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The new type of daytime natural very low frequency (VLF) whistler mode emissions of the magnetospheric origin was found in the VLF observations at Kannuslehto station (L ∼ 5.5) in Northern Finland. The events occurred at the frequencies above 4–5 kHz even up to 15 kHz. These emissions have never been observed earlier in the spectrograms because they were hidden by strong impulsive atmospherics (sferics) in the same frequency range originating in lightning discharges. After filtering out the sferics, we surprisingly discovered completely new types of high frequency right-hand polarized peculiar VLF emissions with various unusual dynamic spectra. These new VLF emissions typically occur as a sequence of a number of short (∼1–3 min) burst-like structures or single short patches with the total duration up to a few hours. The spectral peculiarities of several long-lasting VLF events as well as individual short VLF patches are discussed. The dynamic spectra of these new VLF patches raise the question of the temporal and spatial details of the wave-particle interactions in the magnetospheric plasma. These emissions are observed only under quiet or weakly disturbed space weather conditions, but during small negative values of the Dst index, indicating the presence of a certain excess of the radiation belt electrons. Note, these high frequency VLF patches are observed at the frequencies much higher than the half of the equatorial electron gyrofrequency which is equal to ∼2.7 kHz at L ∼ 5.5. It seems that these emissions are generated deep in the magnetosphere, but the detailed nature, generation region, and propagation behavior of these newly discovered VLF emissions remain unknown. An appearance of high frequency VLF patches could be an indirect indicator of a local enhancement of electron fluxes in the radiation belt that are not directly measured and may occur even in the absence of visible ground-based geomagnetic disturbances. Further researches may shed new lig

The Earth-ionosphere system behaves as a waveguide for the propagation of very low frequency (VLF) radio waves. If in this system the electrical conductivity of its boundaries is disturbed, the propagation of VLF waves is also disrupted, which is observed as phase and amplitude variations of VLF waves with respect to their quiescent levels. There is a diversity of physical phenomena that are able to alter significantly the conductivity of the upper boundary. These phenomena can have their origin at the Earth (e.g., lightning), in the solar system (e.g., solar flares) or even much farther away (e.g., galactic gamma-ray bursts). The aim of this thesis is to study short- and long-term VLF variations measured in Northern Finland (at the Sodankylä Geophysical Observatory) and their associations to different phenomena. The main results are as follows: [i] The minimum energy a solar flare should have in order to produce ionospheric disturbances depends on the solar cycle. This energy is understood as the ionospheric sensitivity and for daytime conditions its value lies in the range (1–12) × 10⁻⁷ J/m². [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] VLF emissions with banded structure were observed in the 16–39 kHz frequency range, which are frequencies not usually used for the study of whistler mode VLF emissions coming from the magnetosphere. All these results are important since the VLF signals are related to variations of electron density in the ionospheric D-region, and thus have been used to identify the processes that influence the behavior, Tiivistelmä Maa-ionosfäärijärjestelmä toimii aaltoputkena erittäin matalataajuisille (VLF) radioaalloille. Jos tässä järjestelmässä sen rajojen sähkönjohtavuus häiriintyy, myös VLF-aaltojen eteneminen häiriintyy, mikä havaitaan VLF-aaltojen vaihe- ja amplitudivaihteluina suhteessa niiden tasoon hiljaisina aikoina. On olemassa useita erilaisia fysikaalisia ilmiöitä, jotka pystyvät muuttamaan merkittävästi ylärajan johtavuutta. Nämä ilmiöt voivat olla peräisin maapallolta (esim. planetaariset aallot), aurinkokunnastamme (esim. auringon roihupurkaukset) tai jopa paljon kauempaa (esim. galaktiset gammapurkaukset). Tässä väitöskirjassa tutkijaan Pohjois-Suomessa (Sodankylän geofysiikan observatoriossa) mitattuja lyhyen ja pitkän aikavälin VLF-vaihteluita ja niiden yhteyksiä eri ilmiöihin. Tärkeimmät tulokset ovat seuraavat: [i] määritettiin, miten ionosfäärin päiväajan herkkyys ulkoisille häiriötekijöille riippuu auringon aktiivisuuden vaiheesta. Tämä tutkimus ymmärretään vähimmäisenergian suhteen, joka ulkoisella tapahtumalla, kuten auringon roihupurkauksella, tulisi olla, jotta se aiheuttaisi signaalin leviämiseen vaikuttavia ionosfäärisiä häiriöitä. [ii] VLF-mittauksissa esiintyvän vuotuisen vaihtelun havaittiin liittyvän mesofäärin lämpötilaan ja auringon Lyman-α-vuohon päivä- ja yöolosuhteissa. Puolivuosittaisen vaihtelun havaittiin liittyvän geomagneettisen aktiivisuuden muutoksiin. Samalla todettiin, että auringon 27 päivän pyörimisjaksoa vastaava värähtely on hallitseva auringon aktiivisuuden laskuvaiheessa. [iii] Auringonnousuvaiheen VLF-häiriöiden pääominaisuuksien havaittiin johtuvan lyhyen aallonpituuden auringon UV-vuon varjostumisesta D-alueen ionosfäärissä johtuen stratosfäärin otsonin absorptiosta auringon noustessa. [iv] VLF-aaltoja, joissa oli raitamainen rakenne, havaittiin taajuusalueella 16–39 kHz. Näitä taajuuksia ei yleensä käytetä magnetosfääristä tulevien vihellysmoodin VLF-aaltojen tutkimiseen. Tuloksemme ovat tärkeitä, koska VLF-signaalit liitt

Thesis. 1978. Elec.E.--Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science., MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING., Includes bibliographical references., Elec.E.

Thesis. 1978. Elec.E.--Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science., MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING., Includes bibliographical references., Elec.E.