In this report, we present our recent effort to understand the cyclic behavior of network magnetic elements based on the unique database from full-disk observations provided by Michelson Doppler Imager on board the Solar and Heliospheric Observatory in the interval including the entire cycle 23. The following results are unclosed. (1) The quiet regions dominate the solar magnetic flux for about 8 years in solar cycle 23, and from the solar minimum to maximum they contribute (0.94-1.44)×1023Mx flux to the solar photosphere. In the entire cycle 23, the magnetic flux of the quiet regions is 1.12 times that of active regions. The occupation ratio of quiet region flux equally characterizes the course of a solar cycle. (2) With the increasing magnetic flux per element, the variations of numbers and total flux of the network elements show three-fold scenario: no-correlation, anti-correlation, and correlation with sunspots, respectively. The anti-correlated elements covering the range of (3-32)×1018Mx occupy 77% of total element number and 37% of quiet Sun flux. (3) The time-latitude distribution of anti-correlated magnetic elements is out of phase with that of sunspots, while the correlated elements display the similar butterfly diagram of sunspots but with wider latitude distribution. These results imply that the correlated elements are the debris of decayed sunspots, and the source of anti-correlated elements is modulated by sunspot magnetic field.

Magnetoconvection in a horizontal layer of incompressible fluid is simulated numerically. The initial magnetic field is assumed to be uniform and horizontal. The interaction of quasi-ordered cellular convection with the magnetic field is shown to be able to produce bipolar (and also diverse more complex) configurations of a substantially amplified magnetic field. The operation of this mechanism, which can be regarded as a modification of the mechanism suggested by Tverskoi (1966), is controlled by the very topology of the cellular flow, should be manifest on various spatial scales, and does not require strong initial fields. Magnetic configurations develop both in the central parts of convection cells, where circulatory fluid motion “winds” magnetic field lines, and in the network formed by their peripheral regions due to the “sweeping” of magnetic field lines.

Quiet-Sun magnetic fields are enigmatic in terms of their properties, and their origin is not well understood. One likely possibility is that they are a consequence of interactions with turbulent convective motions of various temporal and spatial scales. Here we investigate the relationship between small-scale magnetic fields and various convection flows. We demonstrate that in addition to granulation and supergranulation, mesogranulation also plays an important role in structuring quiet-Sun magnetic fields. We also study the vector magnetic fields in the quiet Sun, and propose that emerging granular-scale bipolar loops are major sources of the quiet-Sun magnetic fields.

Helicity characteristics in active regions (ARs) are studied, using so far the most accurate vector magnetic field measurements obtained with SP/Hinode. Our sample includes all ARs observed by SP/Hinode, up to June 2012. The sample is divided into three sub-samples: Cycle 23 (from 2006.11 to 2008.06), Cycle 24a (from 2008.10 to 2010.09) and Cycle 24b (from 2010.10 to 2012.06). We confirm our previous findings that the usual hemispheric helicity sign rule is not obeyed in the descending phase of solar cycle 23 and is obeyed in the ascending phase of solar cycle 24. And we find that the second phase of the solar cycle 24 (Cycle 24b) shows an even stronger evidence of the usual hemispheric helicity sign rule than its first phase (Cycle 24a). It is also found that our previous finding that the strong and weak fields possess the opposite helicity signs is not followed in Cycle 24b, whereas it is weakly followed in Cycle 24a and strongly followed in the descending phase of Cycle 23. This means that this rule also has a solar cycle variation, in addition to the solar cycle variation of the usual hemispheric helicity sign rule, and there is a roughly 2-years time delay between the two.

This work analyzed the morphologic properties of magnetic networks during Carrington Rotations (CRs) 1955 to 2091 by applying the watershed algorithm to magnetograms observed by the Michelson Doppler Interferometer on board the Solar and Heliospheric Observatory spacecraft. We found that the magnetic networks are of fractal and the average fractal dimension is Df=1.253±0.011. We also find that both the fractal dimension and the size of magnetic networks are anti-correlated with the solar magnetic activity.

From the observations of radio and HXR bursts, the escape rate of energetic electrons trapped in the flare loops is studied based on the trap-plus-precipitation model for the kinematics of energetic electrons in solar flares. Coulomb collision is regarded as the main pitch angle scattering of trapped electrons in the decay phase of the event on 2004 December 1. The escape rate of trapped electrons decreases firstly and then increases, which indicates the evolution of the plasma density in the flare loops during the decay phase.

We study the magnetic field structure and evolution for 26 X-ray brightening events in coronal holes and quiet Sun regions, including bright points and jets. We found that all brightening events are associated with bipolar regions and caused by magnetic flux emergence followed by cancellation. The emission fluctuations seen in the X-ray bright points are associated with reoccurring magnetic cancellation in the footpoints. An X-ray jet presents similar magnetic behaviour in the footpoints but its magnetic flux cancellation rate is much higher than in the bright point. Comparing coronal holes and the quiet Sun, we do not find differences in their corresponding magnetic field behavior.

To investigate the characteristics of large scale and long term evolution of magnetic helicity with solar cycles, we use the method of Local Correlation Tracking (LCT) to estimate the magnetic helicity evolution over the 23rd solar cycle from 1996 to 2009 by using 795 MDI magnetic synoptic charts. The main results are: the hemispheric helicity rule still holds in general, i.e. the large-scale negative (positive) magnetic helicity dominates the northern (southern) hemisphere. However, the large scale magnetic helicity fluxes show the same sign in both hemispheres around 2001 and 2005. The global, large scale magnetic helicity flux over the solar disk changes from negative value at the beginning of the 23rd solar cycle to positive value at the end of the cycle, which also shows the similar trend from the normalized magnetic flux by using the magnetic flux. The net accumulated magnetic helicity is negative in the period between 1996 and 2009.

