| 1. |
- Andrews, David J., et al.
(author)
-
MARSIS Observations of Field-Aligned Irregularities and Ducted Radio Propagation in the Martian Ionosphere
- 2018
-
In: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 123:8, s. 6251-6263
-
Journal article (peer-reviewed)abstract
- Knowledge of Mars's ionosphere has been significantly advanced in recent years by observations from Mars Express and lately Mars Atmosphere and Volatile EvolutioN. A topic of particular interest are the interactions between the planet's ionospheric plasma and its highly structured crustal magnetic fields and how these lead to the redistribution of plasma and affect the propagation of radio waves in the system. In this paper, we elucidate a possible relationship between two anomalous radar signatures previously reported in observations from the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument on Mars Express. Relatively uncommon observations of localized, extreme increases in the ionospheric peak density in regions of radial (cusp-like) magnetic fields and spread echo radar signatures are shown to be coincident with ducting of the same radar pulses at higher altitudes on the same field lines. We suggest that these two observations are both caused by a high electric field (perpendicular to B) having distinctly different effects in two altitude regimes. At lower altitudes, where ions are demagnetized and electrons magnetized, and recombination dominantes, a high electric field causes irregularities, plasma turbulence, electron heating, slower recombination, and ultimately enhanced plasma densities. However, at higher altitudes, where both ions and electrons are magnetized and atomic oxygen ions cannot recombine directly, the high electric field instead causes frictional heating, a faster production of molecular ions by charge exchange, and so a density decrease. The latter enables ducting of radar pulses on closed field lines, in an analogous fashion to interhemispheric ducting in the Earth's ionosphere.
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| 2. |
- Blagoveshchenskaya, N. F., et al.
(author)
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Artificial field-aligned irregularities in the nightside auroral ionosphere
- 2006
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In: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 38:11, s. 2503-2510
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Journal article (peer-reviewed)abstract
- Experimental results from two Tromso HF heating experiments in the nightside high latitudinal F region are examined. Bi-static scatter measurements of HF diagnostic signals were carried out on the London-Tromso-St. Petersburg and Pori-Tromso-St. Petersburg paths using a Doppler spectral method. The properties and behaviour of artificial field-aligned small-scale irregularities (striations) in the nightside high latitudinal F-region in course of the Tromso ionospheric modification experiments are studied. Experimental studies have been performed by the use of phased array I with a beamwidth of 6 degrees instead of 12-14 degrees in phased array 2, more often used in Tromso ionospheric modification experiments. The comparison between two experiments carried out in the same background geophysical conditions, shows the strongest striations in the field-aligned position of the heater beam. Possible explanation for this angular dependence is self-focusing of HF pump waves on striations causing the energy to be distributed asymmetrically. A principal question related to HF heating experiments is how is the disturbed auroral ionosphere modified. The results obtained on two paths simultaneously have shown that the strong heater-induced striations were observed along with natural ones. Velocities of heater-induced striations were quite different at different parts of the heated volume. It is suggested that the heater-induced striations can be grouped in two patches at different heights possibly due to the temperature-gradient-driven instability.
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| 3. |
- Blagoveshchenskaya, N. F., et al.
(author)
-
Heater-induced phenomena in a coupled ionosphere-magnetosphere system
- 2006
-
In: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 38:11, s. 2495-2502
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Journal article (peer-reviewed)abstract
- Experimental results from HF pumping experiments in the nightside auroral E and F region are reported. The experiments were carried out by the use of the EISCAT HF heating facility located near Tromso, Norway, allowing HF pumping the ionosphere in a near magnetic field-aligned direction. We present experimental results from multi-instrument observations related to heater-induced phenomena in a coupled ionosphere-magnetosphere system. The following results have been observed on different occasions: a reverberation effect in scattered signals observed simultaneously on two diagnostic paths which is an indication of Alfven wave generation. This phenomenon was seen under specific disturbed background geophysical conditions, namely, a high electron density in the F region up to 8 MHz produced by soft electron precipitation from the magnetosphere along with low electron density in lower ionosphere; increased ionospheric electric fields; ion outflows from the ionosphere. On another occasion a magnetospheric response to heater turning on and off was found from magnetic pulsation observations over a frequency range up to 5 Hz (the upper frequency limit of the sensitive magnetometer at Kilpisjarvi, located near Tromso). The response manifests itself about 1 min after the heater is turned on and off. Other results have shown the modification of a natural auroral arc and local spiral-like formation. It is thought that a local heater-driven current system is formed. An interesting feature is the generation of the heater-induced ion outflows from the ionosphere. They are observed in night hours under both quiet and disturbed magnetic conditions.
