We study the evolution of the decaying active region NOAA 12708, from the photosphere up to the corona using high resolution, multi-wavelength GREGOR observations taken on May 9, 2018. We utilize spectropolarimetric scans of the 10830 Å spectral range by the GREGOR Infrared Spectrograph (GRIS), spectral imaging time-series in the Na ID2 spectral line by the GREGOR Fabry-Pérot Interferometer (GFPI) and context imaging in the Ca IIH and blue continuum by the High-resolution Fast Imager (HiFI). Context imaging in the UV/EUV from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) complements our dataset. The region under study contains one pore with a light-bridge, a few micro-pores and extended clusters of magnetic bright points. We study the magnetic structure from the photosphere up to the upper chromosphere through the spectropolarimetric observations in He II and Si I and through the magnetograms provided by the Helioseismic and Magnetic Imager (HMI). The high-resolution photospheric images reveal the complex interaction between granular-scale convective motions and a range of scales of magnetic field concentrations in unprecedented detail. The pore itself shows a strong interaction with the convective motions, which eventually leads to its decay, while, under the influence of the photospheric flow field, micro-pores appear and disappear. Compressible waves are generated, which are guided towards the upper atmosphere along the magnetic field lines of the various magnetic structures within the field-of-view. Modelling of the He i absorption profiles reveals high velocity components, mostly associated with magnetic bright points at the periphery of the active region, many of which correspond to asymmetric Si I Stokes-V profiles revealing a coupling between upper photospheric and upper chromospheric dynamics. Time-series of Na ID2 spectral images reveal episodic high velocity components at the same locations. State-of-the-art multi-wavelength GREGOR observations allow us to track and understand the mechanisms at work during the decay phase of the active region.

Comparing with solar cycles 21-23, the level of solar activity in the current cycle is very low. So far, there have been only five SARs and 45 X class flares. The monthly smoothed total solar irradiance decreased sharply by 0.09% from the maximum of cycle 23 to the minima between cycles 23 and 24. In this contribution, we present new studies on SARs in Cycle 24. The SARs in the current cycle have relatively smaller flare index (Iflare) and composite vector field index (Icom) comparing with the SARs in cycles 22 and 23. There is a clearly linear relationship between Iflare and Icom. The emphasis of this contribution is put on the similarity and different behaviors of vector magnetic fields of the SARs in the current solar cycle and the previous ones. We try to get a satisfactory account for the general characteristics and relatively lower level of solar flare activity in Cycle 24.

Solar coronal cavities are regions of rarefied density and elliptical cross-section. The Coronal Multi-channel Polarimeter (CoMP) obtains daily full-Sun coronal observations in linear polarization, allowing a systematic analysis of the coronal magnetic field in polar-crown prominence cavities. These cavities commonly possess a characteristic “lagomorphic” signature in linear polarization that may be explained by a magnetic flux-rope model. We analyze the spatial relation between the EUV cavity and the CoMP linear polarization signature.

We give a short introduction to the subject and review advances in understanding the basic ingredients of the mean-field dynamo theory. The discussion includes the recent analytic and numerical work in developments for the mean electromotive force of the turbulent flows and magnetic field, the nonlinear effects of the magnetic helicity, the non-local generation effects in the dynamo. We give an example of the mean-field solar dynamo model that incorporates the fairly complete expressions for the mean-electromotive force, the subsurface shear layer and the conservation of the total helicity. The model is used to shed light on the issues in the solar dynamo and on the future development of this field of research.

Employing wavelet and Fourier methods, we investigate temporal variations of periodicities in the photospheric fields obtained from synoptic magnetograms of the National Solar Observatory at Kitt Peak (NSO/KP) spanning the years 1975-2009. A north-south asymmetry is noticed in the periodicities of photospheric fields in the latitude range, 45°-78°, when the data is grouped into fields prior to and after 1996. This asymmetry when coupled with the fact that both solar fields in the latitude range, 45°-78°, and the micro-turbulence levels in the inner heliosphere began declining ~ 1995-1996 suggests that active changes occurred in the underlying basic solar processes which eventually initiated, at the end of solar cycle 23, the build-up of the deepest solar minimum, in the past 100 years.

