Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...522..518t&link_type=abstract
The Astrophysical Journal, Volume 522, Issue 1, pp. 518-523.
Astronomy and Astrophysics
Astronomy
15
Convection, Instabilities, Magnetohydrodynamics: Mhd, Sun: Faculae, Plages
Scientific paper
We investigate the dependence of convective instability in a vertical photospheric magnetic flux tube, often referred to as convective collapse, over a broad range of parameters. The temporal evolution of the instability is examined by performing one-dimensional magnetohydrodynamic numerical simulations, adopting the thin flux tube approximation. Radiative energy transport is taken into account using Newton's law of cooling. To initiate the instability, the flux tubes are perturbed by a small downflow. The parameters of the instability are the initial photospheric tube radius a_00 and the ratio of gas pressure to magnetic pressure beta_00. It is shown that the parameter plane is divided into convectively stable and unstable regions by a critical curve which is represented as a_00~3x10^2/beta_00 (km). A region of shock formation is found within the most unstable portion of the parameter range. Since a strong downflow due to the instability bounces off high-density plasma in the deeper layers, the resulting upflow leads to a shock wave. It is also shown that an initially unstable weak flux tube evolves into a static intense flux tube. We next investigate the parameter dependence of the collapsed state. In the unstable region without shock formation, the field strength of the collapsed tube at tau=1 increases with a_00 and beta_00. On the other hand, the field strength in the shock-forming region decreases with increasing beta_00 because the shock lifts denser matter from lower layers to upper layers. Thus the field amplification due to convective collapse is suppressed by shock formation.
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