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.

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.

The generation of magnetic flux in the solar interior and its transport to the outer solar atmosphere will be in the focus of solar physics research for the next decades. One key-ingredient is the process of magnetic flux emergence into the solar photosphere, and the reorganization to form the magnetic phenomena of active regions like sunspots and pores.

On July 4, 2009, we observed a region of emerging magnetic flux, in which a proto-spot without penumbra forms a penumbra within some 4.5 hours. This process is documented by multi-wavelength observations at the German VTT: (a) imaging, (b) data with high resolution and temporal cadence acquired in Fe I 617.3 nm with the 2D imaging spectropolarimter GFPI, and (c) scans with the slit based spectropolarimeter TIP in Fe I 1089.6 nm. MDI contiuum maps and magnetograms are used to follow the formation of the proto-spot, and the subsequent evolution of the entire active region.

During the formation of the penumbra, the area and the magnetic flux of the spot increases. The additional magnetic flux is supplied by the adjacent region of emerging magnetic flux: As emerging bipole separate, the poles of the spot polarity migrate towards the spot, and finally merge with it. As more and more flux is accumulated, a penumbra forms. From inversions we infer maps for the magnetic field and the Doppler velocity (being constant along the line-of-sight). We calculate the magnetic flux of the forming spot and of the bipole footpoints that merge with the proto-spot. We witness the onset of the Evershed flow and the associated enhance of the field inclination as individual penumbral filaments form. Prior to the formation of individual penumbral sectors we detect the existence of ‘counter’ Evershed flows. These in-flows turn into the classical radial Evershed outflows as stable penumbra segments form.

A nonlinear force-free magnetic field extrapolation of vector magnetogram data obtained by THEMIS/MTR on 2005 May 27 suggests the simultaneous existence of different magnetic configurations within one active region filament: one part of the filament is supported by field line dips within a flux rope, while the other part is located in dips within an arcade structure. Although the axial field chirality (dextral) and the magnetic helicity (negative) are the same along the whole filament, the chiralities of the filament barbs at different sections are opposite, i.e., right-bearing in the flux rope part and left-bearing in the arcade part. This argues against past suggestions that different barb chiralities imply different signs of helicity of the underlying magnetic field. This new finding about the chirality of filaments will be useful to associate eruptive filaments and magnetic cloud using the helicity parameter in the Space Weather Science.

An assessment on the capabilities of modern spectropolarimeters and magnetographs is in order since most of our astrophysical results rely upon the accuracy of the instrumentation and on the sensitivity of the observables to variations of the sought physical parameters. A contribution to such an assessment will be presented in this talk where emphasis will be made on the use of the so-called response functions to gauge the probing capabilities of spectral lines and on an analytical approach to estimate the uncertainties in the results in terms of instrumental effects. The Imaging Magnetograph eXperiment (IMaX) and the Polarimetric and Helioseismic Imager (PHI) will be used as study cases.

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.

Recent observations of coronal loops in solar active regions show that their heating must be a truly dynamic process. Even though it seems clear that the energy source is the magnetic field that confines the coronal plasma, the details of how it dissipates are still a matter of debate. In this presentation we review the theoretical models of coronal heating, which have been traditionally clasified as DC or AC depending on the electrodynamic response of the loops to the photospheric driving motions.

Also, we show results from numerical simulations of the internal dynamics of coronal loops within the framework of the reduced MHD approximation. These simulations indicate that the application of a stationary velocity field at the photospheric boundary leads to a turbulent stationary regime after several photospheric turnover times. Once this turbulent regime is set, both DC and AC stresses dissipate at faster rates as a result of a direct energy cascade.

M dwarfs produce explosive flare emission in the near-UV and optical continuum, and the mechanism responsible for this phenomenon is not well-understood. We present a near-UV/optical flare spectrum from the rise phase of a secondary flare, which occurred during the decay of a much larger flare. The newly formed flare emission resembles the spectrum of an early-type star, with the Balmer lines and continuum in absorption. We model this observation phenomenologically as a temperature bump (hot spot) near the photosphere of the M dwarf. The amount of heating implied by our model (ΔTphot ~ 16,000 K) is far more than predicted by chromospheric backwarming in current 1D RHD flare models (ΔTphot ~ 1200 K).

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.

Ca II H imaging observations by the Hinode Solar Optical Telescope (SOT) have revealed that the chromosphere is extremely dynamic and that ejections and jets are well observed in moat region around sunspots. X-ray and EUV observations show frequent occurrence of microflaring activities around sunspots; small emerging flux or moving magnetic features approaching opposite pre-existing magnetic flux can be identified on the footpoints for half of microflares studied, while no encounters of opposite polarities are observed at footpoints for the others even with SOT high spatial magnetorams (Kano et al. 2010). Another observations tell the involvement of twisted magnetic fields in the microflares accompanied by no polarity encounters at the footpoints. Some type of sunspot light bridges shows recurrent occurrence of chromospheric ejections, and photospheric vector magnetic field data suggests that twsited magnetic flux tubes lying along light bridge play vital roles in producing such ejections (Shimizu et al. 2009). This presentation reviewed observational findings from these studies. We will need to understand the 3D configuration of magnetic fields for better understanding of activity triggers in the solar atmosphere.