The almost stately evolution of the global heliospheric magnetic field pattern during most of the solar cycle belies the intense dynamic interplay of photospheric and coronal flux concentrations on scales both large and small. The statistical characteristics of emerging bipoles and active regions lead to development of systematic magnetic patterns. Diffusion and flows impel features to interact constructively and destructively, and on longer time scales they may help drive the creation of new flux. Peculiar properties of the components in each solar cycle determine the specific details and provide additional clues about their sources. The interactions of complex developing features with the existing global magnetic environment drive impulsive events on all scales. Predominantly new-polarity surges originating in active regions at low latitudes can reach the poles in a year or two. Coronal holes and polar caps composed of short-lived, small-scale magnetic elements can persist for months and years. Advanced models coupled with comprehensive measurements of the visible solar surface, as well as the interior, corona, and heliosphere promise to revolutionize our understanding of the hierarchy we call the solar magnetic field.

The long-term variability of the sunspot cycle, as recorded by the Wolf numbers, are imprinted in different kinds of statistical relations which relate the cycle amplitudes, duration and shapes. This subject always gets a special attention because it is important for the solar activity forecast. We discuss statistical properties of the mean-field dynamo model with the fluctuating α-effect. Also, we estimate dynamical properties of the model for the long and short time-scale and compare it with the dynamics of the sunspot numbers data sets.

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 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.

Sunspot regions often form complexes of activity that may live for several solar rotations, and represent a major component of the Sun's magnetic activity. It had been suggested that the close appearance of active regions in space and time might be related to common subsurface roots, or “nests” of activity. EUV images show that the active regions are magnetically connected in the corona, but subsurface connections have not been established. We investigate the subsurface structure and dynamics of a large complex of activity, NOAA 10987-10989, observed during the SOHO/MDI Dynamics run in March-April 2008, which was a part of the Whole Heliospheric Interval (WHI) campaign. The active regions in this complex appeared in a narrow latitudinal range, probably representing a subsurface toroidal flux tube. We use the MDI full-disk Dopplergrams to measure perturbations of travel times of acoustic waves traveling to various depths by using time-distance helioseismology, and obtain sound-speed and flow maps by inversion of the travel times. The subsurface flow maps show an interesting dynamics of decaying active regions with persistent shearing flows, which may be important for driving the flaring and CME activity, observed during the WHI campaign. Our analyses, including the seismic sound-speed inversion results and the distribution of deep-focus travel-time anomalies, gave indications of diverging roots of the magnetic structures, as could be expected from Ω-loop structures. However, no clear connection in the depth range of 0-48 Mm among the three active regions in this complex of activity was detected.

Observations of the large-scale magnetic field show that the current extended solar cycle minimum is different from the three previous well-observed minima. The weaker polar fields increase the relative influence of middle and low-latitude flux patterns on the configuration of the corona and heliosphere. A much larger portion of the open flux originates in equatorial coronal holes. Even though the mean magnetic field of the Sun as a star is the weakest since measurements began, the sector structure of the interplanetary field, though smaller in magnitude, reached fairly high latitude until 2009. The emergence of active regions through the cycle and transport of flux from low to high latitudes also show quite different patterns, providing insight into the dynamo that drives the cycle. Long records of synoptic observations provide a rich source of information about solar activity that must be maintained.

