical evolution of its magnetic field. Second, variations in the magnetic-field-strength distributions and the mean values of Fa, Im, Co, An, and Sh are associated with flares and coronal mass ejections, and the variations have considerable amplitudes. Third, the parameter Sh characterizing the degree to which the magnetic field is non-potential in regions adjacent to the main neutral line increases before eruptive events, and is thus particular interest for monitoring the states of ARs in real time. Fourth, the magnetic field of the AR manifests a sort of quasi-elasticity, so that the field structure is restored after active events, on average, within 1–3 h.     

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.     

Cyclic evolution of the global magnetic field observed during the 21st and 22nd solar cycles

The global component of the large-scale magnetic field is identified using two methods applied to Stanford Observatory data on the photospheric magnetic field obtained in 1976–2000. Two significantly different phases are observed in the evolution of the global magnetic field detected during the 11-year cycle. Phase I includes the cycle growth and maximum, while Phase II includes the cycle decay and minimum. During Phases I and II of the 11-year cycle, two different processes dominate both the magnetic-field generation and the solar activity as a whole. At lower latitudes, Phase I demonstrates a longitudinal splitting of the magnetic field, which takes the form of stripes of opposite polarities located parallel to the equator. The long dimensions of these stripes are grouped near 90° and 180°. The global magnetic field rotates as a rigid body with a period that is appreciably shorter than the Carrington-rotation period and reaches about 27.225 days. During reversals of the polarity of the global magnetic field, the field displays opposite signs in antipodal longitude intervals with lengths of about 135°. Thus, the equatorial dipolar field is clearly manifest during Phase I. Transitional longitude intervals with widths of 45° located between 135° intervals correspond to the positions of active longitudes of sunspots, indicating a close relationship between the global and local magnetic fields.     

The pattern of supergranular in a solar active region and the formation of filaments

The formation of filaments in solar bipolar active regions is investigated, giving particular attention to the relationship between this process and the pattern of supergranular convection. SOHO MDI and Kitt Peak magnetograms and Hα filtergrams are used. The large decaying active region NOAA 8525 is considered over the period May 4–7, 1999. The boundaries of supergranules are identified as concentrations of the line-of-sight photospheric field in magnetograms. Filaments in the central part of the active region are studied; as a whole, they are aligned with the supergranule boundaries. Variations in the magnetic field in this period were manifest primarily in the form of “cancellations” and spatial-redistribution processes consistent with the pattern of developing supergranules. These factors created the conditions necessary for the formation of a filament stretched across the entire active region; i.e., the straightening of the polarity-inversion line and reduction of the horizontal gradients of the magnetic field. One possible explanation of the results is that the magnetic-field component along the filament axis is associated with the vortical structure of horizontal flows in the supergranulation cells.     

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.     

The dynamics of photospheric line-of-sight velocities in emerging active regions

The emergence of photospheric magnetic fields and the dynamics of the associated pattern of vertical motions in a developing active region are studied based on SOHO/MDI data. Objects were selected for which complete time series of data were available, so as to make it possible to determine the onset time of the magnetic-field emergence at the surface and tracing the formation of the first pores. The active regions studied originated near the central meridian. The total area of sunspots in these regions exceeded 100 millionths of the hemisphere at the maximum of active region evolution. A generalized evolutionary scenario is constructed for the magnetic field and vertical motions in the emerging active region. An asymmetry in the Doppler velocities is noted at an early stage of the active-region development, which corresponds to a matter flow from the leading to the trailing end of the emerging Ω-shaped tube. A direct relationship is found between the matter-downflow velocity in the area of the pore development and the growth in the strength of the longitudinal magnetic field.     

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.     

