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Opening New Radio Windows and Bending Twisted Beams
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, Department of Physics and Astronomy, Space Plasma Physics. (Physics in Space)
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 [en]
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: urn:nbn:se:uu:diva-158797ISBN: 978-91-554-8160-5 (print)OAI: oai:DiVA.org:uu-158797DiVA: diva2:441192
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
List of papers
1. Transverse and Lateral Shifts of the Centre of Gravity of a Nonparaxial Bessel Beam Propagating Perpendicularly to a Refractive Index Gradient
Open this publication in new window or tab >>Transverse and Lateral Shifts of the Centre of Gravity of a Nonparaxial Bessel Beam Propagating Perpendicularly to a Refractive Index Gradient
(English)Article in journal (Refereed) Submitted
Abstract [en]

By applying geometrical optics (GO) to each plane wave component of a nonparaxial electromagnetic (e-m) Bessel beam carrying spin and orbital angular momentum (SAM/OAM), we calculate the shift of the beam centroid during propagation perpendicular to the refractive index gradient in an isotropic medium. In addition to the transverse spin and orbital Hall shifts expected from paraxial theory, the nonparaxial treatment reveals new shifts in both the transverse and lateral directions.

Keyword
Orbital angular momentum, Hall effect, Nonparaxial beam
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-158792 (URN)
Available from: 2011-09-14 Created: 2011-09-14 Last updated: 2011-11-04Bibliographically approved
2. Transverse and Lateral Shifts of the Centre of Gravity of a Refracted Nonparaxial Bessel Beam Carrying Spin and Orbital Angular Momentum
Open this publication in new window or tab >>Transverse and Lateral Shifts of the Centre of Gravity of a Refracted Nonparaxial Bessel Beam Carrying Spin and Orbital Angular Momentum
(English)Article in journal (Refereed) Submitted
Abstract [en]

By applying geometrical optics (GO) to each plane wave component of a nonparaxial electromagnetic (e-m) Bessel beam carrying spin and orbital angular momentum (SAM/OAM), we calculate the shift of the beam centroid during oblique propagation in an isotropic gradient-index medium. In addition to the transverse spin and orbital Hall shifts expected from paraxial theory, the nonparaxial treatment reveals new shifts in both the transverse and lateral directions. When the propagation is close to perpendicular to the density gradient, the new shifts should be significant also for nearly paraxial beams. Suggestions are given for an experimental verification of the results.

Keyword
Orbital angular momentum, Hall effect, Nonparaxial beam
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-158794 (URN)
Available from: 2011-09-14 Created: 2011-09-14 Last updated: 2011-11-04Bibliographically approved
3. Ray tracing analysis of L mode pumping of the ionosphere, with implications for the magnetic zenith effect
Open this publication in new window or tab >>Ray tracing analysis of L mode pumping of the ionosphere, with implications for the magnetic zenith effect
2010 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 28, no 9, 1749-1759 p.Article in journal (Refereed) Published
Abstract [en]

Using ray tracing of ordinary mode HF waves in an ionosphere with kilometre-scale field aligned density depletions, or ducts, we find that transmission across the plasma resonance becomes possible at new locations as rays are guided into the so-called L mode. Stronger transmitted fields are seen in some directions, notably at inclinations close to the vertical or the magnetic zenith (MZ). It is argued that the results could have implications for the magnetic zenith effect, i.e., the increased plasma response that has been observed around the MZ.

Keyword
Ionosphere, Active experiments, Ionospheric irregularities, Wave propagation
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-147256 (URN)10.5194/angeo-28-1749-2010 (DOI)000282424600012 ()
Available from: 2011-02-25 Created: 2011-02-24 Last updated: 2017-12-11Bibliographically approved
4. Self-focused radio frequency L wave pumping of localized upper hybrid oscillations in high-latitude ionospheric plasma
Open this publication in new window or tab >>Self-focused radio frequency L wave pumping of localized upper hybrid oscillations in high-latitude ionospheric plasma
2009 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 36, no 24, L24105- p.Article in journal (Refereed) Published
Abstract [en]

In experiments on radio frequency pumping of ionospheric plasma it is usually assumed that the pump wave propagates in the ordinary (O) mode. However, it is shown by ray tracing that commonly excited filamentary density inhomogeneities will guide a transmitted O-mode pump wave along the geomagnetic field as an L wave. Nonlinearly guided L-wave pumping of long predicted localized upper hybrid oscillations offers a unified understanding of a range of experimental results, including strong self-focusing for pump beams near geomagnetic zenith.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-148208 (URN)10.1029/2009GL041438 (DOI)000273254900004 ()
Available from: 2011-03-03 Created: 2011-03-03 Last updated: 2017-12-11Bibliographically approved
5. Unprecedentedly strong and narrow electromagnetic emissions stimulated by high-frequency radio waves in the ionosphere
Open this publication in new window or tab >>Unprecedentedly strong and narrow electromagnetic emissions stimulated by high-frequency radio waves in the ionosphere
Show others...
2009 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, 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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-97797 (URN)10.1103/PhysRevLett.102.065003 (DOI)000263389500029 ()
Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2017-12-14Bibliographically approved

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