Other
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009spd....40.0906k&link_type=abstract
American Astronomical Society, SPD meeting #40, #9.06; Bulletin of the American Astronomical Society, Vol. 41, p.818
Other
Scientific paper
Effect of the horizontal radial outflow in a sunspot penumbra (called "Evershed effect") has a 100-year history of investigations, but its physical nature is not clear yet. The Evershed flows begin at bright penumbral grains and propagate outward along penumbra filaments with the mean velocity of 1 - 2 km/s. High-resolution observations reveal that the Evershed flows are non-stationary, and that the strongest, 4 - 5 km/s, flows appear in quasi-periodic patches, "Evershed clouds". To study the nature of the Evershed effect we simulate behavior of convective motions in the presence strong inclined magnetic field. We use a 3D radiative non-linear MHD code, which describes realistic physical properties: compressible fluid flow in a highly stratified and magnetized plasma, 3D multi-group radiative energy transfer, a real-gas equation of state, and sub-grid scale turbulence models. We present a set of numerical experiments, which include the upper solar convection zone and lower atmosphere for different magnetic field strength (600 - 2000 Gauss) and inclination (0 - 90 degrees). The results show the development of filamentary magnetic structures and systematic flows in the direction of field inclination, strongly resembling the Evershed effect in penumbra. In particular, the simulations reproduce the high-speed "Evershed clouds", relationships between the flow velocity and the field strength and inclination, and other observational characteristics. We discuss the simulation results in the context of previously models, such as the embedded flux tube model, the magnetic gap model and the overturning magnetoconvection model, and argue that the physical mechanism of the Evershed effect is in a non-linear interaction between the narrow overturning convective motions and traveling magnetoconvection waves, formed in highly inclined strong magnetic field regions.
Kitiashvili Irina
Kosovichev Aleksandr G.
Mansour Nagi N.
Wray Alan A.
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