We briefly review the observations of the solar photosphere and pinpoint some open questions related to the magnetohydrodynamics of this layer of the Sun. We then discuss the current modelling efforts, addressing among other problems, that of the origin of supergranulation.

In this contribution, we present our recent effort in understanding the solar small-scale magnetic field. Firstly, with the unique data from MDI/SOHO in an interval embodying solar cycle 23, we find three categories of small-scale magnetic elements, and their number variations shows no correlation, anti-correlation and correlation with sunspots, respectively. The possible sources for the three categories of small-scale magnetic elements are discussed. Secondly, by the observations from SOT on board Hinode, we study the properties of vector magnetic field in quiet Sun. The following results are summarized. (1) Two categories of horizontal magnetic elements are disclosed, and most of horizontal elements are associate with the emergence of U-loop. (2) The magnetic field of quiet region is obviously non-potential. The filigrees and network bright points are characterized by strong longitudinal field, large electric helicity and free energy density. (3) For quiet region, the intra-network region displays a predominance of weak field concentration, but network region shows the coexisting of weak field with strong field; For polar region, the strong field occupies 6.7% of the region. (4) On quiet region, the magnetic field mainly resides in the inter-granular lanes. By the observation, we reconstruct an average granular model, for which the detailed distributions of vector fields, Doppler velocity and intensity are obtained.

Using data from the Helioseismic and Magnetic Imager and the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we study the property of emerging flux regions and the response of upper solar atmosphere to the flux emergence. Property of the EFRs is delineated by the total emerged flux, flux growth rate, the maximum area, the duration of the emergence and the separation speed of the opposite polarities. The response of the upper atmosphere is addressed by the response of the atmosphere at different wavelengths (and thus at different temperatures). The total emerged fluxes are in the range of (0.48−11.2) × 1019Mx while the maximum area ranges from 17 to 182 arcsec2. The durations of the emergence are between 1 and 12 hours and positively correlated to both the total emerged flux and the maximum area. The maximum distances between the opposite polarities are 7−25 arcsec and the separation speeds are from 0.05 to 1.08 km s-1, negatively correlated to the duration. The derived flux growth rates are (0.1−1.1)  × 1019Mx/hr, which are positively correlated to the total emerging flux. The upper atmosphere responds to the flux emergence in the 1600 Å chromospheric line first, and then tens and hundreds of seconds later, in coronal lines, such as the 171 Å (T = 105.8K) and 211 Å (T = 106.3K) lines almost simultaneously, suggesting the successively heating of atmosphere from the chromosphere to the corona.

With the observations from the Solar Dynamics Observatory, we investigate the boundary evolution of a coronal hole (CH). Small-scale solar activity (e.g., coronal jet) can lead to gradual change of CH boundaries (CHBs). Besides gradual change of CHBs, a quiet Sun region can change into a CH region on a large scale when magnetic fields in the photosphere evolve from mixed-polarities into one-polarity dominance. CH regions can also evolve into quiet regions on a large scale when the underlying photosphere becomes mixed-polarity fields from one-polarity dominated fields.

With three examples of forward modeling of spectral intensity formation we illustrate some of the difficulties encountered in the use of spectra for the determination of physical properties of highly structured magnetized plasmas in general, and the Sun in particular. We demonstrate that the average spectrum of a structured atmosphere cannot be use for an accurate determination of the atmosphere’s average properties, show that why it is difficult to measure the chromospheric magnetic vector field from Zeeman polarization, and make clear from forward modeling of spectral lines that it will be hard to fully recover the physical structure of a sunspot.

The presentation is intended to describe results obtained with the UNNOFIT inversion of spectropolarimetric data. Quiet regions of the Sun are studied on the one hand, where the structure of the magnetic field remains unresolved and is approached via a magnetic filling factor. It is got that the field is structured in thin flux tubes connected by loops at any altitude in the solar atmosphere. The fluxtube diameter size is evaluated. On the other hand, active regions are studied, in particular sunspots where the magnetic field horizontal gradient is found one order of magnitude weaker than the vertical gradient, as in other observations reported in the literature. This is a difficulty to ensure \hbox{${\rm div}\vec{B}=0$}$\mathrm{div}\begin{array}{}{}_{\mathit{􏿻}}\\ \mathit{B}\end{array}\mathrm{=}\mathrm{0}$. Before entering the results, the particular features of the UNNOFIT inversion are recalled, which concern essentially the magnetic filling factor modeling.

