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Various approaches to forecasting the solar cycle based on solar dynamo models are considered. The importance of separating predictions of catastrophic events such as the Maunder minimum, predictions of general trends in solar activity between Grand minima, and forecasts of the next cycle, based on current knowledge of previous cycles is noted. The role of fluctuations of dynamo drivers for the forecast is emphasized. The polar magnetic field in the current cycle is considered as a natural predictor of the amplitude of the following cycle. [ABSTRACT FROM AUTHOR]

We investigate the frequency of all (X-ray flare events higher than class B1.0), B, C, M and X-class flares, respectively, derived from the National Geophysical Data Center (NGDC) list of solar flares between May 1983 and September 2014, which corresponds to the two complete solar cycles (SCs) 22 and 23 as well as the rise and maximum phases of SC 24. Analysis shows that the temporal behavior for these various class flares is quite different. The main findings of this study, confirmed by using the Hinode flare catalog where possible, are as follows. (1) The B-class flares are in complete antiphase with all, C, M and X-class flares. (2) While, there is a small decreasing trend in the peak values of the smoothed monthly C-class flare numbers from SC 22 to 24, the occurrence rate of M and X-class flares dropped by almost half and two-thirds, respectively, during SC 23 and remained almost the same during SC 24. This class-dependent temporal behavior provides support for dynamo models that involve the coexistence of a deep global and a superficial local dynamo. [ABSTRACT FROM AUTHOR]

Abstract: The common methodologies used to predict the smooth sunspot number (SSN) at peak (Rmax) and the rise time (Tr) for a cycle are noted. The estimates based on geomagnetic precursors give the best prediction of Rmax for five SSN cycles (20–24). In particular, an empirical technique invoking three-cycle quasi-periodicity (TCQP) in Ap index has made accurate predictions of Rmax and Tr for two consecutive SSN cycles (23 and 24). The dynamo theories are unable to account for TCQP. If it endures in the 21st century the Sun shall enter a Dalton-like grand minimum. It was a period of global cooling. The current status of the ascending phase of cycle 24 is described and the delayed reversal of the solar polar field reversal in the southern hemisphere in September 2013 is noted. [Copyright &y& Elsevier]

Abstract: This review provides an introduction to the generation and evolution of the Sun’s magnetic field, summarising both observational evidence and theoretical models. The eleven year solar cycle, which is well known from a variety of observed quantities, strongly supports the idea of a large-scale solar dynamo. Current theoretical ideas on the location and mechanism of this dynamo are presented. The solar cycle influences the behaviour of the global coronal magnetic field and it is the eruptions of this field that can impact on the Earth’s environment. These global coronal variations can be modelled to a surprising degree of accuracy. Recent high resolution observations of the Sun’s magnetic field in quiet regions, away from sunspots, show that there is a continual evolution of a small-scale magnetic field, presumably produced by small-scale dynamo action in the solar interior. Sunspots, a natural consequence of the large-scale dynamo, emerge, evolve and disperse over a period of several days. Numerical simulations can help to determine the physical processes governing the emergence of sunspots. We discuss the interaction of these emerging fields with the pre-existing coronal field, resulting in a variety of dynamic phenomena. [Copyright &y& Elsevier]

Abstract: The correlation between geomagnetic activity and the sunspot number in the 11-year solar cycle exhibits long-term variations due to the varying time lag between the sunspot-related and non-sunspot related geomagnetic activity, and the varying relative amplitude of the respective geomagnetic activity peaks. As the sunspot-related and non-sunspot related geomagnetic activity peaks are caused by different solar agents, related to the solar toroidal and poloidal fields, respectively, we use their variations to derive the parameters of the solar dynamo transforming the poloidal field into toroidal field and back. We find that in the last 12 cycles the solar surface meridional circulation varied between 5 and 20m/s (averaged over latitude and over the sunspot cycle), the deep circulation varied between 2.5 and 5.5m/s, and the diffusivity in the whole of the convection zone was ∼108 m2/s. In the last 12 cycles solar dynamo has been operating in moderately diffusion dominated regime in the bulk of the convection zone. This means that a part of the poloidal field generated at the surface is advected by the meridional circulation all the way to the poles, down to the tachocline and equatorward to sunspot latitudes, while another part is diffused directly to the tachocline at midlatitudes, “short-circuiting” the meridional circulation. The sunspot maximum is the superposition of the two surges of toroidal field generated by these two parts of the poloidal field, which is the explanation of the double peaks and the Gnevyshev gap in sunspot maximum. Near the tachocline, dynamo has been operating in diffusion dominated regime in which diffusion is more important than advection, so with increasing speed of the deep circulation the time for diffusive decay of the poloidal field decreases, and more toroidal field is generated leading to a higher sunspot maximum. During the Maunder minimum the dynamo was operating in advection dominated regime near the tachocline, with the transition from diffusion dominated to advection dominated regime caused by a sharp drop in the surface meridional circulation which is in general the most important factor modulating the amplitude of the sunspot cycle. [Copyright &y& Elsevier]

