Abstract Details

Name: Avijeet Prasad
Affiliation: Physical Research Laboratory, Ahmedabad
Conference ID: ASI2017_1130
Title : Magnetic helicity and force-free properties of astrophysical magnetic fields
Authors and Co-Authors :
Abstract Type : Thesis
Abstract Category : Thesis
Abstract : The Thesis applies novel techniques and formulae for magnetic helicity and non-linear force-free field (NLFFF) to two different astrophysical settings: the solar corona and disc galaxies. The solutions obtained in the solar case provide alternate and verifiable means to calculate the structure and energetics of active regions (ARs). Further the topological methods provide a deeper understanding of the sources of coronal heating. In the case of the disc galaxies, our analytic treatment provided a transparent and powerful use of eigen-functions that yielded a full global solution to the galactic dynamo and its route to saturation. Complementing these solutions with future numerical models can give further insight and drive us to better models in both cases. The specific novel results are: We provided a new formulation for relative helicity in arbitrary geometries using the toroidal-poloidal representation. In the case of the solar corona, we solved linear and NLFFF equation using photospheric boundary conditions to obtain simple axisymmetric magnetic field configurations in spherical geometry and applied it to the AR 10930 and we confirmed in both models a substantial decrease in free energy and relative helicity after the flare, which is in agreement with those obtained from other numerical methods. We tested a model of self-organized criticality for the distribution of coherent braid sequences by comparing the resulting distribution of peak-flare energies with those obtained from NLFFF extrapolation. The results indicate that a significant component of the energy budget for coronal heating can potentially be supplied by nano-flares during reconnection of magnetic braids in the case of the active Sun. For the galactic dynamo, we have introduced the following novelties: building a three-dimensional model of the global field of the disc and corona using a simplified treatment of reconnecting the small-scale field to describe a large-scale force-free coronal field and balancing the global helicity by the use of gauge-free descriptions of absolute helicity and incorporated the radial dependence in the supernovae and MRI- driven turbulence parameters under a common formalism. The time-dependent solution is expressed in a separable form, with the radial part in terms of the steady-state counterparts. These global analytic solutions allowed us to calculate the field structure and evolution of relative helicity for both the disc and the corona. The resulting saturated quadrupolar magnetic field is of the order of its equipartition value (the 99% value is reached in about a Gyr, which is faster than the timescales reported in previous simulations) and proportional to the advective and diffusive fluxes.