Propagation of a fast magnetoacoustic shock wave in the magnetosphere of an active region

The propagation of a fast magnetoacoustic shock wave the magnetosphere of a solar active region is considered the nonlinear geometrical acoustics approximation. The magnetic field is modeled as a subphotospheric magnetic dipole embedded in the radial field of the quiet corona. The initial parameters of the wave are specified at a spherical surface in the depths of the active region. The wave propagates asymmetrically and is reflected from regions of the strong magnetic field, which results in the radiation of the wave energy predominantly upwards. Substantial gradients in the Alfvén speed facilitate appreciable growth in the wave intensity. Non-linear damping of the wave and divergence of the wave front lead to the opposite effect. Analysis of the joint action of these factors shows that a fast magnetoacoustic perturbation outgoing from an active region can correspond to a shock wave of moderate intensity. This supports the scenario in which the primary source of the coronal wave is an eruptive filament that impulsively expands in the magnetosphere of an active region.     

The structure of the photospheric velocity field near Hα filaments

The velocity field of the plasma in the solar photosphere beneath chromospheric Hα filaments is studied. Observations were conducted in 1999–2000 using the magnetograph and tachometer of the tower telescope of the Institute of Terrestrial Magnetism, Ionosphere, and Radio Propagation, recently upgraded to improve both its sensitivity and spatial resolution. The results confirm that, as noted earlier, filaments are frequently found near velocity-inversion lines between regions of upward and downward motion of solar material, and lie predominantly above regions with upward motion of photospheric material. This tendency is characteristic of both the stable filaments of active regions and quiescent filaments far from active regions, though it is more distinct for the former case. The upward motion of photospheric material beneath filaments may play an important role in supporting the filaments against gravity.     

Twist of magnetic fields in solar active regions

We study the twist properties of photospheric magnetic fields in solar active regions using magnetographic data on 422 active regions obtained at the Huairou Solar Observing Station in 1988–1997. We calculate the mean twist (force-free field α_f) of the active regions and compare it with the mean current-helicity density of these same active regions, h _c =B _∥·(?×B)_∥. The latitude and longitude distributions and time dependence of these quantities is analyzed. These parameters represent two different tracers of the α effect in dynamo theory, so we might expect them to possess similar properties. However, apart from differences in their definitions, they also display differences associated with the technique used to recalculate the magnetographic data and with their different physical meanings. The distributions of the mean α_f and h _c both show hemispherical asymmetry—negative (positive) values in the northern (southern) hemisphere—although this tendency is stronger for h _c. One reason for these differences may be the averaging procedure, when twists of opposite sign in regions with weak fields make a small contribution to the mean current-helicity density. Such transequatorial regularity is in agreement with the expectations of dynamo theory. In some active regions, the average α_f and h _c do not obey this transequatorial rule. As a whole, the mean twist of the magnetic fields α_f of active regions does not vary significantly with the solar cycle. Active regions that do not follow the general behavior for α_f do not show any appreciable tendency to cluster at certain longitudes, in contrast to results for h _c noted in previous studies. We analyze similarities and differences in the distributions of these two quantities. We conclude that using only one of these tracers, such as α_f, to search for signatures of the α effect can have disadvantages, which should be taken into account in future studies.     

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).     

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.     

The three-dimensional structure of the corona above active region NOAA 9591 from microwave observations

We study the active region NOAA 9591 using observations at 1.92–10.17 cm obtained on two large Russian radio instruments: the RATAN-600 radio telescope and the Siberian Solar Radio Telescope. The active region was associated with an isolated spot at the photospheric level, whose magnetic field had a well-defined Delta configuration. The radio observations show that the structure of the coronal source located above the spot cannot be described in a simple (unipolar) cyclotron model. A comparison with X-ray observations indicates that the three-dimensional structure of the corona above the spot can be represented as a strongly elongated loop whose apex resembles a cusp brightening (a qualitative model for the structure is presented). Unexpectedly, radiation with a high degree of polarization (～25%) was detected far (～100 000 km) from the photosphere. The need for a quantitative model for coronal sources above the strong Delta-configuration magnetic fields, which are known to play an important role in active solar processes (flares, phenomena such as coronal-mass ejections) is outlined. Thanks to its simple morphology, which enabled the identification of a pure Delta configuration, the active region NOAA 9591 provides high-quality observational material for the creation of such a model.     

