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Transient dynamics of secondary radiation from an HF pumped magnetized space plasma
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
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2007 (English)In: Journal of Geophysical Research, ISSN 0148-0227, Vol. 112, no A9, A09303- p.Article in journal (Refereed) Published
Abstract [en]

In order to systematically analyze the transient wave and radiation processes that are excited when a high-frequency (HF) radio wave is injected into a magnetized space plasma, we have measured the secondary radiation, or stimulated electromagnetic emission (SEE), from the ionosphere, preconditioned such that geomagnetic field-aligned plasma irregularities are already present. The transient dynamics experiments were made using a duty cycle of the HF radio wave of 200 ms (180 ms on and 20 ms off) and 100 ms (80 ms on and 20 ms off) for various frequencies near the fifth harmonic of the local ionospheric electron cyclotron frequency. Within the first 10 ms after the radio pulse turn-on, frequency downshifted structures of the SEE exhibit an overshoot with a maximum at 3 ms < t < 8 ms, whereas the upshifted spectral components do not exhibit this feature. The relative magnitude of the overshoot is strongly dependent on the frequency offset of the pump from the harmonic of the electron cyclotron frequency. A transient blue-shifted frequency component is identified. This component is upshifted from the pump by 14 kHz < Δ f< 55 kHz and exists only within the first 10 ms after the radio pulse turn-on. On a longer time scale we analyze the amplitude modulation, or "ringing," of the reflected radio wave, (also known as "quasi-periodic oscillations" or "spikes"). The ringing has a frequency of the order 15-20 Hz and we show that this phenomenon is also present in the SEE sidebands and is synchronized with the ringing of the reflected HF wave itself.

Place, publisher, year, edition, pages
2007. Vol. 112, no A9, A09303- p.
Keyword [en]
Quasi periodic variation, Amplitude modulation, Frequency shift, Blueshift, exhibits, harmonics, Electron cyclotron frequency, cycles, experimental studies, geomagnetic field, ionosphere, electromagnetic emission, high frequency, Radio wave, Plasma, radiation, dynamics, transient phenomena
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-97795DOI: 10.1029/2007JA012454ISI: 000249373600012OAI: oai:DiVA.org:uu-97795DiVA: diva2:172868
Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2011-01-21Bibliographically approved
In thesis
1. Secondary Electromagnetic Radiation Generated by HF Pumping of the Ionosphere
Open this publication in new window or tab >>Secondary Electromagnetic Radiation Generated by HF Pumping of the Ionosphere
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Electromagnetic waves can be used to transmit information over long distances and are therefore often employed for communication purposes. The electromagnetic waves are reflected off material objects on their paths and interact with the medium through which they propagate. For instance, the plasma in the ionosphere can refract and even reflect radio waves propagating through it.

By increasing the power of radio waves injected into the ionosphere, the waves start to modify the plasma, resulting in the generation of a wide range of nonlinear processes, including turbulence, in particular near the reflection region. By systematically varying the injected radio waves in terms of frequency, power, polarisation, duty cycle, inclination, etc. the ionosphere can be used as an outdoor laboratory for investigating fundamental properties of the near-Earth space environment as well as of plasma turbulence. In such ionospheric modification experiments, it has been discovered that the irradiation of the ionosphere by powerful radio waves leads to the formation of plasma density structures and to the emission of secondary electromagnetic radiation, a phenomenon known as stimulated electromagnetic emission. These processes are highly repeatable and have enabled systematic investigations of the nonlinear properties of the ionospheric plasma.

In this thesis we investigate features of the plasma density structures and the secondary electromagnetic radiation. In a theoretical study we analyse a certain aspect of the formation of the plasma structures. The transient dynamics of the secondary radiation is investigated experimentally in a series of papers, focussing on the initial stage as well as on the decay. In one of the papers we use the transient dynamics of the secondary radiation to reveal the intimate relation between certain features of the radiation and structures of certain scales. Further, we present measurements of unprecedentedly strong secondary radiation, attributed to stimulated Brillouin scattering, and report measurements of the secondary radiation using a novel technique imposed on the transmitted radio waves.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. 59 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 576
Keyword
space physics, ionospheric modification experiments, plasma turbulence, stimulated electromagnetic emission, ionospheric irregularities, radio waves, parametric processes, thermal parametric instability, transient dynamics, wave-wave interactions
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-9393 (URN)978-91-554-7344-0 (ISBN)
Public defence
2008-12-12, Siegbahnsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15
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Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2011-01-21Bibliographically approved

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