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| 4. |
- Eliasson, B., et al.
(author)
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Numerical study of upper hybrid to Z-mode leakage during electromagnetic pumping of groups of striations in the ionosphere
- 2015
-
In: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 33:8, s. 1019-1030
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Journal article (peer-reviewed)abstract
- We investigate numerically the interaction between ionospheric magnetic field-aligned density striations and a left-hand circularly polarized (L)-mode wave. The L-mode wave is scattered into upper hybrid (UH) waves which are partially trapped in the striations, but leak energy to electromagnetic waves in the Z-mode branch. For small-amplitude (1 %) striations, this loss mechanism leads to a significant reduction in amplitude of the UH waves. For several striations organized in a lattice, the leaking of Z-mode waves is compensated by influx of Z-mode radiation from neighboring striations, leading to an increased amplitude of the weakly trapped UH waves. For large-amplitude (10 %) striations the trapped UH waves rapidly increase in amplitude far beyond the threshold for parametric instabilities, and the Z-mode leakage is less important. The results have relevance for the growth of striations and the onset of UH and lower hybrid turbulence during electromagnetic high-frequency pumping of ionospheric plasma, which require large-amplitude UH waves.
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| 5. |
- Gustavsson, B., et al.
(author)
-
Electron gyroharmonic effects in ionization and electron acceleration during high-frequency pumping in the ionosphere
- 2006
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 97:19, s. 195002-
-
Journal article (peer-reviewed)abstract
- Optical emissions and incoherent scatter radar data obtained during high-frequency electromagnetic pumping of the ionospheric plasma from the ground give data on electron energization in an energy range from 2 to 100 eV. Optical emissions at 4278 angstrom from N-2(+) that require electrons with energies above the 18 eV ionization energy give the first images ever of pump-induced ionization of the thermosphere. The intensity at 4278 angstrom is asymmetric around the ionospheric electron gyroharmonic, being stronger above the gyroresonance. This contrasts with emissions at 6300 angstrom from O(D-1) and of electron temperature enhancements, which have minima at the gyroharmonic but have no apparent asymmetry. This direct evidence of pump-induced ionization contradicts previous indirect evidence, which indicated that ionization is most efficiently produced when the pump frequency was below the gyroharmonic.
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| 6. |
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| 7. |
- Hall, Jan-Ove, et al.
(author)
-
Excitation of localized rotating waves in plasma density cavities by scattering of fast magnetosonic waves
- 2004
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 92:25 pt.1, s. 255002-
-
Journal article (peer-reviewed)abstract
- An analytic description of electromagnetic waves in an inhomogeneous plasma is applied to investigate excitation of localized rotating waves below the lower hybrid frequency through scattering of fast magnetosonic waves on a density cavity. The magnetosonic wave is focused to left-handed rotating oscillations. We find the amplitude of the localized oscillations, resonance frequencies, and the width of the resonances. The theory is relevant for the lower hybrid solitary structures observed in space plasmas and is shown to be consistent with observations by the Freja satellite.
