6 research outputs found
Theoretical Models of Sunspot Structure and Dynamics
Recent progress in theoretical modeling of a sunspot is reviewed. The
observed properties of umbral dots are well reproduced by realistic simulations
of magnetoconvection in a vertical, monolithic magnetic field. To understand
the penumbra, it is useful to distinguish between the inner penumbra, dominated
by bright filaments containing slender dark cores, and the outer penumbra, made
up of dark and bright filaments of comparable width with corresponding magnetic
fields differing in inclination by some 30 degrees and strong Evershed flows in
the dark filaments along nearly horizontal or downward-plunging magnetic
fields. The role of magnetic flux pumping in submerging magnetic flux in the
outer penumbra is examined through numerical experiments, and different
geometric models of the penumbral magnetic field are discussed in the light of
high-resolution observations. Recent, realistic numerical MHD simulations of an
entire sunspot have succeeded in reproducing the salient features of the
convective pattern in the umbra and the inner penumbra. The siphon-flow
mechanism still provides the best explanation of the Evershed flow,
particularly in the outer penumbra where it often consists of cool, supersonic
downflows.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
Three-Dimensional Magnetic Reconnection
The importance of magnetic reconnection as an energy release mechanism in
many solar, stellar, magnetospheric and astrophysical phenomena has long been
recognised. Reconnection is the only mechanism by which magnetic fields can
globally restructure, enabling them to access a lower energy state. Over the
past decade, there have been some major advances in our understanding of
three-dimensional reconnection. In particular, the key characteristics of 3D
magnetohydrodynamic (MHD) reconnection have been determined. For instance, 3D
reconnection (i) occurs with or without nulls, (ii) occurs continuously and
continually throughout a diffusion region and (iii) is driven by counter
rotating flows.
Furthermore, analysis of resistive 3D MHD magnetic experiments have revealed
some intriguing effects relating to where and how reconnection occurs. To
illustrate these new features, a series of constant-resistivity experiments,
involving the interaction of two opposite-polarity magnetic sources in an
overlying field, are considered. Such a simple interaction represents a typical
building block of the Sun's magnetic atmosphere. By following the evolution of
the magnetic topology, we are able to explain where, how and at what rate the
reconnection occurs. Remarkably there can be up to five energy release sites at
anyone time (compared to one in the potential case) and the duration of the
interaction increases (more than doubles) as the resistivity decreases (by a
factor of 16). The decreased resistivity also leads to a higher peak ohmic
dissipation and more energy being released in total, as a result of a greater
injection of Poynting flux.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200