The Quiet Solar Photosphere: Dynamics and Magnetism

Abstract

The Sun is the only solar-type star where the dynamics and the magnetism can be studied in detail and the physical process involved understood, in particular those which occur at very small scales. This lecture is restricted to the quiet solar photosphere. The properties of the three cellular scales of motions observed at the solar surface (granulation, mesogranulation, supergranulation) are presented, as well as the numerical simulations which reproduce most granulation properties very satisfactorily. The granular convection is driven by radiative cooling through the surface. In these simulations, the mesogranulation appears to be an extension to deeper layers of the surface granular convection. The mesogranulation also appears as convective, indirectly driven by the surface radiative cooling. However, several alternative origins for both the mesogranulation and the supergranulation, have been proposed too. On the same way, the magnetic structure of the quiet photosphere is described, including the network and the fields. Their origin is discussed on the basis of the properties of the magnetic elements and of the results of numerical simulations of magneto-convection and of local dynamo. In the network, that means at the supergranular boundaries, the field is concentrated in the form of vertical flux tubes of sizes smaller than a few hundred kilometers and of magnetic field strength $1\text{--}2\,\hbox{kG}.$ They are visible as bright points located in the intergranular lanes. The presence of magnetic field is also ubiquitous inside the supergranules, where it is known as IntraNetwork magnetic field. This field is much different from the network field, consisting of small loops of size $1^{\prime\prime}\text{--}2^{\prime\prime},$ closely related to granules. The field strength is much lower than in the network, not exceeding a few hundreds Gauss, and of flux lower by one or two orders of magnitude. The time scale of both kinds of field is short, less than 10 min, determined by the evolution of the neighbouring granules. The origin of the intranetwork field is not yet clarified: are they fragments of magnetic flux rising from deep layers and reprocessed close to the surface by convective motions, or generated near the surface by a fast local dynamo?