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| 8. |
- Hall, Jan-Ove, 1971-
(author)
-
Interaction between Electromagnetic Waves and Localized Plasma Oscillations
- 2004
-
Doctoral thesis (other academic/artistic)abstract
- This thesis treats interaction between electromagnetic waves and localized plasma oscillations. Two specific physical systems are considered, namely artificially excited magnetic field-aligned irregularities (striations) and naturally excited lower hybrid solitary structures (LHSS). Striations are mainly density depletions of a few percent that are observed when a powerful electromagnetic wave, a pump wave, is launched into the ionosphere. The striations are formed by upper hybrid (UH) oscillations that are localized in the depletion where they are generated by the linear conversion of the pump field on the density gradients. However, the localization is not complete as the UH oscillation can convert to a propagating electromagnetic Z mode wave. This process, termed Z mode leakage, causes damping of the localized UH oscillation. The Z mode leakage is investigated and the theory predicts non-Lorentzian skewed shapes of the resonances for the emitted Z mode radiation. Further, the interaction between individual striations facilitated by the Z mode leakage is investigated. The LHSS are observed by spacecraft in the ionosphere and magnetosphere as localized waves in the lower hybrid (LH) frequency range that coincides with density cavities. The localized waves are immersed in non-localized wave activity. The excitation of localized waves with frequencies below LH frequency is modelled by scattering of electromagnetic magnetosonic (MS) waves off a preexisting density cavity. It is shown analytically that an incident MS wave with frequency less than the minimum LH frequency inside the cavity is focused to localized waves with left-handed rotating wave front. In addition, the theory is shown to be consistent with observations by the Freja satellite. For frequencies between the minimum LH frequency inside the cavity and the ambient LH frequency, the MS wave is instead mode converted and excites pressure driven LH oscillations. This process is studied in a simplified geometry.
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| 9. |
- Istomin, Yakov, et al.
(author)
-
Electromagnetic interaction of localized upper hybrid oscillations in a system of density depletions
- 2006
-
In: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 38:11, s. 2516-2517
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Journal article (peer-reviewed)abstract
- We present the results of the investigation of interaction of different depletions of plasma density by Z-mode radiation. It is shown that more profitable disposition of cavities in space is random. The amplitude of the radiated Z-mode in this case is of the order of the amplitude of the pumping ordinary wave, but the amplitude of trapped into cavities upper hybrid wave is maximum and can be significantly higher than the amplitude of the pumping wave.
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| 10. |
- Istomin, Ya. N., et al.
(author)
-
Enhanced nonlinear interaction of powerful electromagnetic waves with ionospheric plasma near the second electron gyroharmonic
- 2013
-
In: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 20:5, s. 052904-
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Journal article (peer-reviewed)abstract
- Plasma experiments in which a powerful electromagnetic pump wave is transmitted into the ionosphere from the ground give access to a rich range of phenomena, including gyroharmonic effects when the pump frequency is near an harmonic of the ionospheric electron gyrofrequency. For pump frequencies close to the second gyroharmonic, experiments show a strong enhancement, as observed in radar scatter from pump-induced geomagnetic field-aligned density striations and optical emissions. This is in contrast to the case at the third harmonic and higher at which most of the effects are instead suppressed. We show theoretically that electrostatic oscillations can be localized in density inhomogeneities associated with small scale striations. The localized field is a mixture of the electron Bernstein and upper hybrid modes when the pump frequency is near the second gyroharmonic. The coupling of the modes is enabled by a symmetry feature of the linear electron Bernstein and upper hybrid dispersion properties that occur only near the second gyroharmonic. Electron acceleration inside the density inhomogeneities by localized azimuthal electrostatic oscillations is more efficient near the second gyroharmonic than at higher frequencies, consistent with the observed enhancements.
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| 11. |
- Istomin, Ya. N., et al.
(author)
-
Kinetics of density striations excited by powerful electromagnetic waves in the ionosphere
- 2010
-
In: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 17:3, s. 032903-
-
Journal article (peer-reviewed)abstract
- One of the most important effects observed when pumping ionospheric plasma by powerful radio waves from the ground is the excitation of filamentary density striations that are stretched along the ambient geomagnetic field. The kinetics of the striations present in the pump electromagnetic field is studied theoretically. The density irregularities cause inhomogeneities in the pump field, which result in a ponderomotive force acting on the striations that makes the density depressions move perpendicular to the geomagnetic field. Striations moving with different velocities can collide, thereby merging to produce larger scale striations. The merging of striations constitutes a cascade process that distributes the energy over the spatial spectrum of the striations. The resulting inhomogeneity spectrum as well as the obtained outward radial drift of a few meters per second is consistent with experimental results.
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| 12. |
- Leyser, Thomas B., et al.
