One researcher’s noise is another’s data: using radio astronomy observations to study small-scale structures in the Earth’s ionosphere

Boyde, Ben ORCID: 0000-0002-2953-9734 (2025). One researcher’s noise is another’s data: using radio astronomy observations to study small-scale structures in the Earth’s ionosphere. University of Birmingham. Ph.D.

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Abstract

The LOw Frequency ARray (LOFAR) provides a unique opportunity to observe the mid-latitude ionosphere. Although it is intended for astronomical observation, the signals from astronomical radio sources are distorted as they pass through the ionosphere. The nature of these variations in intensity and phase can provide detailed information on structures in the ionosphere at a range of scales, and the distributed network of LOFAR stations provides opportunities to characterise the propagation, spatial extent and temporal evolution of these structures. Because of the frequencies observed, wide bandwidth, spatially distributed network and high sensitivity provided by LOFAR, the observations of ionospheric phenomena made using LOFAR are highly complementary to other established techniques for observing the ionosphere.

Using just intensity measurements, it is shown that LOFAR can detect structures on spatial scales which are inaccessible with many other common techniques. A case study is presented in which LOFAR broadband intensity measurements from a bright cosmic radio source are used to infer the presence of a travelling ionospheric disturbance with a wavelength of \[\sim\] 20 km. This is achieved by comparing the observed intensity as a function of frequency and time to the modelled intensity, considering the ionosphere as a 1-dimensional thin screen containing a sinusoidal perturbation. Extensions to the model are also used to identify deviations from a simple sine wave shape. Other data sources such as Global Navigation Satellite System (GNSS) Total Electron Content (TEC) and ionosonde measurements are used to confirm that the model parameters used are physically reasonable.

The calibration process for interferometric observations provides another means of studying the ionosphere with LOFAR, as it produces a direct measure of TEC. This is sensitive to disturbances up to an order of magnitude smaller than those detected using GNSS signals. A method is presented for identifying waves in this data and calculating their wavelengths, amplitudes and propagation directions, including robust characterisation of the uncertainties in these parameters. The range of wave parameters which can be identified in this way is shown to be complementary to previous work using networks of GNSS receivers, extending to lower amplitudes and shorter timescales. Using this analysis method with \[\sim\] 2,700 hours of observations, the statistics of ionospheric waves observed using LOFAR are investigated, showing a dominant population with directions tracking the neutral wind, which are associated with atmospheric gravity waves (AGWs). The precise relationship between wave direction and wind direction is found to be period dependent, indicating a previously unreported feature of AGW propagation in the thermosphere. A secondary population of field-aligned plasmaspheric disturbances is also found to be present in the observations.

Within the regular ionospheric scintillation observations made with LOFAR, a class of scintillation features referred to here as `U-shaped scintillation' are identified. These are shown to occur on northward lines of sight at low elevation. They show an increased occurrence rate at night during winter relative to other seasons and times, and an elevated occurrence rate during geomagnetically active conditions. Detailed investigation of one of these events indicates that the structures responsible are strongly field-aligned and likely occur within the auroral oval or the poleward wall of the mid-latitude trough. Possible explanations are proposed for the frequency dependent behaviour of this scintillation.

Type of Work:	Thesis (Doctorates > Ph.D.)
Award Type:	Doctorates > Ph.D.
Supervisor(s):	Supervisor(s) Email ORCID Wood, Alan G. UNSPECIFIED orcid.org/0000-0002-7527-4757 Themens, David R. UNSPECIFIED orcid.org/0000-0003-2567-8187 Dorrian, Gareth UNSPECIFIED orcid.org/0000-0002-5308-6894
Licence:	All rights reserved
College/Faculty:	Colleges > College of Engineering & Physical Sciences
School or Department:	School of Engineering, Department of Electronic, Electrical and Systems Engineering
Funders:	Leverhulme Trust
Subjects:	Q Science > QB Astronomy Q Science > QC Physics T Technology > TK Electrical engineering. Electronics Nuclear engineering
URI:	http://etheses.bham.ac.uk/id/eprint/16247

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