Convective intensification of magnetic fields in the quiet Sun - AstrophysicsReport as inadecuate

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Abstract: Kilogauss-strength magnetic fields are often observed in intergranular lanesat the photosphere in the quiet Sun. Such fields are stronger than theequipartition field $B e$, corresponding to a magnetic energy density thatmatches the kinetic energy density of photospheric convection, and comparablewith the field $B p$ that exerts a magnetic pressure equal to the ambient gaspressure. We present an idealised numerical model of three-dimensionalcompressible magnetoconvection at the photosphere, for a range of values of themagnetic Reynolds number. In the absence of a magnetic field, the convection ishighly supercritical and is characterised by a pattern of vigorous,time-dependent, ``granular- motions. When a weak magnetic field is imposedupon the convection, magnetic flux is swept into the convective downflows whereit forms localised concentrations. Unless this process is significantlyinhibited by magnetic diffusion, the resulting fields are often much greaterthan $B e$, and the high magnetic pressure in these flux elements leads totheir being partially evacuated. Some of these flux elements containultra-intense magnetic fields that are significantly greater than $B p$. Suchfields are contained by a combination of the thermal pressure of the gas andthe dynamic pressure of the convective motion, and they are constantlyevolving. These ultra-intense fields develop owing to nonlinear interactionsbetween magnetic fields and convection; they cannot be explained in terms of``convective collapse- within a thin flux tube that remains in overallpressure equilibrium with its surroundings.

Author: P. J. Bushby, S. M. Houghton, M. R. E. Proctor, N. O. Weiss


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