High sensitivity spectropolarimetric observations of the four Stokes parameters of the solar Ca i 4227 Å line have been performed in October 2010 at IRSOL with the ZIMPOL polarimeter, near the disk center, outside an active region (Bianda et al. 2011). They were analyzed in Anusha et al. 2011 with a combination of detailed radiative transfer modelling of the Hanle effect for the linear polarization and weak field Zeeman approximation for the circular polarization. This approach made possible a unique determination of the magnetic field vector at various positions along the slit of the spectrograph. A summary of the observations and of their analysis is presented here.

Molecular lines formed in the upper photosphere of the Sun show significant degree of linear scattering polarization, when one observes close to the solar limb. Those lines prove to be important tools for turbulent magnetic field diagnostic via the Hanle effect. In order to correctly model the line formation in regions close to the Solar limb, one has to take into account the sphericity of the atmosphere and to model depolarizing collisions and NLTE line formation in details. We present computations of scattering polarization in C2 lines in a 1D spherically symmetric medium described by FALC and FALX models, and compare those computations to observations of the Solar flash spectrum.

In order to clear up the origin and possibly explain some solar limb and disc spicule quasi-periodic recurrences produced by overlapping effects, we present a simulation model assuming quasi- random positions of spicules. We also allow a set number of spicules with different physical properties (such as: height, lifetime and tilt angle as shown by an individual spicule) occurring randomly. Results of simulations made with three different spatial resolutions of the corresponding frames and also for different number density of spicules, are analyzed. The wavelet time/frequency method is used to obtain the exact period of spicule visibility. Results are compared with observations of the chromosphere from i/ the Transition Region and Coronal Explorer (TRACE) filtergrams taken at 1600 angstrom, ii/ the Solar Optical Telescope (SOT) of Hinode taken in the Ca II H-line and iii/ the Sac-Peak Dunn’s VTT taken in H? line. Our results suggest the need to be cautious when interpreting apparent oscillations seen in spicule image sequences when overlapping is present, i.e.; when the spatial resolution is not enough to resolve individual components of spicules.

Using spectroscopic data from Hinode/SOT we have obtained monochromatic images of the solar granulation at different levels in the Fe i 603 nm lines. The granulation contrast inversion is clearly detected as a negative peak in the cross-correlation of images at the continuum level and at line center levels. Taking advantage of the perspective effect which appears in images taken out of the solar disk center, we were able to measure the formation heights of the lines and the height of the contrast inversion layer.

A new inversion method derived from central moments of Zeeman line profiles (ICM), is used to determine magnetic field vectors (Mein et al. 2011). Two quantities A1 and A2 combining moments of profiles I ± S (S = Q,U,V) are nearly linear functions of the longitudinal and transverse components and lead to the field components through very fast iterations. Optimized exponents reduce noise effects. The ICM inversion does not require Milne Eddington approximation and can be used in a wide range of solar models.

Movies of magnetograms of sunspots often show small-size magnetic patches that move radially away and seem to be expelled from the field of the spot. These patches are named Moving Magnetic Features (MMFs). They have been mostly observed around spots and have been interpreted as manifestations of penumbral filaments. Nevertheless, few observations of MMFS streaming out from spots without penumbra have been reported. He we investigate the physical properties of MMFs observed around the field of a pore derived by the analyses of high spectral, spatial and temporal resolution data acquired at the Dunn Solar Telescope with IBIS. We find that the main properties of the investigated features agree with those reported for MMFs observed around regular spots. These results indicate that an improvement of current numerical simulations is required to understand the generation of MMFs in the lack of penumbrae.