(author)
-
Plasma angular momentum effects and twisted incoherent scatter radar beams
- 2012
-
In: Radio Science. - 0048-6604 .- 1944-799X. ; 47, s. RS5004-
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Journal article (peer-reviewed)abstract
- Phased arrays provide new possibilities for remote sensing with radars. By imposing an azimuthal phase variation, electromagnetic beams that carry orbital angular momentum can be formed. Such beams have a phase structure that appears twisted and as a result an intensity null in the center of the beam cross section. Here we numerically investigate twisted beams for incoherent scatter radars that are used to study the ionosphere. We discuss the possibility of utilizing such radar beams to probe twisted beams of plasma waves and flows transverse to the beam axis, such as associated with auroral arcs. Transverse plasma flows may give rise to a rotational frequency shift of the scatter from a twisted beam and Doppler broadening due to the beam divergence, the latter also occurring with regular beams. Although the angular momentum effects of the considered large scale flows are generally small, sheared and vortical flows transverse to the beam axis can in principle be discriminated from unidirectional flows with beams carrying orbital angular momentum.
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| 13. |
- Leyser, Thomas B., et al.
(author)
-
Powerful electromagnetic waves for active environmental research in geospace
- 2009
-
In: Reviews of geophysics. - 8755-1209 .- 1944-9208. ; 47, s. RG1001-
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Research review (peer-reviewed)abstract
- Powerful electromagnetic (EM) waves can exert well-defined influence on the atmosphere, ionosphere, and magnetosphere. These active EM interactions can provide spatiotemporal information on the near-Earth space environment (geospace). Objectives include remote monitoring and controlling of a wide range of parameters of geospace, controlling properties of the ionosphere and magnetosphere, as well as interaction with large-scale natural energy sources. In addition, applications such as mitigation of atmospheric pollutants and solar power satellites are discussed. Studies of EM wave interactions also contribute to the knowledge of anthropogenic effects in the geospace environment, such as the increasing use of EM radiation.
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| 14. |
- Leyser, Thomas B., et al.
(author)
-
Radio Pumping of Ionospheric Plasma with Orbital Angular Momentum
- 2009
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:6, s. 065004-
-
Journal article (peer-reviewed)abstract
- Experimental results are presented of pumping ionospheric plasma with a radio wave carrying orbital angular momentum (OAM), using the High Frequency Active Auroral Research Program (HAARP) facility in Alaska. Optical emissions from the pumped plasma turbulence exhibit the characteristic ring-shaped morphology when the pump beam carries OAM. Features of stimulated electromagnetic emissions (SEE) that are attributed to cascading Langmuir turbulence are well developed for a regular beam but are significantly weaker for a ring-shaped OAM beam in which case upper hybrid turbulence dominates the SEE.
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| 15. |
- Leyser, Thomas
(author)
-
Deterministic Chaos in Ionospheric Plasma Pumped by Radio Waves
- 2021
-
In: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 48:12
-
Journal article (peer-reviewed)abstract
- A commonly observed spectral feature of electromagnetic emissions stimulated by powerful radio waves in ionospheric plasma is found to have an exponential frequency spectrum. This so called broad upshifted maximum (BUM) is excited for pump frequencies near and slightly above an harmonic of the ionospheric electron gyro frequency. A simplified model of lower hybrid oscillations driven by parametric four-wave interaction localized in a cylindrical density depletion shows how the plasma drift can exhibit deterministic chaos. The associated exponential spectra are consistent with the experimental results.
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| 16. |
- Leyser, Thomas, et al.
(author)
-
Electron heating by HF pumping of high-latitude ionospheric F-region plasma near magnetic zenith
- 2020
-
In: Annales Geophysicae. - : COPERNICUS GESELLSCHAFT MBH. - 0992-7689 .- 1432-0576. ; 38:2, s. 297-307
-
Journal article (peer-reviewed)abstract
- High-frequency electromagnetic pumping of ionospheric F-region plasma at high and mid latitudes gives the strongest plasma response in magnetic zenith, antiparallel to the geomagnetic field in the Northern Hemisphere. This has been observed in optical emissions from the pumped plasma turbulence, electron temperature enhancements, filamentary magnetic field-aligned plasma density irregularities, and in self-focusing of the pump beam in magnetic zenith. We present results of EISCAT (European Incoherent SCAT-ter association) Heating-induced magnetic-zenith effects observed with the EISCAT UHF incoherent scatter radar. With heating transmitting a left-handed circularly polarized pump beam towards magnetic zenith, the UHF radar was scanned in elevation in steps of 1.0 and 1.5 degrees around magnetic zenith. The electron energy equation was integrated to model the electron temperature and associated electron heating rate and optimized to fit the plasma parameter values measured with the radar. The experimental and modelling results are consistent with pump wave propagation in the L mode in magnetic zenith, rather than in the O mode.
