Solar magneto-convection

An overview is given about recent developments and results of comprehensive simulations of magneto-convective processes in the near-surface layers and photosphere of the Sun. Simulations now cover a wide range of phenomena, from whole active regions, over individual sunspots and pores, magnetic flux concentrations and vortices in intergranular lanes, down to the intricate mixed-polarity structure of the magnetic field generated by small-scale dynamo action. The simulations in concert with high-resolution observations have provided breakthroughs in our understanding of the structure and dynamics of the magnetic fields in the solar photosphere.Comment: invited talk, IAU-Symposium 294, Solar and Astrophysical Dynamos and Magnetic Activity, eds. A.G. Kosovichev, E.M. de Gouveia Dal Pino, Y.Yan, Beijing 201

The crucial role of surface magnetic fields for the solar dynamo

Sunspots and the plethora of other phenomena occuring in the course of the 11-year cycle of solar activity are a consequence of the emergence of magnetic flux at the solar surface. The observed orientations of bipolar sunspot groups imply that they originate from toroidal (azimuthally orientated) magnetic flux in the convective envelope of the Sun. We show that the net toroidal magnetic flux generated by differential rotation within a hemisphere of the convection zone is determined by the emerged magnetic flux at the solar surface and thus can be calculated from the observed magnetic field distribution. The main source of the toroidal flux is the roughly dipolar surface magnetic field at the polar caps, which peaks around the minima of the activity cycle.Comment: This manuscript has been accepted for publication in Science. This version has not undergone final editing. Please refer to the complete version of record at http://www.sciencemag.org/. The manuscript may not be reproduced or used in any manner that does not fall within the fair use provisions of the Copyright Act without the prior, written permission of AAA

Understanding solar cycle variability

The level of solar magnetic activity, as exemplified by the number of sunspots and by energetic events in the corona, varies on a wide range of time scales. Most prominent is the 11-year solar cycle, which is significantly modulated on longer time scales. Drawing from dynamo theory together with empirical results of past solar activity and of similar phenomena on solar-like stars, we show that the variability of the solar cycle can be essentially understood in terms of a weakly nonlinear limit cycle affected by random noise. In contrast to ad-hoc `toy models' for the solar cycle, this leads to a generic normal-form model, whose parameters are all constrained by observations. The model reproduces the characteristics of the variable solar activity on time scales between decades and millennia, including the occurrence and statistics of extended periods of very low activity (grand minima). Comparison with results obtained with a Babcock-Leighton-type dynamo model confirms the validity of the normal-mode approach.Comment: ApJ, accepte

Origin of the hemispheric asymmetry of solar activity

The frequency spectrum of the hemispheric asymmetry of solar activity shows enhanced power for the period ranges around 8.5 years and between 30 and 50 years. This can be understood as the sum and beat periods of the superposition of two dynamo modes: a dipolar mode with a (magnetic) period of about 22 years and aquadrupolar mode with a period between 13 and 15 years. An updated Babcock-Leighton-type dynamo model with weak driving as indicated by stellar observations shows an excited dipole mode and a damped quadrupole mode in the correct range of periods. Random excitation of the quadrupole by stochastic fluctuations of the source term for the poloidal field leads to a time evolution of activity and asymmetry that is consistent with the observational results.Comment: Astronomy & Astrophysics, accepte

Mechanisms for MHD Poynting flux generation in simulations of solar photospheric magneto-convection

We investigate the generation mechanisms of MHD Poynting flux in the magnetised solar photosphere. Using radiative MHD modelling of the solar photosphere with initial magnetic configurations that differ in their field strength and geometry, we show the presence of two different mechanisms for MHD Poynting flux generation in simulations of solar photospheric magneto-convection. The weaker mechanism is connected to vertical transport of weak horizontal magnetic fields in the convectively stable layers of the upper photosphere, while the stronger is the production of Poynting flux in strongly magnetised intergranular lanes experiencing horizontal vortex motions. These mechanisms may be responsible for the energy transport from the solar convection zone to the higher layers of the solar atmosphere.Comment: 5 pages, 5 figures, accepted for ApJ

Does the butterfly diagram indicate asolar flux-transport dynamo?

We address the question whether the properties of the observed latitude-time diagram of sunspot occurence (the butterfly diagram) provide evidence for the operation of a flux-transport dynamo, which explains the migration of the sunspot zones and the period of the solar cycle in terms of a deep equatorward meridional flow. We show that the properties of the butterfly diagram are equally well reproduced by a conventional dynamo model with migrating dynamo waves, but without transport of magnetic flux by a flow. These properties seem to be generic for an oscillatory and migratory field of dipole parity and thus do not permit an observational distinction between different dynamo approaches.Comment: 4 pages, 1 figur

Magnetic field intensification: comparison of 3D MHD simulations with Hinode/SP results

Recent spectro-polarimetric observations have provided detailed measurements of magnetic field, velocity and intensity during events of magnetic field intensification in the solar photosphere. We consider the temporal evolution of the relevant physical quantities for three cases of magnetic field intensification in a numerical simulation. We determine the evolution of the intensity, magnetic flux density and zero-crossing velocity derived from the synthetic Stokes parameters by taking into account the spectral and spatial resolution of the spectropolarimeter (SP) on board Hinode. The three events considered show a similar evolution: advection of magnetic flux to a granular vertex, development of a strong downflow, evacuation of the magnetic feature, increase of the field strength and the appearance of the bright point. We find that synthetic and real observations are qualitatively consistent and, for one of the cases considered, agree very well also quantitatively. The effect of finite resolution (spatial smearing) is most pronounced in the case of small features, for which the synthetic Hinode/SP observations miss the bright point formation and also the high-velocity downflows during the formation of the smaller magnetic features.Comment: accepted in A&