UK MHD 2016 will feature two Invited Lectures on major directions of current research in magnetohydrodynamics and applications. We are glad to present our distinguished Invited Speakers:
- Professor Peter A Davidson, Department of Engineering, University of Cambridge, UK
How do planetary dynamos work?
We argue that the underlying mechanisms for planetary dynamo action should not depend on the presence of a mantle or on the consequences of viscous stresses, such as Ekman pumping. This lies in stark contrast to many of the numerical geodynamo simulations. If we abandon Ekman pumping as a source of kinetic helicity in planetary cores, we are obliged to find an alternative source which is robust (in the sense that it will necessarily manifest itself in both the terrestrial planets and the gas giants) and also provide the require antisymmetric north-south helicity distribution. We argue that helical waves (either inertial waves or magnetostrophic waves) launched from the equatorial regions may provide the required asymmetric helicity distribution that drives the planetary dynamos we observe.
- Professor Philippa Browning, Jodrell Bank Centre for Astrophysics, University of Manchester, UK
Relaxation modelling and MHD simulations of energy release in kink-unstable coronal loops
Twisted magnetic flux ropes are fundamental building blocks of the solar coronal magnetic field and provide a reservoir of free magnetic energy which can be released in large-scale solar flares – as well as in smaller, but much more frequent, nanoflare events associated with coronal heating. We first describe 3D MHD simulations of individual twisted solar coronal loops in which magnetic reconnection is triggered by the ideal kink instability, extending earlier models in cylindrical geometry to more realistic coronal loop geometries. Magnetic reconnection, both within the loop and between the loop and the surrounding field, dissipates magnetic energy. Test-particles can be incorporated into the model, allowing prediction of energetic particle acceleration, and forward-modelling of the observational signatures of both thermal and non-thermal plasma. We then show how instability in a single unstable loop may trigger reconnection with stable neighbouring loops, releasing the stored energy in these loops and leading to an "avalanche" of heating events. Thus, an avalanche is demonstrated in a MHD model, as previously proposed in more idealised Cellular Automaton models. For both individual and interacting loops, the field relaxes towards a lower-energy equilibrium which is well-approximated as linear force-free field. We present a relaxation model for merging coronal loops, and show how this may be used to predict the energy release and heating for a wide parameter space.