The Australian Mid-latitude Continental Ionosphere with Respect to Low-frequency Radio Astronomy

This thesis will show that the Murchison Radio Observatory (MRO) site is an excellent location from which to conduct radio astronomy at VHF frequencies. This result is supported by the Eindings of a Square Kilometre Array (SKA) site selection committee mandated investigation into the RFI environment and ionospheric variability of the Murchison region of Western Australia. In this work, the ionosphere is characterised and its response to impinging VHF radio waves clearly established. This activity will contribute to an operational requirement (calibration) of the Murchison WideEield Array radio telescope (MWA) and delivery of the project’s scientiEic goals (high Eidelity imaging). Characterisation involved the acquisition and mapping of ionospheric absolute total electron content (aTEC) measured as total electron content units, 1016 electron m-2 (TECU) and scintillation (as S4) data over a period of the Eirst half of solar cycle 24. Readings were performed at Eirst (epoch 1 E1) on the MRO (2008-2009) and later (epoch 2 E2) at (1) Boolardy homestead, adjacent to the MRO and (2) on a site 519 km south of the MRO on the Australian Space Academy campus at Meckering (2012-2014). Observations revealed annual, inter-annual and diurnal variability on timescales of minutes and greater. Rapid, anomalous, event-related changes over short (minute to hourly) timescales were detected. These included the response of the ionosphere to a signiEicant CME event (March 17th 2013) and a transient event (October 15th 2013). Clear evidence for large amplitude (several TECU) travelling ionospheric disturbances (TIDs), was produced and described in detail. The directions of 2 TIDs were calculated with ±12% and ±9% uncertainty and their speeds to ±19% and ±8% accuracy respectively. During E1, TEC levels rarely exceeded 20 TECU (99.0% < 20 TECU). During E2, TEC at Boolardy rarely exceeded 60 TECU (99.5% < 60 TECU). At the same time, TEC at Meckering rarely exceeded 50 TECU (98.1% < 50 TECU). The signiEicance of these TEC levels is discussed with respect to imaging requirements for the MWA including iii implications for Faraday rotation (FR), which is the tendency for the plane of polarisation of linearly polarised radio signals to be rotated by free electrons in the presence of a magnetic Eield (the Earth’s) and refraction effects. At both sites over both E1 and E2, S4 was reliably undetectable. Returned values fell typically within the range 0.0 to <<0.1. Occasional excursions in S4 are discussed and are shown to be the result of signal interferences through multi-path effects. Differences between the disposition of the ionosphere at Boolardy and at Meckering, 519 km to the south, are discussed. Whereas TEC levels at Meckering were typically very close in value to those at Boolardy, background levels at the latter over E2 were often ranged a little higher (by a few TECU). Modelling of the response of the ionosphere to VHF (50-300 MHz) radio waves was undertaken. In the radio astronomy community, these frequencies are regarded and discussed as being of low-frequency. This expression is employed occasionally throughout this work. The consequences for the selection of parameters used in the calibration of the MWA, including thin-shell ionosphere height, were studied. Many radio astronomy calibration models conEine the ionospheric TEC contribution to a thin-shell located at some altitude above the Earth’s surface. The effective altitude however, was demonstrated to vary over not greater than hourly timescales and should be recalculated accordingly. Dual frequency GPS systems provided accurate line-of-sight (slant) aTEC readings that were converted to vertical TEC values. These data, incorporated into an appropriate modelling process, demonstrated that a single thin-shell ionosphere height returns such inaccurate FR values that all polarisation information in an image can quickly be lost. For a change of 1 TECU, FR ranges from ~4π radians at an observation frequency of 50 MHz and elevation of 30° to 0.03π radians at 300 MHz and 90° elevation (zenith). Improving model thin-shell height selection with the aid of the International Reference Ionosphere model (IRI-2012) was investigated. The difference in output of this model with measured TEC values over E1 and E2 was studied and reported. E1 incorporated a period of time