We study the ephemeral regions (ERs) in the quiet Sun observed by the Solar Dynamics Observatory, and find that they can be classified into two types: normal ERs (NERs) and self-cancelled ERs (SERs). We identify 2988 ERs among which there are 190 SERs, about 6.4% of the ERs. The total self-cancelled flux is 9.8% of the total ER flux. We suggest that the self-cancellation of SERs is caused by the submergence of magnetic loops connecting the dipolar patches, without magnetic energy release.

We review some of the recent progress on modeling planetary and stellar dynamos. Particular attention is given to the dynamo mechanisms and the resulting properties of the field. We present direct numerical simulations using a simple Boussinesq model. These simulations are interpreted using the classical mean-field formalism. We investigate the transition from steady dipolar to multipolar dynamo waves solutions varying different control parameters, and discuss the relevance to stellar magnetic fields. We show that owing to the role of the strong zonal flow, this transition is hysteretic. In the presence of stress-free boundary conditions, the bistability extends over a wide range of parameters.

In this paper we first discuss observational evidence of longitudinal concentrations of magnetic activity in the Sun and rapidly rotating late-type stars with outer convective envelopes. Scenarios arising from the idea of rotationally influenced anisotropic convective turbulence being the key physical process generating these structures are then presented and discussed - such effects include the turbulent dynamo mechanism, negative effective magnetic pressure instability (NEMPI) and hydrodynamical vortex instability. Finally, we discuss non-axisymmetric stellar mean-field dynamo models, the results obtained with them, and compare those with the observational information gathered up so far. We also present results from a pure α2 mean-field dynamo model, which show that time-dependent behavior of the dynamo solutions can occur both in the form of an azimuthal dynamo wave and/or oscillatory behavior related to the alternating energy levels of the active longitudes.

Using results of 3D MHD numerical simulations we investigate the generation of the magnetic field in an accretion disk of a close binary star. Analysis of the numerical simulations shows that the magnetic field, which is mainly toroidal, is intensively generated in the accretion disk. In the disk, we can distinguish three regions: inner region of the intensive toroidal field generation due to the differential rotation; region of the current sheets; and outer region of the magnetic field dissipation. In the outer regions of the accretion disk of a magnetic close binary system two types of the dynamo may exist. One of them is the laminar dynamo that occurs as a result of the nonaxisymmetric motions. The other type is the turbulent αω-dynamo. Numerical simulations show that during the disk lifetime the dynamo effect in the outer region of the disk can be significant.

We present new measurements of the rotational profile of 56 nearby stars of spectral types A-F obtained by line profile analysis. Together with earlier work, we now know of 33 stars in which Sun-like differential rotation was identified by line profile analysis. We find evidence of two populations of differential rotators, one group of rapidly rotating A stars at the granulation boundary with strong horizontal shear, and another group of mid- to late-F type stars with moderate rates of rotation and less shear. There is a gap between A and F stars, in which the stars appear to exhibit very little shear. Apparently, the physical conditions of differential rotation change at early-F spectral types.

Rotation is a fundamental physical parameter in stellar astrophysics, playing an important role on the formation and evolution of stars. This parameter may also offer valuable information on stellar magnetism, mixing in the stellar interior, tidal interactions in close binary, as well as on angular momentum transfer and rotational breaking due to planets. The present work brings the results of an unprecedented study on the behavior of the distribution of the projected rotational velocity (v sin i) as a function of galactic position, on the basis of an unique sample of 14000 main-sequence field stars, along the spectral regions F and G. The vsini measurements used in this analyses were obtained from observations carried out with the CORAVEL spectrometers, with a precision better than about 1 km/s.

The present study demonstrates that behavior of rotational velocity as a function of stellar age is consistent using Tsallis' nonextensive formalism, resulting in a new approach to understanding the stellar rotational scenario.

The present study reports measurements of the rotation period of a young solar analogue, estimates of its surface coverage by photospheric starspots and of its chromospheric activity level, and derivations of its evolutionary status. It compares the chromospheric activity level of this young star with a model of chromospheric activity evolution established by combining relationships between the R′HK index and the Rossby number with a recent model of stellar rotation evolution on the main-sequence.

To study the effect of magnetic fields on lithium depletion and explain the characters of lithium evolution in solar-type stars, stellar models including Tayler-Spruit dynamo-type field are constructed. We compare the theoretical results with the time scales of lithium depletion suggested by Sestito & Randich (2005) and obtain very good agreements.

The lithium abundance of KIC 11395018 and KIC 10920273 are not compatible with their age, which is deduced by asteroseismology. To explain this phenomenon, we investigate the possible evolutionary status and perform seismological analysis of the three stars KIC 11395018, KIC 10273246 and KIC 10920273. Using the Yale Rotating Stellar Evolution Code (YREC), we constructed a grid of evolutionary tracks with different input physics and rotation rates. In addition to the conventional observed properties, we added two observed constraints: lithium abundance and rotational period. As a result, the lithium abundance of our rotation models agrees well with the observation. Meanwhile, we obtained a set of more accurate stellar fundamental parameters than previous studies.

We report on cross-matching the ROSAT All-Sky Survey and the XMM-Newton catalogs with the Kepler Input Catalog (KIC). For several stars associated with X-ray sources, we provide also an access to our high-resolution spectroscopic observations.

In order to investigate how magnetic activity influences a stellar evolution, we undertake a detailed study of photospheric abundances in a sample of RS CVn stars. Using the Nordic Optical Telescope we investigate abundances of more than 20 chemical elements, including carbon isotopes, nitrogen and other mixing-sensitive species. In this presentation we report on current results of the analysis, among which there are first evidences that extra-mixing processes may start acting in low-mass chromospherically active stars below a bump of the luminosity function of red giants.