Ionospheric models for use in radio astronomy

Abstract

This thesis describes a variety of techniques for modelling the ionosphere of the Earth to improve low-frequency radio observations. The ionosphere of the Earth is a dynamic and inhomogeneous plasma which distorts low-frequency radio signals by causing absorption and scattering of the incoming radio wave and changes to the measured Faraday rotation and phase error. Within this thesis the phase errors, absorption and changes to the measured Faraday rotation caused by the ionosphere are investigated.

The assumption of a thin-layer model to correct for the phase errors is a method that is becoming more commonly used throughout radio astronomy. In this thesis we explore the errors arising from this assumption and the conditions under which the assumption can be applied to radio telescopes including the GMRT, VLA, LOFAR, MWA and SKA1-LOW.

Absorption can strongly affect faint low-frequency radio signals such as the 21 cm hydrogen line from the epoch of reionisation. It is therefore integral to studies of the epoch of reionisation to be able to accurately measure and model the amount of absorption caused by the ionosphere. In this thesis we use radio telescopes as multi-frequency riometers to study the amount of absorption that occurs in the lower ionosphere. We find that this method is not accurate enough to measure the absorption that occurs in a quiet, mid-latitude ionosphere. However we have designed and tested software that recreates electron density height profiles from high-latitude multi-frequency absorption measurements which is capable of recovering information about the lower ionosphere as well as information about the physical processes that affect the electron density within it. This method of measuring the electron density of the lower ionosphere is useful to ionospheric physicists as this region of the ionosphere is difficult to observe with existingmethods.

Finally we investigate the structure of the turbulence within the ionosphere by observing changes to the measured Faraday rotation of pulsars. We first investigate whether information about the turbulent spectrum can be inferred from variations in the Faraday rotation of a simulated radio signal, finding that we can recover the power of the input turbulent spectrum. We then implement a pulsar observing mode at KAIRA that successfully observes full Stokes polarisation data at a high temporal resolution and develop a pipeline that converts the pulsar data into a format readable by pulsar analysis programs. We use the change in the rotation measures of the observed pulsars to investigate the turbulent spectrum above KAIRA.

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