Fluid Dynamics in Earth and Planetary Sciences (FDEPS) Second FDEPS Workshop Dec 04 - Dec 08, 2000 Graduate School of Mathematical Sciences, University of Tokyo
Magnetohydrodynamics in Solar Plasmas
`Convection, Dynamo and Reconnection'
KUSANO, Kanya (ADSM, Hiroshima University)
On the solar surface, a wide range of the magnetohydrodynamic activity appears intermittently and some of them influence the electromagnetic environment in the whole solar system. The origin of the energetic activity is the solar magnetic field which is periodically recreated by dynamo process operating in the outer region of the sun so called the convection zone. In this talk, three typical topics in the solar physics, those are convection, dynamo and reconnection, are reviewed about the current status of study as well as the remaining open questions.
From the theoretical point of view, it is widely believed that the elementary processes for the solar dynamo are the omega effect caused by the differential rotation of the sun as well as the alpha effect created by the kinematic helicity. Although the convection and the dynamo in the rotating spherical shell have been intensively investigated by direct numerical simulations based on the Boussinesq and the anelastic approximations for the last two decades, even basic properties of the oscillating solar activity has not yet been well reproduced numerically. On the other hand, the recent progress in the helioseismology, which reconstructs the inside structure of the sun from the oscillation on the solar surface as the inversion problem, may reveal the mean flow structure in a deep area of the sun, and the observed results are much different from the numerical predictions. Several new models, those are in progress to understand the modern observations, will be reviewed.
When the magnetic flux created by the dynamo process in the convection region emerges on the solar surface, it forms an active region which is the main stage of the energy relaxation activities like solar flares. However, as a result of the magnetic energy relaxation itself, the magnetic Reynolds number in the coronal plasma is extremely high, so that the energy relaxation must be accompanied by magnetic reconnection, in which the resistive heating may work only in the limited region. Also a number of X-ray observations strongly supports that the solar flare appears as a result of magnetic reconnection. However, the most basic question `what is the cause of reconnection' is still open. We will discuss about some theoretical possibility for the answer and introduce the future plan of flare study.