Abstract: In this work we use a low order dynamo model and study under which conditions can it reproduce solar grand minima. We begin by building the phase space of a proxy for the toroidal component of the solar magnetic field and we develop a model, derived from mean field dynamo theory, that gives the time evolution of the toroidal field. This model is characterized by a non-linear oscillator whose coefficients retain most of the physics behind dynamo theory. In the derivation of the model we also include stochastic oscillations in the effect. We found evidences that stochastic fluctuations in effect can trigger grand minima episodes in this model under some considerations. We also explore other ways of creating grand minima by looking into the physical mechanisms that compose the coefficients of the oscillator. The balance between meridional circulation and magnetic diffusivity as well as the field intensification by buoyancy driven instabilities, might have a crucial role in inducing grand minima. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the dynamo underpinning of solar cycle precursor schemes based on direct or indirect measures of the solar surface magnetic field. We do so for various types of mean-field-like kinematic axisymmetric dynamo models, where amplitude fluctuations are driven by zero-mean stochastic forcing of the dynamo number controlling the strength of the poloidal source term. In all stochastically forced models considered, the surface poloidal magnetic field is found to have precursor value only if it feeds back into the dynamo loop, which suggests that accurate determination of the magnetic flux budget of the solar polar fields may hold the key to dynamo model-based cycle forecasting. [ABSTRACT FROM AUTHOR]

Abstract: The differential rotation of the solar corona has been analyzed using as the input data the brightness of the coronal green line Fe xiv 530.3nm for more than five activity cycles. It is found that the character of rotation of the solar corona changes during the activity cycle. Approximately at the middle of the descending branch the differential rotation is weakly pronounced, while the greatest differential gradient is observed at the ascending branch and, occasionally, at the maximum of the cycle. An explanation of this difference has been suggested. The total rotation rate of the corona can be represented as a superposition of two rotation modes (components) – the fast and slow ones. The synodic period of the fast mode near the equator is about 27 days, increasing slightly with latitude. The synodic period of the slow mode exceeds 30 days. The changing relative fraction of these two modes results in variation of the latitude dependence of the observed rotation rate during the activity cycle. The characteristics of two principal types of differential rotation of the solar corona have been determined. The first type consists of the fast mode alone and is established approximately at the middle of the descending branch of the cycle. The second type is the sum of both modes with the fast mode dominating at low latitudes and the slow mode at high latitudes. The results obtained can be used for in-depth study of interaction of the velocity field and dynamo mechanism in the Sun and stars. [Copyright &y& Elsevier]

Abstract: It is well accepted that the solar cycle originates from a magnetohydrodynamics dynamo deep inside the Sun. Many dynamo models have long been proposed based on a lot of observational constraints. In this paper, using 342 NSO/Kitt Peak solar synoptic charts we study the solar cycle phases in different solar latitudinal zones to set further constraints. Our results can be summarized as follows: (1) The variability of solar polar regions’ area has a correlation with total unsigned magnetic flux in advance of 5years. (2) The high-latitude region mainly appears unipolar in the whole solar cycle and its flux peak time lags sunspot cycle for 3years. (3) For the activity belt, it is not surprised that its phase be the same as sunspot’s. (4) The flux peak time of the low-latitude region shifts forward with an average gradient of 32.2day/deg. These typical characteristics may provide some hints for constructing an actual solar dynamo. [Copyright &y& Elsevier]

Abstract: Current helicity data for solar active region provide the unique observational information for so-called α-effect which plays a key role in astrophysical dynamos producing large-scale magnetic fields of celestial bodies. We discuss problems arising in comparison between helicity data and predictions of solar dynamo theory. In particular, a significant excess of active regions with the “wrong” sign of helicity is seen to occur just at the beginning of the 11-year solar cycle. We attribute the excess partially to additional current helicity production from the twisting of rising magnetic flux tubes. We demonstrate that the excess is much less pronounced in the twist data. Future perspectives of helicity observations are discussed. [Copyright &y& Elsevier]

Measurements of the general magnetic field of the Sun seen as a star were performed over last 45 years by the CrAO and five other observatories (1968–2012, nearly 23 thousand daily strengths B ). Analysis of the B time series showed that the most substantial long-term period of the field variation is the Hale’s cycle 22 years, which cannot be explained by dynamo theory. It reveals a saw-edged profile, indicating perhaps a cosmic origin of the cycle. [ABSTRACT FROM AUTHOR]