On, Alvina Yee Lian; (2021) Observational signatures and effects of magnetic fields in astrophysical systems. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Magnetic fields are ubiquitous, permeating across all scales from interstellar space to voids. Their origins and evolution still remain as open questions. On galactic scales and beyond, Faraday rotation measure (RM) at radio wavelengths is commonly used to diagnose magnetic fields, and its spatial correlation gives the characteristic length scales of the field variation. This work shows how the RM is derived from the polarised radiative transfer equations under restrictive conditions and assesses the merit of RM fluctuations (RMF) for large-scale magnetic field diagnostics. The interpretation of RMF analyses is ambiguous for an ill-defined characteristic density, such as lognormal-distributed and fractal-like structures. The RMF approach also falls short under radiative absorption, emission, Faraday mixing and the contribution of non-thermal electrons. Notably, correlations along the line-of-sight and across the sky plane are generally dissimilar, therefore the context of RMF must be clarified when inferring from observations. Magnetic fields can also imprint observational signatures in the radio synchrotron emission, whose total intensity reveals the field strength and polarisation traces the field orientation. A point-by-point comparison between the X-ray and radio emissions of a simulated galaxy cluster follows a linear best-fit slope of almost unity, indicating that the magnetic field scales with density locally. On smaller scales, magnetic fields may have an important role during the formation and evolution of molecular clouds. The effects of magnetic fields on the ionisation and heating rates of cosmic rays in IC 5146 are quantified, assuming that the fields are traceable via optical and near-infrared starlight polarisations. While the ionisation rate is fairly constant across the cloud, cosmic-ray heating is capable of raising temperatures by order of 1 K in a Galactic environment, or even higher in actively star-forming regions. This may lead to an increase in the Jeans mass and consequently affect the onset of star formation.

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