The Solar Magnetic Field Telescope (SMFT) at the Huairou Solar Observing Station of the National Astronomical Observatory of China has been obtaining photospheric vector magnetograms on a daily basis for more than 20 years. In this brief review, I will first introduce our magnetograph, together with some efforts on correcting for a few known problems in filter-type magnetograph measurement. Then, I will focus on what we can learn from these vector magnetic field measurements with an emphasis on the helicity of active regions. It is generally believed that magnetic helicity is produced by solar dynamo in the convection zone and its accumulation in the corona will influence coronal dynamics. Our observations of helicity in active regions can thus be used to constrain dynamo models and suggest favorable conditions for coronal eruptions.

The helicity is an important quantity to present the basic topological configuration of magnetic field in the solar atmosphere. In this paper, we present the observational magnetic helicity in solar active regions with solar cycles and the corresponding possible model.

We performed 2.5D compressible resistive MHD simulations of magnetic reconnection with gravity considered. The background magnetic field is a canopy-type configuration which is rooted at the boundary of the solar supergranule. By changing the bottom boundary conditions in the simulations, an emerging flux rises up at the center of the supergranule and reconnects with the canopy magnetic field. We successfully simulate the coronal and chromospheric microflares, whose current sheets locate at the corona and chromosphere respectively. The coronal microflare is triggered by the reconnection at the corona, whose size and temperature enhancement is bigger and higher than the chromospheric one. We also found a hot jet (~1.8 × 106 K) relating to the observational EUV/SXR jet and a cold jet (~104 K) corresponding to the observational Hα/Ca surge or brightening in the coronal case. Whereas there is only Hα/Ca bright point in the chromospheric one.

Magnetic flux emergence as the mechanism leading to the formation of magnetized structures in the solar atmosphere plays a key role in the dynamic of the Sun. Observed as a whole, emerging flux regions show clear signs of twisted structure, bearing the magnetic free energy necessary to power active events. The high resolution observations of the recent solar observatories (e.g. Hinode, SDO) have revealed how intermittent the magnetic field appears and how various active events induced by flux emergence are. Magnetic field reconstructions methods show that the topology of the field in interspot regions presents a serpentine structure, i.e. field lines having successive U and Ω parts. Associated with the appearance of magnetic polarities, a tremendous number of brief small scale brightening are observed in different photospheric and chromospheric lines, e.g. Ellerman Bombs, along with small scale jet-like structures. These events are believed to be the observational signatures of the multiple magnetic reconnections which enable the magnetic field to emerge further up and magnetically structure the corona above active region. Meanwhile a world-wide effort to numerically model the emergence of magnetic field forming solar active region is been carried on. Using different types of physical paradigm – e.g. idealized magnetohydrodynamic model, advanced treatment of the physical equations, data-driven simulations – these numerical experiments highlight how electric currents can build-up during flux emergence, lead to reconnection and thus explain the formation of the different observed transients.

We study the evolution of the emerging and eruptive Active Region (AR) 10930 for which the coronal magnetic field is reconstructed as a nonlinear force-free field along with vector magnetograms obtained from Hinode/SOT/SP data. We show that the AR evolves quasi-statically during several days through a sequence of equilibria until the time of the eruption. In addition this sequence emphasizes the formation and expansion of a coronal Twisted Flux Rope (TFR) which is related to other observed coronal structures. On the other hand, the pre-eruptive and post-eruptive magnetic configurations show that the pre-eruptive TFR plays a crucial role during the eruption.

A new method is proposed for automatic choosing the so-called magnetic “charges” that are necessary to model the coronal magnetic field in solar ARs. The method makes the quantitative analysis of large-scale magnetic field fast, easy, and reliable that allows us to study the evolution of ARs on short time scales.

In this presentation, we first introduced the definition and classification of rotating sunspots. Using multi-wavelength data, we investigated the evolution of some rotating sunspots and the response of upper atmosphere relevant to the rotating sunspots. Moreover, the distribution of rotating sunspots in solar cycle 23 and the statistics of flares relevant to rotating sunspots are also presented.