We study the distribution of magnetic shear in an emerging flux region using the high-resolution Hinode/SOT SP observations. The distribution of mean magnetic shear angle across the active region shows large values near region of flux emergence i.e., in the middle of existing bipolar region and decreases while approaching the periphery of the active region.

The magnetohydrodynamic (MHD) sausage tube waves are excited in the magnetic flux tubes by p-mode forcing. These tube waves thus carry energy away from the p-mode cavity which results in the deficit of incident p-mode energy. We calculate the loss of incident p-mode energy as a damping rate of f- and p-modes. We calculate the damping rates of f- and p-modes by a model Sun consisting of an ensemble of many thin magnetic flux tubes with varying plasma properties and distributions. Each magnetic flux tube is modelled as axisymmetric, vertically oriented and untwisted. We find that the magnitude and the form of the damping rates are sensitive to the plasma-β of the tubes and the upper boundary condition used.

In this paper, we compare components of the horizontal flow below the solar surface in and around regions consisting of rotating and non-rotating sunspots. Our analysis suggests that there is a significant variation in both components of the horizontal flow at the beginning of sunspot rotation as compared to the non-rotating sunspot. The flows in surrounding areas are in most cases relatively small. However, there is a significant influence of the motion on flows in an area closest to the sunspot rotation.

We observe the acoustic velocity oscillations in and near active region NOAA 10960 on 8 June, 2007 using observations from the IBIS instrument at the Dunn Solar Telescope at NSO/Sacramento Peak and simultaneous Hinode BFI/SP data. Inversions were performed on the spectropolarimetric datasets in order to get magnetic field information for the AR. A time series of Doppler maps from line bisectors and Stokes V zero-crossing was constructed and allowed us to construct power maps for the AR. Past works by various authors have shown that acoustic power in the solar atmosphere is strongly influenced by magnetic field strength and inclination. Our study also explores this, but in addition, we also discuss the role of oscillations due to purely magnetized gas in the photosphere. Our preliminary results for this study are presented.

Assuming that the torsional oscillation is driven by the Lorentz force of the magnetic field associated with the sunspot cycle, we use a flux transport dynamo to model it and explain its initiation at a high latitude before the beginning of the sunspot cycle.

Motivated by increasingly more advanced solar observations, we recently develop a method of coronal magnetic field extrapolation, especially for an active region (sunspot region). Based on a more complex variational principle, the principle of minimum (energy) dissipation rate (MDR), we adopt and solve a more complex equation governing the coronal magnetic field that is non-force-free in general. We employ the vector magnetograms from multiple instruments, including Hinode, NSO, and HSOS, and particularly observations at both photospheric and chromospheric levels for one active region. We discuss our results in the context of quantitative characterization of active region magnetic energy and magnetic topology. These quantitative analyses aid in better understanding and developing prediction capability of the solar activity that is largely driven by the solar magnetic field.

An urgent problem in modern solar physics, which is not completely solved up to now, is to obtain realistic magnetic field strength values from parameters measured magnetographs or Stokes-meter instruments. One of the important tools on this way is a comparison of observations made in different spectral lines with the same or with the different telescopes. This issue is an actual task in the analysis of the new data sets provided by the space missions SOHO and Hinode, which measurements are available for several years already, and SDO, which data appeared recently. The main aim of this study is a cross-comparison of magnetic field observations made in different spectral lines used on the above mentioned space observatories: Ni i λ676.77 nm (SOHO/MDI), Fe i λ630.152 nm and Fe i λ630.25 nm (Hinode/SP), and Fe i λ617.33 nm (SDO/HMI). Full-disk high-precision Stokes-meter measurements with the STOP telescope at the Sayan observatory in these lines are used basically, as well as some observations in other spectral lines having a great diagnostic impact, such as Fe i λ525.02 nm, Fe i λ523.29 nm and Fe i λ532.42 nm. The difference between one-instrument (STOP) simultaneous or quasi-simultaneous observations in different spectral lines do not exceed the factor of 2-3 depending on the combination of spectral lines and the position on the solar disk. This is significantly less than in some other studies devoted to cross-comparison of different data sets. Importance and consequences of the obtained results are discussed.

Numerical computations of wave propagation through sunspot-like magnetic field structures are critical to developing and testing methods to deduce the subsurface structure of sunspots and active regions. We show that helioseismic analysis applied to the MHD sunspot simulations of Rempel and collaborators, as well as to translation-invariant models of umbral-like fields, yield wave travel-time measurements in qualitative agreement with those obtained in real sunspots. However, standard inversion methods applied to these data fail to reproduce the true wave-speed structure beneath the surface of the model. Inversion methods which incorporate direct effects of the magnetic field, including mode conversion, may be required.

