7 Interaction with Magnetic Fields

The strongest magnetic fields near the solar surface are in approximate pressure balance with their surroundings. That is, the magnetic plus gas pressure inside the magnetic concentration is nearly equal to the gas pressure in its neighborhood. This means that the magnetic energy density can be much larger than the kinetic energy of convective motions. However, as one descends into the convection zone the ratio of gas to magnetic pressure quickly becomes very large and the magnetic fields in the quiet Sun get moved around by the convective flows. Diverging upflows sweep magnetic flux to intergranular downflow lanes (Figure 44View Image) (Tao et al., 1998Thelen and Cattaneo, 2000Emonet and Cattaneo, 2001Jump To The Next Citation PointWeiss et al., 2002Vögler et al., 2005Jump To The Next Citation PointStein and Nordlund, 2006Jump To The Next Citation Point). Larger scale, slower flows – mesogranulation and supergranulation – sweep the flux along the intergranular lanes on longer time scales to the boundaries of increasingly larger scale horizontal patterns, while new flux, with mixed polarity, continually emerges throughout the quiet Sun. Eventually a balance is reached where the rate of emergence of new flux balances the rate at which flux is swept to larger horizontal scales, where it encounters existing magnetic flux with which it either cancels or augments (Simon et al., 2001Krijger and Roudier, 2003Isobe et al., 2008). This balance empirically occurs at supergranulation scales and produces the magnetic network (Schrijver et al., 1997b, point out that ephemeral active regions are actually a more important source of flux for the quiet-Sun network.). In general, the size and shape of a pattern produced by launching finite life time “corks” in the solar multi-scale velocity field depends both on the amplitude spectrum of the velocities, the morphology of the flows, and on the distribution of life times of the corks.

Note that the transportation and concentration of magnetic flux in the solar photosphere – and more generally at a free surface – is a different process than the “flux expulsion” mechanism (Weiss, 1966Maheswaran, 1969Peckover and Weiss, 1978Galloway and Weiss, 1981). Convective motions inside a box with closed boundaries tends to – over a period of time and in the presence of magnetic diffusion – expel magnetic fields from the interior of convection cells. Magnetic field lines penetrating a free surface boundary are efficiently concentrated by purely advective transport, and can subsequently be further concentrated due the suppression of convection associated with a strong magnetic field (Spruit, 1976Jump To The Next Citation Point1977a1979Spruit and Zweibel, 1979Bushby et al., 2008).

View Image

Figure 44: Granulation image and overlaid magnetogram contours at 30, 50, 70 and 90 G in the Fe i λ6302.5 line (Domínguez Cerdeña et al., 2003Jump To The Next Citation Point). Tickmarks at 1” intervals.
 7.1 Effects of magnetic fields on convection
 7.2 Center-to-limb behavior
 7.3 Magnetic flux emergence
 7.4 Convection as a driver of chromospheric and coronal heating

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