Particle acceleration in complex, turbulent electromagnetic fields

The student will work on the transport and acceleration of charged particles in turbulent, complex electromagnetic fields. Such fields are known to occur in both the solar atmosphere, extensively studied in Glasgow University, and the terrestrial magnetosphere, of major importance for the BAS Natural Complexity Programme. We hope that it will still be possible to consider this late application. Successful Earth System (ES) science and policy requires us to identify, measure and explain the natural complexity of Earth. One of the main spheres of the ES is the plasma environment of the terrestrial magnetosphere. Theoretically, plasmas are expected to be inherently complex (Sato, 1996, Phys. Plasmas, 3, 2135), and research at BAS and elsewhere has shown evidence for complexity of the magnetosphere. Mixing and merging magnetic field structures will arise from the multi-scale intermittent turbulence of overlapping plasma resonances in the magnetotail current sheet; these contribute critically to magnetosphere energetic phenomena. Other, astrophysical plasmas will behave in a similarly complex manner, particularly the solar phenomena studied in Glasgow. Evidence of, and information about, the complex magnetic field structure could be inferred by studying the velocity distributions of charged particles that have traveled through it, whether via in situ measurements in the magnetosphere or remotely sensed via their X-ray and radio signatures in astrophysics. The study of the trajectories of charged particles in complex, electromagnetic fields is thus essential in understanding magnetospheric and solar plasma dynamics. It will increase our appreciation and understanding of complexity and nothermal phenomena in plasmas, and will allow us to deal better with their benefits and hazards. By 'complex' we mean here magnetic structures including several null points, with a magnetic topology consequently partitioned into many disjoint regions. Such complex fields may arise in plasmas driven externally by inflowing magnetic flux and boundary flows. By 'turbulent' we mean fields that include a random component, modelled as realisations of a multifractal or a fractional Levy motion model (Watkins et al., 2005, Space Sci. Rev., 121, 271; Saichev & Sornette, 2006, Phys. Rev. E 74, 011111) that may describe turbulent fields including coherent structures and intermittency. We have previously studied particle trapping, heating and acceleration at isolated null points (Petkaki and MacKinnon, 2007, A&A 472, 623), formation of multiple null points via superposition of force free fields (Malara, Petkaki, Veltri, 2000, ApJ 533, 523) and stochastic particle transport (MacKinnon and Craig, 1991, A&A 251, 693). The proposed project will draw on all this pre-existing expertise to determine particle distributions in highly fragmented plasmas believed to emerge in the process of reconnection. The student will: construct magnetic fields with multiple nulls, with and without realisations of a turbulent component; develop a code to integrate test particle trajectories in the presence of these complex fields (building on the work of MacKinnon and Craig); determine via many numerical experiments the circumstances (multiplicity of nulls; statistical properties of turbulence) that may account for observed particle acceleration in solar and magnetospheric contexts. The representation of the stochastic component of the field here distinguishes this approach from modelling using particular MHD simulations, and should lead to results of wider applicability than e.g. Turkmani et al., 2006, A&A 449, 749). The student will be inducted into research in solar and space plasmas via solution of a timely problem using state-of-the-art numerical tools. He or she will gain skills in numerical programming that can be applied in many other scientific and financial disciplines; knowledge and understanding of plasma physics.