Phase relations is extracted at different latitudes between the weak background solar magnetic (poloidal) field and strong magnetic field associated with sunspots (toroidal field) by comparing low-resolution images from Wilcox Solar Observatory (WSO) and the high-resolution SOHO/MDI magnetograms. Sunspot areas and excess flux in all latitudinal zones (averaged with a sliding 1 year filter) reveal a strong positive correlation with the absolute and excess solar magnetic fields with a timelag of zero and ∼ 3 years. The residuals of a sunspot magnetic excess flux averaged by one year from those by 4 years are shown to have well defined periodic temporal and spatial structures. The periods of these structures are close to π/4 (π≈ 11 years). The structures have maxima at −40^ and +40^ and reveal spatial drifts with time either towards the equator or the poles depending on a latitude of sunspot occurences.

The momenta and start times measured from the TD diagrams in 3 seismic sources observed in the flare of 28 October 2003 are compared with those delivered to the photosphere by different kinds of high energy particles as well as by the hydrodynamic shocks caused by these particles. The energetic protons with energy power laws combined with quasi-thermal ones are shown to form hydrodynamic shocks deeply in a flaring atmosphere which deliver the required momentum to the photosphere within a measured timescale. The seismic waves observed in two sources associated with γ-rays can be explained by the momenta produced by hydrodynamic shocks caused by mixed proton beams and jets. The seismic wave in the source asociated with HXR only and delayed by 4 and 2 minutes from the first and second HXR bursts is likely to be associated with a hydrodynamic shock occurring from precipitation of a very powerful and hard electron beam possibly mixed with quasi-thermal lower energy protons.

Typical latitude zones and longitude sectors with a dominant occurrence of newly emerging magnetic flux were systematically detected during three last activity cycles. Two long time persistent longitude sectors with the preferred occurrence of a new strong magnetic flux are characteristic for magnetic flux distribution and their rotation rate is latitude dependent in the relationship with differential rotation rate. Recent new information about the large-scale flows in convection zone relates to a concept of the expected giant cells and jets and show a new relationship with different scales of the large-scale circulation. Non-axially symmetric horizontal flow in upper part of convection zone gives a good motivation for an extension of the existing axially symmetric 2D models into new 3D concept.

We extrapolated observed photospheric magnetic field before the 3B/X1.2 flare occurred in NOAA 10486 on 2003 October 26. Two magnetic null points exist above the active region, which have no crucial role in triggering the flare, even though enhanced TRACE EUV/UV emission was observed before the main flare due to magnetic reconnection near the lower-altitude null point. We conclude that this flare results from the loss of equilibrium due to persistent flux emergence and photospheric motion, and strong shear. The accompanying fast CME with velocity of $\sim$ 800 km $s^{-1}$ shows quick mass pickup and energy increase in the low corona. Its kinetic energy is always larger than its potential energy.

Solar coronal heating is a complex problem due to the variety of scales and physical phenomena involved, and intricacy of “boundary conditions”. Lattice models and self-organized criticality provide means to model phenomenologically some of the physics involved over a wide range of scales, and reproduce certain statistical features of solar flares. Furthermore, these models offer a basis for the study of Parker's hypothesis of coronal heating by nanoflares. We provide a short review of this approach pioneered by Lu & Hamilton (1991) and related more recent works involving lattice models.

The combined action of the subadiabatic ambient stratification in the overshoot region below the convection zone and the inertial forces associated with the solar rotation is shown to lead to the suppression of the escape of magnetic flux in the form of toroidal flux tubes both toward the surface and toward higher latitudes. We show that a flux ring initially in thermal equilibrium with its environment and rotating with the ambient angular velocity moves radially and latitudinally towards an equilibrium configuration of lower internal temperature and larger internal rotation rate with respect to the surrounding, field-free gas. We conclude that flux rings with B≲ 105 G can be kept within the overshoot region if the superadiabaticity is sufficiently negative, i.e. δ = ▿ – ▿ad ≲–10−5; below that field strength the poleward drift is also reduced to a latitudinal oscillation of moderate amplitude, δθ ≲ 20 deg. Flux rings with significantly larger field strength cannot be kept in the equatorial parts of the overshoot region: their equilibrium configuration is located at high latitudes far outside the solar activity belts and, at any rate, requires unrealistic values of δ.