The Instability of Magnetic Fields (Astrophysical and Geophysical Fluid Dynamics)

One of the most striking features of the solar surface is the appearance of sunspots, concentrated bipolar regions of magnetic flux erupting through the solar surface. The Sun's magnetic field is believed to be generated quite deep down in the solar interior, and it is therefore an outstanding issue to explain the initial escape and subsequent rise of strong magnetic fields from deep within the Sun. The mechanism responsible for the instability of the field is known as 'magnetic buoyancy', and results from the capacity of magnetic fields to exert a pressure and thus become buoyant. We have performed a number of studies of both the linear and nonlinear evolution of instabilities driven by magnetic buoyancy. Recent helioseismological observations of the internal solar rotation rate have added a new dimension to the problem, with the location of a region of strong velocity shear (the 'tachocline') coincident with the region of strong magnetic field. We are therefore now investigating the interaction between shear flows and magnetic buoyancy instabilities, so as to shed further light on the observed solar magnetic field.

keywords: applied mathematics, MHD, fluid dynamics, magnetic buoyancy, solar magnetic field

Astrophysical and Geophysical Fluid Dynamics
The group in Leeds is one of the leading groups in the field of Astrophysical and Geophysical Fluid Dynamics, with international reputation in dynamo theory, astrophysical MHD and convection. The strength of the group is recognised by the award of several prizes and special fellowships. The group also holds one of the largest grants ever awarded to the University of Leeds. The nine permanent members of staff work with eighteen postdocs and postgraduate students.

The group is actively engaged in research in a wide-range of areas of astrophysical and geophysical fluid dynamics: from planetary dynamics (the geodynamo and planetary dynamos) through solar, stellar and galactic dynamics to highly compressible and relativistic dynamics on the largest scales. Magnetic fields are a strong theme, and the group is interested in how planets (like the Earth), stars (like the Sun), neutron stars, black holes and galaxies generate their magnetic fields through dynamo action. On the Sun, the well-known eleven-year sunspot cycle is a manifestation of the solar dynamo; indeed the solar magnetic field underlies all solar magnetic phenomena such as solar flares, coronal mass ejections and the solar wind. In the Earth, magnetic fields are generated by convection in the molten iron core, and it has recently become possible to solve the fundamental equations that govern the motion of fluids and the generation of magnetic fields, and successfully reproduce many of the observed features of the geomagnetic field. At the other end of the scale, magnetic fields are implicated in the formation of spectacular jets coming from neutron stars, black holes and galaxies. Without magnetic fields, the group has interests in waves and hydrodynamic instabilities in rotating stratified fluids, with applications to the Earth's atmosphere and ocean (and with application to other planets).