We discuss recent progress in the helioseismic probing of the subsurface structure of solar magnetic regions. To simulate the interaction of helioseismic waves with magnetic fields and thermal perturbations we use a simple model that is translation invariant in the horizontal directions, has a realistic stratification in the vertical direction, and has physically consistent boundary conditions for the waves at the upper and lower boundaries of the computational domain. Using this model we generate synthetic helioseismic data and subsequently measure time-distance travel times. We evaluate a model for the wave-speed perturbation below sunspots that replaces the sound speed in a non-magnetic model by the fast-mode speed from a magnetic model; our results indicate that this approach is unlikely to be useful in modeling wave-speed perturbations in sunspots. We develop and test an inversion algorithm for inferring the sound-speed perturbation in magnetic regions. We show that this algorithm retrieves the correct sound-speed perturbation only when the sensitivity kernels employed account for the effects of the magnetic field on the waves and the subsurface structure.

We present a study of the temporal changes in the sensitivities of the frequencies of the solar p-mode oscillations to corresponding changes in the levels of solar activity during Solar Cycle 23. From MDI and GONG++ full-disk Dopplergram three-day time series obtained between 1996 and 2008 we have computed a total of 221 sets of m-averaged power spectra for spherical harmonic degrees ranging up to 1000. We have then fit these 284 sets of m-averaged power spectra using our WMLTP fitting code and both symmetric Lorentzian profiles for the peaks as well as the asymmetric profile of Nigam and Kosovichev to obtain 568 tables of p-mode parameters. We then inter-compared these 568 tables, and we performed linear regression analyses of the differences in p-mode frequencies, widths, amplitudes, and asymmetries as functions of the differences in as many as ten different solar activity indices. From the linear regression analyses that we performed on the frequency difference data sets, we have discovered a new signature of the frequency shifts of the p-modes. Specifically, we have discovered that the temporal shifts of the solar oscillation frequencies are positively correlated with the changes in solar activity below a limiting frequency. They then become anti-correlated with the changes in activity for a range of frequencies before once again becoming positively-correlated with the activity changes at very high frequencies. We have also discovered that the two frequencies where the sensitivities of the temporal frequency shifts change sign also change in phase with the average level of solar activity.

The striking similarity between the cyclic equatorward migration of the torsional oscillation belts and the sunspot activity latitudes inspired several attempts to seek an explanation of the torsional phenomenon in terms of interactions between flux ropes and plasma motions. The aim of the present work is to examine the spatial and temporal coincidences of the torsional waves and the emergence of sunspot magnetic fields. The locations of the shearing latitudes have been compared with the distributions of certain sunspot parameters by using sunspot data of more than two cycles. The bulges of the sunspot number and area distributions tend to be located within the ‘forward’ belts close to their poleward shearing borders. A possible geometry of the magnetic and velocity field interacion is proposed.

Collision of the magnetic flux tubes in the Quiet Sun was proposed as one of the possible sources for the heating of the solar atmosphere (Furusawa and Sakai, 2000). The solar photosphere was observed using the New Solar Telescope ad Big Bear Solar Observatory. In TiO spectral line at 705.68 nm we approached resolution of 0.1″. The horizontal plasma wave was observed spreading from the larger bright point. Shorty after this wave an increase in the oscillatory power appeared at the same location as the observed bright point. This behavior matches some of the results from the simulation of the collision of the two flux tubes with a weak current.

The primary task of the Debrecen Observatory is the most detailed, reliable and precise documentation of the solar photospheric activity. This long-term effort started with the continuation of the Greenwich photoheliograph program, this is the Debrecen Photoheliographic Data (DPD) sunspot catalogue based on ground-based observations. The profile of the work has later been extended to space-borne observations (SOHO/MDI and SDO), to magnetic fields and faculae as well as to higher temporal resolution (one hour) and nearly real-time data supply. The database also includes historical observations. The web-presentation developed for the material is easy to search and browse. We describe the main characteristics of these catalogs, and their advantages. We summarize the recent advances in the procedure of their compilation, and the available sets of the data and images.

Millimeter emission is known to be a sensitive diagnostic of temperature and density in the solar chromosphere. In this work we use millimeter wave data to distinguish between various atmospheric models of sunspots, whose temperature structure in the upper photosphere and chromosphere has been the source of some controversy. From mm brightness simulations we expect a radio umbra to change its appearance from dark to bright (compared to the Quiet Sun) at a given wavelength in the millimeter spectrum (depending on the exact temperature in the model used). Thereby the millimeter brightness observed above an umbra at several wavelengths imposes strong constraints on temperature and density stratification of the sunspot atmosphere, in particular on the location and depth of the temperature minimum and the location of the transition region. Current mm/submm observational data suggest that brightness observed at short wavelengths is unexpectedly low compared to the most widely used sunspot models such as of Maltby et al. (1986). A successful model that is in agreement with millimeter umbral brightness should have an extended and deep temperature minimum (below 3000 K), such as in the models of Severino et al. (1994). However, we are not able to resolve the umbra cleanly with the presently available observations and better resolution as well as better wavelength coverage are needed for accurate diagnostics of umbral brightness at millimeter wavelengths. This adds one more scientific objective for the Atacama Large Millimeter/Submillimeter Array (ALMA).

