Solar and Planetary Physics at Newcastle University

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The proposed research programme consists of two projects that aim to explain some of the surprising features of the magnetic fields of the Sun and planets.In the first project, we shall address one of the fundamental questions relating to solar magnetism. The solar surface often contains dark features, known as sunspots, that are the sites of strong, localised magnetic fields. The surface distribution of sunspots follows a well-known cyclic pattern, with zones of sunspot emergence (which are restricted to low latitudes) migrating equatorwards over an 11 year period. It is believed that sunspots are the surface manifestation of an underlying large-scale magnetic field that is buried deep with the solar interior. This magnetic field is generated and maintained by the motions of the plasma within the solar interior, via a (so-called) dynamo mechanism. However, this solar dynamo process is not fully understood. Our aim is to determine the extent to which the latitudinal distributions of sunspots truly reflects the latitudinal distribution of the subsurface large-scale field (and hence the extent to which these surface observations constrain models of the solar dynamo). To achieve this, we will be studying magnetic buoyancy, which is the process by which magnetic flux is transported from the deep interior to the solar surface. This will be investigated via a combination of analytical and numerical techniques. For the first time, the initial magnetic field distributions for the numerical simulations of this process will be derived from existing dynamo models. We shall determine which of these could produce a sunspot distribution that is consistent with solar observations (which would be an important step forward for this area). A better understanding of the solar dynamo and the cyclic activity that it produces would considerably enhance our understanding of space weather and many other solar magnetic phenomena. Our second project concerns planetary magnetic fields. The magnetic field of a planet is generated deep in its interior, in the core which is composed of an electrically conducting fluid (mainly liquid iron for rocky planets and metallic hydrogen for gas giants). This conducting fluid swirls around due to convection as the planet gradually cools down. These motions generate electric currents that induce the magnetic field through a dynamo process. Planetary evolution models indicate that, in many planets of our solar system, such as Mercury, the Earth and Saturn, the outermost part of the core might not be convective, that is the mean variation of density with depth is stable to overturning convection. This stable layer is not at rest though: the density perturbations depend both on the temperature and the chemical composition of the fluid and this can lead to vertical flows. This happens in addition to horizontal flows that are driven by latitudinal variations of density. All these motions can distort the magnetic field produced deeper in the core by the dynamo effect and passing through the stable layer. The effects of the horizontal flows on the magnetic field have been studied for several decades and they are fairly well understood. In particular, they are used to explain a surprising feature of Saturn's and Mercury's magnetic fields, namely that their magnetic field is axisymmetric, i.e. it lacks longitudinally-dependent features. However, the vertical motions have never been studied before in the context of planetary cores, so we do not know what their effect is on the magnetic field. In this project, we will address this issue by studying the motions of the stable layer in computer simulations that model the outer region of the core. The main objectives of the work are to determine the size and flow speed of the motions, and to predict how they modify the planet's magnetic field.