Radio propagation through ionospheric sporadic-E

: (Alternative Format Thesis)

Abstract

Midlatitude sporadic-E (Es) is an intermittent phenomenon of the E region of the ionosphere. Es clouds are thin, transient, and patchy layers of intense ionisation, with electron densities which can be much higher than those in the background ionosphere. Oblique reflection of radio signals in the very high frequency (VHF) range is regularly supported, but the mechanism for it has never been clearly established - specular reflection, scattering, and magnetoionic double refraction have all been suggested. This thesis uses the polarisation behaviour of signals reflected from intense midlatitude sporadic-E clouds as a way of differentiating between the three possible mechanisms.

Initial observation of the amplitude of 50 MHz signals reflected from Es layers confirmed that they often exhibited a strong axis of polarisation which rotated on a timescale of seconds or minutes. Following this initial work, a more advanced system was developed for accurate polarimetric and fading measurements of 50 MHz radio waves arriving at low elevation angles, such as those obliquely reflected from midlatitude Es layers. The overall sensitivity of the system was optimized by reducing environmental electromagnetic noise, giving the ability to observe weak and short-lived 50 MHz Es reflection events.

In support of this work, a novel approach was developed for the use of consolidated amateur radio reception reports as indicators of the presence of intense sporadic-E. A case study is presented which demonstrates and validates the approach, showing that the use of amateur data can provide an important supplement to other techniques. The new approach allows the detection and tracking of Es where no suitable ionosonde or other measurements are available.
The advanced polarimetric system was used to conduct a polarisation measurement campaign over the summer of 2018.

The results show that in all cases the signals received were elliptically polarised, despite being transmitted with linear polarisation; there are also indications that polarisation behaviour varies systematically with the orientation of the path to the geomagnetic field. This represents, for all the examples recorded, clear evidence that signals were reflected from midlatitude Es by magnetoionic double refraction.

To explore the observed polarisation behaviour in more detail and to link that behaviour to the physical properties of the Es layers, a geometric optics-based ray trace simulation has been developed using the PHaRLAP raytracing toolkit. The changes in the polarisation state of the characteristic modes as they pass through the ionosphere have been modelled, and useful insights gained into the relative ellipticity of the characteristic waves at VHF. This represents a sound basis for further theoretical work.

The results of this research could have important implications for users of the radio spectrum. Trans-ionospheric services are frequently disrupted by sporadic-E, and this research has shown that Es not only causes amplitude and phase disturbances but can also significantly change the polarisation of a signal. In addition, terrestrial services in the low-VHF range can regularly suffer interference from outof-area signals, but it is clear from this work that there is no single choice of receiver polarisation which will reliably protect them. By contrast, terrestrial low-VHF services seeking to use sporadic-E to extend their range can potentially optimise their effectiveness by exploiting selective polarisation transmission and/or reception to increase signal strength and reduce fading.

Date of Award	4 Dec 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Cathryn Mitchell (Supervisor) & Robert Watson (Supervisor)