Evolution of sunspot structure and photospheric magnetic fields are important to understand how the flare energy is built up and released. With high-resolution optical data, it is possible to examine in details the optical flows of the photosphere and their relationship to the flaring process. Using G-band and Stokes-V data obtained with Hinode Solar Optical Telescope (SOT), we study the sunspot motion and flow fields associated with the 2006 December 13 X3.4 flare in NOAA AR 10930. We calculate the centroids of the delta spot umbrae lying in opposite magnetic polarities, and use two different methods to derive the photospheric flow fields of the AR. We find that the shearing motion before the flare changes to unshearing motion associated with the eruption. A decrease of average velocity of shear flow is found to be associated with the flare, with a magnitude of 0.2 km s−1.

As a related study, we also test implementing the recently developed differential affine velocity estimator for vector magnetograms (DAVE4VM; Schuck, P. W 2008) technique for the magnetic field observations obtained by the Big Bear Solar Observatory (BBSO) and Helioseismic Magnetic Imager (HMI) on board the Solar Dynamic Observatory (SDO). Using this method to analyze changes of active region magnetic fields associated with flares may shed new light on the cause and effect of flaring process.

How magnetic field structure changes with eruptive events (e.g., flares and CMEs) has been a long-standing problem in solar physics. Here we present the analysis of eruption-associated changes in the magnetic inclination angle, the transverse component of magnetic field and the Lorentz force. The analysis is based on an observation of the X3.4 flare on Dec.13 2006 and a numerical simulation of a solar eruption made by Yuhong Fan. Both observation and simulation show that (1) the magnetic inclination angle in the decayed peripheral penumbra increases, while that in the central area close to flaring polarity inversion line (PIL) deceases after the flare; (2) the transverse component of magnetic field increases at the lower altitude near flaring PIL after the flare. The result suggests that the field lines at flaring neutral line turn to more horizontal near the surface, that is in agreement with the prediction of Hudson, Fisher & Welsch (2008).

Numerical 3D simulations of MHD waves in magnetized regions with background flows are very important for the understanding of propagation and transformation of waves in sunspots. Such simulations provide artificial data for testing and calibration of helioseismic techniques used for analysis of data from space missions SOHO/MDI, SDO/HMI, and HINODE. We compare with helioseismic observations results of numerical simulations of MHD waves in different models of sunspots. The simulations of waves excited by a localized source provide a detailed picture of the interaction of the MHD waves with the magnetic field and background flows (deformation of the waveform, wave transformation, amplitude variations and anisotropy). The observed cross-covariance function represents an effective Green's function of helioseismic waves. As an initial step, we compare it with simulations of waves generated by a localized source. More thorough analysis implies using multiple sources and comparison of the observed and simulated cross-covariance functions. We plan to do such calculations in the nearest future. Both, the simulations and observations show that the wavefront inside the sunspot travels ahead of a reference “quiet Sun” wavefront, when the wave enters the sunspot. However, when the wave passes the sunspot, the time lag between the wavefronts becomes unnoticeable.

Today's Solar Physics comes across of different type of fine structures in solar atmosphere including umbral dots and penumbral grains in sunspots, and G-band bright points in quiet Sun. In this report, we present evidence that umbral dots, penumbral grains, and, possibly, G band bright points are related to a common type of features in solar atmosphere magnetic flux tubes.

We propose that at the beginning of the Maunder minimum the poloidal field or amplitude of meridional circulation or both fell abruptly to low values. With this proposition, a flux transport dynamo model is able to reproduce various important aspects of the historical records of the Maunder minimum remarkably well.

We have derived the latitude motions of sunspots classified into three area categories using the measures of positions and areas of their umbrae from the white – light images of the Sun for the period 1906 – 1987 from the Kodaikanal Observatory archives. The latitude motions are directed equator – ward in all the three area classes. We interpret that these equator – ward latitude motions reflect the meridional flows at the three depths in the convection zone where the magnetic flux loops of the spots of the three area classes are anchored. We obtain estimates of the anchor depths through a comparison of the rotation rates of the spots in each area class with the rotation rate profiles from helioseismic inversions. The equator – ward flows measured by us thus provide evidence of the return meridional flows in the convection zone as required in the flux transport solar dynamo models. We have done an identical analysis using a similar data set derived from the photoheliogram collections of the Mt.Wilson Observatory for the period 1917 – 1985. There is good agreement between the results from the data sets of the two observatories.