@proceedings {852, title = {Why is sporadic-E propagation so weird?}, year = {2024}, month = {03/2024}, publisher = {HamSCI}, address = {Cleveland, OH}, abstract = {

Mid-latitude sporadic-E ("Es") clouds are transient, thin layers of dense but patchy ionization which appear in the E region of the ionosphere. The process of formation of Es is different from that of the background ionosphere and can, by comparison, produce much higher ionization densities. Sporadic-E propagation is well known to radio amateurs because it allows communication at higher frequencies and/or over shorter skip distances than is possible via the background ionosphere, but Es is also currently the subject of much academic research. This is partly because of the disruptive impact of Es on satellite communications and satellite radars, but there is also growing scientific interest in the Mesosphere /\ Lower Thermosphere region of the upper atmosphere, which is where Es mainly occurs. The thin, intense, and variable nature of Es means that reflected signals can have quite extreme temporal, spatial, and polarization characteristics. My PhD research showed that the reflection process at 50 MHz is primarily magnetoionic in nature, but many detailed features remain to be explained. To explore the observed polarization behaviour in more detail and to link that behaviour to the physical properties of the Es layers, a PHaRLAP-based raytrace simulation has been developed which predicts polarization parameters a signal passes through an Es cloud. As a case study, the observed significant and systematic differences in the polarization of the signals received in the UK over very similar paths from beacons in Hungary and Slovenia have been investigated in detail. Each of the two beacons shows strongly defined elliptical polarization, but the sense of rotation and predominant tilt angle are consistently opposite from each other, over multiple Es reflection events and on multiple days. This presentation will summarize the earlier work and then describe the building and testing of the PHaRLAP simulation model and the case study results obtained so far. Finally, outstanding questions about the weird nature of Es propagation will be discussed and opportunities for further work described.

}, author = {Chris Deacon} } @proceedings {603, title = {Consolidated Amateur Radio Reports as Indicators of Intense Sporadic-E Layers}, year = {2022}, month = {03/2022}, publisher = {HamSCI}, address = {Huntsville, AL}, abstract = {

A case study is presented to demonstrate the usefulness and validity of consolidated amateur ({\textquoteleft}ham{\textquoteright}) radio signal reports as indicators of the presence of ionospheric sporadic-E (Es). It is shown that amateur data can provide an important supplement to other techniques, allowing the detection of Es where no suitable ionosonde or satellite radio occultation measurements are available. The effectiveness of the approach is demonstrated by reference to ionosonde data, and the advantages and limitations of the technique are discussed.

}, author = {Chris Deacon and Cathryn Mitchell and Robert Watson} } @proceedings {468, title = {"Sprinkles" or "Mirrors"? Exploring the true nature of VHF propagation via sporadic-E}, year = {2021}, month = {03/2021}, publisher = {HamSCI}, address = {Scranton, PA (Virtual)}, abstract = {

Mid-latitude sporadic-E clouds (commonly abbreviated as {\textquoteleft}Es{\textquoteright}) are a transient feature consisting of thin layers of dense, but patchy, ionization which occur in the E region of the ionosphere. The process of formation is different from that of the rest of the ionosphere and it can produce much higher electron densities, sometimes permitting oblique reflection of radio waves up to 150 MHz.
The mechanism for the oblique reflection of VHF waves from Es layers has not been well described, with candidates including specular reflection, scattering, and magneto-ionic double refraction. The polarization and fading characteristics of waves reflected from Es layers are proposed as a marker for the presence or absence of magneto-ionic effects.\ 
An experimental system has been developed for rapid and accurate polarization and fading measurements at 50 MHz. The overall sensitivity of the system has been optimized by reducing environmental electromagnetic noise, giving the ability to observe weak, short-lived Es propagation events. The effect of the ground reflection on observed polarization has been analyzed and the induced amplitude and phase biases compensated for.
A measurement campaign in the summer of 2018 gathered a large quantity of data, using amateur 50 MHz beacons, at distances between 1,000 km and 1,650 km, as signal sources. The results provide compelling evidence that Es-layer propagation at 50 MHz exhibits the characteristics of magneto-ionic double refraction, but the thin, intense and variable nature of the reflecting region means that the reflected signals can have quite extreme characteristics. Some of the results are surprising, and are yet to be explained convincingly.
In this presentation, an overview of the experimental technique will be given, and the results described. Some of this information has already been published, but much of it is new.

}, author = {Chris Deacon and Ben Witvliet and Cathryn Mitchell and Simon Steendam} } @article {450, title = {Rapid and Accurate Measurement of Polarization and Fading of Weak VHF Signals Obliquely Reflected from Sporadic-E Layers}, journal = {IEEE Transactions on Antennas and Propagation}, year = {2020}, abstract = {

In the E-region of the ionosphere, at heights between 90 and 130 km, thin patches of enhanced ionization occur intermittently. The electron density in these sporadic-E (Es) clouds can sometimes be so high that radio waves with frequencies up to 150 MHz are obliquely reflected. While this phenomenon is well known, the reflection mechanism itself is not well understood. To investigate this question, an experimental system has been developed for accurate polarimetric and fading measurements of 50 MHz radio waves obliquely reflected by mid-latitude Es layers. The overall sensitivity of the system is optimized by reducing environmental electromagnetic noise, giving the ability to observe weak, short-lived 50 MHz Es propagation events. The effect of the ground reflection on observed polarization is analyzed and the induced amplitude and phase biases are compensated for. It is found that accurate measurements are only possible below the pseudo-Brewster angle. To demonstrate the effectiveness of the system, initial empirical results are presented which provide clear evidence of magneto-ionic double refraction.

}, keywords = {Brewster angle, ionosphere, radio noise, Radio wave propagation, VHF}, issn = {0018-926X}, url = {https://researchportal.bath.ac.uk/en/publications/rapid-and-accurate-measurement-of-polarization-and-fading-of-weak}, author = {Chris Deacon and Witvliet, Ben A. and Cathryn Mitchell and Simon Steendam} }