New contributions on VLF radio wave perturbations measured at high-latitudes

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Remote connection, Zoom link: https://oulu.zoom.us/j/528757122 (password if needed: 346529)

Topic of the dissertation

New contributions on VLF radio wave perturbations measured at high-latitudes

Doctoral candidate

Master of Science Edith Liliana Macotela Cruz

Faculty and unit

University of Oulu Graduate School, Faculty of Science, Sodankylä Geophysical Observatory

Subject of study

Physics

Opponent

Professor Paul Cannon, University of Birmingham

Custos

Professor Eija Tanskanen, University of Oulu

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Terrestrial and extraterrestrial phenomena that disturbs the propagation of radio signals

You probably have experienced that sound can propagate in a metal pipe much clearer and farther away than in open air. The same goes with light in a fiber optic, where light signals (Internet!) travel long distances with minimal distortion. The reason for such behavior is that they both are waveguides carrying these signals from their source to wherever a receiver is placed.

In analogous manner, the Earth and the lower ionosphere (starting approx. 70 km up there, above airplane routes and below satellite orbits) form a natural waveguide for radio wave signal propagation called the Earth-ionosphere waveguide. The radio signal that can propagate inside this waveguide is in the frequency range of 3–30 (kHz), which is called very low frequency (VLF). These waves can be produced by a building-tall radio antenna and received by pole type (electric component) or loop type (magnetic component) antenna. Moreover, VLF waves can also be produced naturally in the Earth environment, for example by lightning activity.

If, for whatever reason, the upper boundary of this waveguide moves down or up, then the radio propagation is disturbed and the radio reception is not be the same. There is a diversity of physical phenomena that can significantly alter the low ionosphere. These phenomena may have their origin on Earth (e.g., lightning), in the solar system (e.g., space weather phenomena), or come from more remote locations (e.g., galactic gamma-ray emissions). Therefore, by using the signal propagation we can study the effects of different phenomena on the lower ionosphere.

The aim of this thesis is to analyze short- and long-term VLF variations measured in Northern Finland and their associations to different phenomena. The main results are:
(i) The minimum energy a solar flare should have in order to produce ionospheric disturbances is inversely related to the solar activity cycle.
(ii) The semiannual oscillation that appears in VLF measurements was determined to be related to geomagnetic activity variations. At the same time, it was found that the 27-day solar rotation oscillation is dominant during the declining phase of the solar cycle.
(iii) The main characteristics of the observed VLF sunrise phase perturbation are derived from the shadowing of short wavelength solar UV radiation due to stratospheric ozone absorption when the Sun rises.
(iv) Banded-structure VLF emissions are observed in the frequency range 16–39 kHz. These are frequencies not usually used for the study of natural VLF emissions coming from the magnetosphere, i.e., from a region dominated by the Earth’s magnetic field. To explain these observations, two different hypotheses were put forward. First, they might be coming from the magnetosphere, as in the case of auroral hiss. Second, they could be formed in the Earth‐ionosphere waveguide, as in the case of long‐distance propagation of lightning generated VLF emissions.

The results can be used to identify those times when satellite communication or satellite geolocation should not be trusted. Therefore, these results have the potential to contribute to maintain reliable communication and accurate navigation systems. Especially since humanity today is becoming increasingly dependent on technology, and therefore reliable communication and navigation systems have become of central importance in many aspects of human life and industry.
Last updated: 15.4.2020