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.     

Sources of the global magnetic field of the Sun

Data on the global magnetic field (GMF) of the Sun as a star for 1968–1999 are used to determine the correlation of the GMF with the radial component of the interplanetary magnetic field (IMF) |B _r|; all data were averaged over a half year. The time variations in the GMF |H| are better correlated with variations in |B _r|; than the results of extrapolating the field from the “source surface” to the Earth’s orbit in a potential model based on magnetic synoptic maps of the photosphere. Possible origins for the higher correlation between the GMF and IMF are discussed. For both the GMF and IMF, the source surface actually corresponds to the quiet photosphere—i.e., background fields and coronal holes—rather than to a spherical surface artificially placed ≈2.5 R _⊙ from the center of the Sun, as assumed in potential models (R _⊙ is the solar radius). The mean effective strength of the photospheric field is about 1.9 G. There is a nearly linear dependence between |H| and |B _r|. The strong correlation between variations in |H| and |B _r| casts doubt on the validity of correcting solar magnetic fields using the so-called “saturation” factor δ^?1 (for magnetograph measurements in the λ 525.0 nm FeI line).     

The altitude structure of the coronal magnetic field of AR 10933

The magnetic fields of solar active regions are analyzed using a method based on comparing the spatial structures of the reconstructed magnetic field and of the radio emission of the active region. Two approaches are used: comparing the radio size of the active region and the corresponding size calculated using the reconstructed magnetic field, and comparing the radio spectra that are observed and calculated using the reconstructed magnetic field. Overall, the calculated sizes and spectra correspond fairly well to the observational data, making it possible to estimate physical parameters of the emitting region, such as the electron density and temperature.     

Some properties of the joining of solar filaments

Some possibilities for the reconnection of magnetic-field lines of solar filaments that approach when the photospheric polarity inversion lines change their positions, are discussed. The interaction between filaments depends on their internal properties, which are determined by the filament chirality, or the sign of the helicity of the filament magnetic field. In quadrupolar magnetic configurations, filaments with the same chirality can exchange their halves. Filaments with opposite chirality rupture after the reconnection of the polarity inversion lines, since the two fragments of the different filaments cannot be connected continuously. The morphology and connectivity of the filaments are analyzed using daily Hα filtergrams obtained over the period of maximum activity of the 23rd solar cycle. Examples of alterations of the filament connectivity occuring during the evolution of photospheric fields are presented.     

Tectonic deformation and subsurface active fault trends of El-Faiyum province,Egypt as deduced by seismicity and potential field data

We utilized the seismicity and the potential field data to study the tectonic deformation and to delineate the seismically active subsurface tectonic trends of El-Faiyum area. To accomplish these goals, we analyzed and interpreted the seismicity data, the reduced to pole total magnetic intensity, and the Bouguer anomaly maps. We also used the spatial distribution of the recent seismic events and the focal mechanism to outline the local seismic zones that control the seismicity of the study area and to determine the sense of the motion along the subsurface active faults. The focal mechanism of the recent seismic events and the interpreted subsurface tectonic faults from the potential field data reflect strike-slip movements with normal components along the subsurface active faults. The gravity and magnetic maps show a NE-SW regional trend with low gravity and magnetic values. The NE-SW regional trend extends across the whole area and could be related to the Pelusium Megashear fault. A NE-SW trend with high gravity and magnetic exists at the northern part and coincides with the Kattania Uplift and the basaltic flows in Gabal Qatrani area. The gravity and magnetic maps also reveal several local anomalies with different polarities, amplitudes, and extensions, which reflect anticlinal and synclinal structures on the basement surface. The seismotectonic map, generated by linking the basement structure map and the spatial distribution of the recent earthquake foci, reveals the dominant tectonic trends and the subsurface active faults.