In some cases the magnetic inversion line is tilted and no longer perpendicular to the axis of the active region. The line separating the two opposite polarities is called the magnetic field inversion line and has commonly a North-South direction, perpendicular to the generally East/West axis linking the center of the opposite magnetic polarities (for example AR 10487 in Figure 1). Progressively the leading sunspot and the trailing sunspot of opposite polarities grow and take on structure of a mature sunspot-a dark, circular umbra surrounded by a lighter penumbra with radial spokes. The flux in an active region, for sunspots to be observed, is usually larger than 10 21 Mx in each polarity.ĭuring this growing phase, the distribution of the emerging magnetic polarities tends to adopt an ellipsoidal-shaped pattern (as AR 10488 in Figure 1). The growing phase of the EFR lasts about 3-5 days. The newly emerged magnetic polarities separate and reach the already emerged main polarities with high velocities: larger spots form by the coalescence of smaller magnetic elements. New flux emerges continuously in the central part between the main polarities. The two opposite magnetic polarities of each bipole move apart at a relatively large speed (~5 km/s) in the initial phase and then slow down. The birth of an active region is manifested by the appearance of small magnetic bipoles at the limit of the spatial resolution of present-day solar telescopes. The active regions are observed at the photospheric level with sunspots ( top left panel: white light image), at the chromospheric level with sunspots, plages, filaments ( bottom left panel: Hα image bottom right panel: Ca K1 image) and present an intense magnetic flux ( top right panel: photosphere longitudinal magnetogram, blue (resp. The AR 10488 is an emerging stressed active region, the AR 10486 is a mature, fully developed and complex one, the AR 10487 is a simple bipole. Magnetic flux emergence is responsible for the formation of sunspots and active regions.įigure 1: Typical examples of active regions in the solar atmosphere. This article deals with observable properties of magnetic field emergence in emerging flux regions (EFR) in the solar atmosphere and is complementary with the article on numerical simulations of magnetic field emergence. This process is beyond the scope of the present discussion of magnetic flux emergence. These bipoles are due to the recycling of magnetic field by convection ( granules and supergranules). The solar surface is also covered by the so-called magnetic carpet with small magnetic bipoles scattered everywhere over the solar surface. The occurrence of large-scale magnetic flux emergence follows the solar cycle periodicity and is governed by the solar dynamo process. Such a group of sunspots forms what is called an active region. Measurements of magnetic fields at the solar surface shows that sunspots tend to be grouped in pairs, one with positive and one with negative magnetic polarity. The magnetic flux emergence is directly traced on the solar surface (in visible-white light) by the presence of dark, mainly round-shaped areas, called sunspots, surrounded by brighter regions called plages. Magnetic flux emergence corresponds to the mechanism leading to the establishment of magnetized structures in the solar atmosphere. Etienne Pariat, LESIA CNRS, Observatoire de Paris, UPMC, Université Paris Diderot, Meudon, France Brigitte Schmieder, Observatoire de Paris Meudon, Franceĭr.
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