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Unprecedentedly strong and narrow electromagnetic emissions stimulated by high-frequency radio waves in the ionosphere
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.
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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2009 (English)In: Physical Review Letters, ISSN 0031-9007, Vol. 102, no 6, 065003- p.Article in journal (Refereed) Published
Abstract [en]

Experimental results of secondary electromagnetic radiation, stimulated by high-frequency radio waves irradiating the ionosphere, are reported. We have observed emission peaks, shifted in frequency up to a few tens of Hertz from radio waves transmitted at several megahertz. These emission peaks are by far the strongest spectral features of secondary radiation that have been reported. The emissions are attributed to stimulated Brillouin scattering, long predicted but hitherto never unambiguously identified in high-frequency ionospheric interaction experiments. The experiments were performed at the High-Frequency Active Auroral Research Program (HAARP), Alaska, USA.

Place, publisher, year, edition, pages
2009. Vol. 102, no 6, 065003- p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-97797DOI: 10.1103/PhysRevLett.102.065003ISI: 000263389500029OAI: oai:DiVA.org:uu-97797DiVA: diva2:172870
Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2011-11-04Bibliographically 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
Opponent
Supervisors
Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2011-01-21Bibliographically approved
2. Opening New Radio Windows and Bending Twisted Beams
Open this publication in new window or tab >>Opening New Radio Windows and Bending Twisted Beams
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In ground based high frequency (HF) radio pumping experiments, absorption of ordinary (O) mode pump waves energises the ionospheric plasma, producing optical emissions and other effects. Pump-induced or natural kilometre-scale field-aligned density depletions are believed to play a role in self-focussing phenomena such as the magnetic zenith (MZ) effect, i.e., the increased plasma response observed in the direction of Earth's magnetic field.

Using ray tracing, we study the propagation of ordinary (O) mode HF radio waves in an ionosphere modified by density depletions, with special attention to transmission through the radio window (RW), where O mode waves convert into the extraordinary (X, or Z) mode. The depletions are shown to shift the position of the RW, or to introduce RWs at new locations. In a simplified model neglecting absorption, we estimate the wave electric field strength perpendicular to the magnetic field at altitudes normally inaccessible. This field could excite upper hybrid waves on small scale density perturbations.

We also show how transmission and focussing combine to give stronger fields in some directions, notably at angles close to the MZ, with possible implications for the MZ effect.

In a separate study, we consider electromagnetic (e-m) beams with helical wavefronts (i.e., twisted beams), which are associated with orbital angular momentum (OAM). By applying geometrical optics to each plane wave component of a twisted nonparaxial e-m Bessel beam, we calculate analytically the shift of the beam's centre of gravity during propagation perpendicularly and obliquely to a weak refractive index gradient in an isotropic medium. In addition to the so-called Hall shifts expected from paraxial theory, the nonparaxial treatment reveals new shifts in both the transverse and lateral directions. In some situations, the new shifts should be significant also for nearly paraxial beams.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2011. 77 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 854
Keyword
Orbital angular momentum, Hall effect, Nonparaxial beam, Geometrical optics, High frequency radio waves, Ray tracing, Magnetic zenith effect, Ionospheric irregularities, Wave propagation
National Category
Fusion, Plasma and Space Physics Atom and Molecular Physics and Optics
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-158797 (URN)978-91-554-8160-5 (ISBN)
Public defence
2011-10-28, Polhemssalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2011-10-07 Created: 2011-09-14 Last updated: 2011-11-04Bibliographically approved

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