| 26. |
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| 27. |
- Mohammadi, Siavoush, M, et al.
(author)
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Interactive visualization of new electromagnetic quantities
- 2008
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In: SIGRAD 2008. - : Linköping University Electronic press. ; , s. 71-74
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Conference paper (peer-reviewed)abstract
- Recent development in classical electrodynamics has demonstratedthe usefulness of different rotational and topological modes in theelectromagnetic fields (angular momentum, polarization, vorticityetc.). Unfortunately, the visualization tools available to illustratethese electrodynamic quantities have hitherto been inadequate.Therefore we have developed a VTK and Python based interactivevisualization tool, with working name EMVT (ElectroMagnetic VisualizationTool), targeted at visualizing precisely these modes.In the near future, EMVT will be further developed to visualize andcontrol live antenna systems, where electromagnetic field data isinstantly received, calculated, and visualized from an antenna or asystem of antennas. It will then be possible to see how the antennaproperties change through direct user interaction in real time.
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| 28. |
- Norin, Lars, et al.
(author)
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Ionospheric plasma density irregularities measured by stimulated electromagnetic emission
- 2008
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A9, s. A09314-
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Journal article (peer-reviewed)abstract
- It is well known that ionospheric plasma turbulence can be conveniently generated by controlled injection of powerful high-frequency radio beams from the ground. Irradiation of the ionosphere with such radio waves leads to the formation of plasma density structures, striations, and the generation of secondary electromagnetic radiation, a phenomenon known as stimulated electromagnetic emission (SEE). In this paper we present experimental results of the dependence of SEE on decreasing excitation levels of the striations. In the experiments the frequency of the injected radio beam was varied near the fifth harmonic of the local ionospheric electron gyro frequency. We use the SEE measurements to obtain transverse length scales of the striations involved in the generation of the SEE. Our results show that different spectral features of the SEE display different temporal dynamics, suggesting that they are related to striations with different transverse length scales (1 less than or similar to L-perpendicular to less than or similar to 25 m).
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| 29. |
- Norin, Lars, et al.
(author)
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On the linear stage of thermal parametric instabilities in the ionosphere excited by HF pumping near electron gyroharmonics
- 2006
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In: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 38:11, s. 2527-2532
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Journal article (peer-reviewed)abstract
- Thresholds and growth rates of the thermal parametric instability in the ionosphere were studied taking into account the regular inhomogeneity (altitude dependence) of the plasma density and of the magnetic field strength. A noticeable decrease of the pump wave threshold power and an increase of the growth rate occurs when f0 approaches the double resonance frequency fd = nfce(zd) = fUH(zd), zd being the altitude for double resonance. Particularly, the threshold power decreases relative to the non-resonant case by a factor 3-7 at |f0 - fd(zd)| ≲ 15 kHz for n = 3 and up to a factor 3 for n = 4. The instability growth rate can increase with a factor of 103-104 relative to the non-resonant case. The decrease of the threshold and increase of the growth rate occur for quite small scales of the striations, l⊥ ∼ 1 m for n = 3 and l⊥ ∼ 0.5 m for n = 4.
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| 30. |
- Norin, Lars, 1976-
(author)
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Secondary Electromagnetic Radiation Generated by HF Pumping of the Ionosphere
- 2008
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Doctoral thesis (other academic/artistic)abstract
- 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.
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| 31. |
- Norin, Lars, et al.
(author)
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Transient dynamics of secondary radiation from an HF pumped magnetized space plasma
- 2007
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 119:A9
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Journal article (peer-reviewed)abstract
- 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.
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| 32. |
- Norin, Lars, et al.
(author)
-
Unprecedentedly strong and narrow electromagnetic emissions stimulated by high-frequency radio waves in the ionosphere
- 2009
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:6, s. 065003-
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Journal article (peer-reviewed)abstract
- 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.
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| 33. |
- Stål, Oscar, et al.
(author)
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Lunar Satellite Detection of Ultra-High Energy Neutrinos with the Use of Radio Methods
- 2005
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In: DGLR Interational Symposium: To Moon and Beyond, Bremen, Germany, 15-16 Sep 2005..
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Conference paper (peer-reviewed)abstract
- Neutrinos interact with matter only through weak processes with low cross-section. To detect cosmic neutrinos most efforts have relied on the detection of visible Vavilov-Cerenkov light in detectors embedded in the target volumes. To access the decreasing flux of ultra-high energy neutrinos, far above 1 PeV, ideas on how to increase the detection volume by observing coherent radio frequency emission caused by the Askaryan effect have been put forward. Here we describe how a satellite in lunar orbit equipped with an electromagnetic vector sensor could detect Askaryan pulses induced by neutrinos interacting with the moon. The threshold neutrino energy is found to be 50 EeV for this setup, and the sensitivity is determined from simulations. A model dependent event rate of 2.2 events per year is calculated.
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| 34. |
- Stål, Oscar, et al.
(author)
-
Prospects for Lunar Satellite Detection of Radio Pulses from Ultrahigh Energy Neutrinos Interacting with the Moon
- 2007
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 98:7
-
Journal article (peer-reviewed)abstract
- The Moon provides a huge effective detector volume for ultrahigh energy cosmic neutrinos, which generate coherent radio pulses in the lunar surface layer due to the Askaryan effect. We report systematic Monte Carlo simulations which show that radio instruments on board a Moon-orbiting satellite can detect Askaryan pulses from neutrinos with energies above 10^{19} eV, i.e. near and above the interesting GZK limit, at the very low fluxes predicted in different scenarios.
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| 35. |
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| 36. |
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| 37. |
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| 38. |
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| 39. |
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| 40. |
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| 41. |
- Thidé, Bo
(author)
-
Nonlinear physics of the ionosphere and LOIS/LOFAR
- 2007
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In: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 49:12B, s. B103-B107
-
Journal article (peer-reviewed)abstract
- The ionosphere is the only large-scale plasma laboratory without walls that we have direct access to. Here we can study, both in situ and from the ground, basic small- and large-scale processes and fundamental physical principles that control planet Earth's interaction with its space environment. From results obtained in systematic, repeatable experiments, where we can vary the stimulus and observe its response in a controlled, laboratory-like manner, we can draw conclusions on similar physical processes occurring naturally in the Earth's plasma environment as well as in parts of the plasma universe that are not easily accessible to direct probing. Of particular interest is electromagnetic turbulence excited in the ionosphere by beams of particles (photons, electrons) and its manifestation in terms of secondary radiation (electrostatic and electromagnetic waves), structure formation (solitons, cavitons, alfveons, hybrons, striations) and the associated exchange of energy, linear momentum and angular momentum. The primarily astrophy sic s-oriented, distributed radio telescope Low Frequency Array (LOFAR) currently under construction in the Netherlands, Germany and France, will operate in a frequency range (10-240 MHz), close to fundamental ionospheric plasma resonance/cut-off frequencies, with a sensitivity that is orders of magnitude higher than any radio (or radar) facility used so far. The LOFAR Outrigger in Scandinavia (LOIS) radio and radar facility, with one station in Vaxjo in southern Sweden and three more planned in the same area (Ronneby, Kalmar, Lund) plus one near Poznan in Poland, supplements LOFAR with optimized Earth and space observing extensions. For this purpose LOIS will operate in the same frequency range as LOFAR (but extended on the low-frequency side) and will augment the observation capability to enable direct radio imaging of plasma vorticity.
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| 42. |
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