In this paper we study the dynamic evolution of emerging magnetic fields
in the
solar atmosphere by deriving a model based on
a three-dimensional MHD simulation.
The simulation shows that magnetic field lines
initially forming a twisted flux tube below the solar surface emerge
into the
atmosphere by magnetic buoyancy. Outer field lines of the flux tube are
almost
free to expand in a wide fan shape without strong confinement by
surrounding
field lines. On the other hand, inner field lines are subject to strong
confinement by adjacent twisted field lines that prevent the lateral
expansion
of inner field lines. By examining the result of the simulation,
we derive a model of emerging field lines to demonstrate that
the height of an emerging field line increases at
the average rate of 0.5 H (g_0 κ)^{1/2}, where g_0, κ,
and H are the gravitational acceleration, curvature of the emerging field
line, and a
scale height of magnetic field strength. Applying this model to the
outer and inner field lines, we show that inner field lines are less
dynamical than outer field lines and inner field lines are likely to
form quasi-static structure in the corona such as prominences and
sigmoids. We
also discuss that the shape of inner field lines is
reflected in the chirality of a modeled sigmoid.
Dynamic evolution of an emerging flux tube
(Click the picture for a larger size)
A model of sigmoid
Reference
Magara, T.
2004 ApJ, 605, 480