Helicity measures complexity in the field. Magnetic helicity is given by a volume integral over the scalar product of magnetic field B and its vector potential A. A direct computation of magnetic helicity in the solar atmosphere is not possible due to unavailability of the observations at different heights and also due to non-uniqueness of A. The force-free parameter α has been used as a proxy of magnetic helicity for a long time. We have clarified the physical meaning of α and its relationship with the magnetic helicity. We have studied the effect of polarimetric noise on estimation of various magnetic parameters. Fine structures of sunspots in terms of vertical current (Jz) and α have been examined. We have introduced the concept of signed shear angle (SSA) for sunspots and established its importance for non force-free fields. We find that there is no net current in sunspots even in presence of a significant twist, showing consistency with their fibril-bundle nature. The finding of existence of a lower limit of SASSA for a given class of X-ray flare will be very useful for space weather forecasting. A good correlation is found between the sign of helicity in the sunspots and the chirality of the associated chromospheric and coronal features. We find that a large number of sunspots observed in the declining phase of solar cycle 23 do not follow the hemispheric helicity rule whereas most of the sunspots observed in the beginning of new solar cycle 24 do follow. This indicates a long term behaviour of the hemispheric helicity patterns in the Sun. The above sums up my PhD thesis.

The most promising model for explaining the origin of solar magnetism is the flux transport dynamo model, in which the toroidal field is produced by differential rotation in the tachocline, the poloidal field is produced by the Babcock–Leighton mechanism at the solar surface and the meridional circulation plays a crucial role. After discussing how this model explains the regular periodic features of the solar cycle, we come to the questions of what causes irregularities of solar cycles and whether we can predict future cycles. Only if the diffusivity within the convection zone is sufficiently high, the polar field at the sunspot minimum is correlated with strength of the next cycle. This is in conformity with the limited available observational data.

We combine photometric data from field stars, plus over a dozen open clusters and associations, to explore how the maximum photometric amplitude (Amax) and the distribution of amplitudes varies with stellar properties. We find a complex variation of Amax with inverse Rossby number Ro−1, which nevertheless can be modeled well with a simple model including an increase in Amax with rotation for low Ro−1, and a maximum level. Amax may then be further affected by differential rotation and a decline at the highest Ro−1. The distribution of Aspot below Amax varies with Ro−1 : it peaks at low Aspot with a long tail towards Amax for low Ro−1, but is more uniformly distributed at higher Ro−1. We investigate further dependences of the Aspot distributions on stellar properties, and speculate on the source of these variations.

Independent of the normal solar cycle, a decrease in the sunspot magnetic field strength has been observed using the Zeeman-split 1564.8nm Fe I spectral line at the NSO Kitt Peak McMath-Pierce telescope. Corresponding changes in sunspot brightness and the strength of molecular absorption lines were also seen. This trend was seen to continue in observations of the first sunspots of the new solar Cycle 24, and extrapolating a linear fit to this trend would lead to only half the number of spots in Cycle 24 compared to Cycle 23, and imply virtually no sunspots in Cycle 25.

We examined synoptic observations from the NSO Kitt Peak Vacuum Telescope and initially (with 4000 spots) found a change in sunspot brightness which roughly agreed with the infrared observations. A more detailed examination (with 13,000 spots) of both spot brightness and line-of-sight magnetic flux reveals that the relationship of the sunspot magnetic fields with spot brightness and size remain constant during the solar cycle. There are only small temporal variations in the spot brightness, size, and line-of-sight flux seen in this larger sample. Because of the apparent disagreement between the two data sets, we discuss how the infrared spectral line provides a uniquely direct measurement of the magnetic fields in sunspots.

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.

A simple nonlinear model is introduced here to describe the rotational evolution of main sequence cool (FGKM) stars. It is formulated only in terms of the ratio of a star's rotation period, P, to its convective turnover timescale, τ, and two dimensionless constants which are specified using solar- and open cluster data. The model explains the origin of the two sequences, C/fast and I/slow, of rotating stars observed in open cluster color-period diagrams, and describes their evolution from C-type to I-type through the rotational gap, g, separating them. It explains why intermediate-mass open cluster stars have the longest periods, while higher- and lower-mass cool stars have shorter periods. It provides an exact expression for the age of a rotating cool star in terms of P and τ, thereby generalizing gyrochronology. The possible range of initial periods is shown to contribute upto 128 Myr to the gyro age errors of solar mass field stars. A transformation to color-period space shows how this model explains some detailed features in the color-period diagrams of open clusters, including the shapes and widths of the sequences, and the observed number density of stars across these diagrams.

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.

The supergiant star Alpha Orionis (Betelgeuse) is the only star other than the Sun to be spatially resolved either through direct imaging or through reconstruction of interferometric observations. Centimeter-radio wavelength, infrared and ultraviolet images reveal a few bright hot spots in the photosphere and chromosphere that possess characteristics different from sunspots. Large photospheric spots on Betelgeuse appear to result from convective motions, consistent with radiative hydrodynamic modeling; the chromospheric hot spots may be produced by shock waves in the chromosphere excited by the convective motions or pulsation in the photosphere. Bright chromospheric spots that cluster around the pole of Betelgeuse could be a natural result of shock breakout in a rotating star.

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).

From radiation magnetohydrodynamic (RMHD) simulations we track the temporal evolution of a vertical magnetic flux sheet embedded in a two-dimensional non-stationary atmosphere that reaches all the way from the upper convection zone to the low chromosphere. Examining its temporal behavior near the interface between the convection zone and the photosphere, we describe the excitation of propagating longitudinal waves within the magnetic element as a result of convective motion in its surroundings.