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| 17. |
- Leyser, Thomas, et al.
(author)
-
Evidence of L - mode electromagnetic wave pumping of ionospheric plasma near geomagnetic zenith
- 2018
-
In: Annales Geophysicae. - : COPERNICUS GESELLSCHAFT MBH. - 0992-7689 .- 1432-0576. ; 36:1, s. 243-251
-
Journal article (peer-reviewed)abstract
- The response of ionospheric plasma to pumping by powerful HF (high frequency) electromagnetic waves transmitted from the ground into the ionosphere is the strongest in the direction of geomagnetic zenith. We present experimental results from transmitting a left-handed circularly polarized HF beam from the EISCAT (European Incoherent SCATter association) Heating facility in magnetic zenith. The CASSIOPE (CAScade, Smallsat and IOnospheric Polar Explorer) spacecraft in the topside ionosphere above the F-region density peak detected transionospheric pump radiation, although the pump frequency was below the maximum ionospheric plasma frequency. The pump wave is deduced to arrive at CASSIOPE through L -mode propagation and associated double (O to Z, Z to O) conversion in pump-induced radio windows. L -mode propagation allows the pump wave to reach higher plasma densities and higher ionospheric altitudes than O -mode propagation so that a pump wave in the L -mode can facilitate excitation of upper hybrid phenomena localized in density depletions in a larger altitude range. L -mode propagation is therefore suggested to be important in explaining the magnetic zenith effect.
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| 18. |
- Leyser, Thomas, et al.
(author)
-
On mechanisms for high-frequency pump-enhanced optical emissions at 557.7 and 630.0 nm from atomic oxygen in the high-latitude F-region ionosphere
- 2023
-
In: Annales Geophysicae. - : European Geosciences Union (EGU). - 0992-7689 .- 1432-0576. ; 41:2, s. 589-600
-
Journal article (peer-reviewed)abstract
- The EISCAT (European Incoherent Scatter Scientific Association) Heating facility was used to transmit powerful high-frequency (HF) electromagnetic waves into the F-region ionosphere to enhance optical emissions at 557.7 and 630.0 nm from atomic oxygen. The emissions were imaged by several stations of ALIS (Auroral Large Imaging System) in northern Sweden, and the EISCAT UHF incoherent scatter radar was used to obtain plasma parameter values. The ratio of the 557.7 to 630.0 nm column emission rates changed from I5577/I6300 ≈ 0.2 for the HF pump frequency f0=6.200 MHz ≈ 4.6 f(e) to I5577/I6300 ≈ 0.5 when f0=5.423 MHz less than or similar to 4fe , where fe is the ionospheric electron gyro frequency. The observations are interpreted in terms of decreased electron heating efficiency and thereby weaker enhancement at 630.0 nm for f0=5.423 MHz less than or similar to 4fe. The emissions at 557.7 nm are attributed to electron acceleration by upper hybrid waves of metre-scale wavelengths that can be excited with f0=5.423 MHz less than or similar to 4fe.
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| 19. |
- Leyser, Thomas, et al.
(author)
-
Self-focused radio frequency L wave pumping of localized upper hybrid oscillations in high-latitude ionospheric plasma
- 2009
-
In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 36:24, s. L24105-
-
Journal article (peer-reviewed)abstract
- 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.
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| 20. |
- McCrea, Ian, et al.
(author)
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The science case for the EISCAT_3D radar
- 2015
-
In: Progress in Earth and Planetary Science. - : Springer Science and Business Media LLC. - 2197-4284. ; 2:1
-
Research review (peer-reviewed)abstract
- The EISCAT (European Incoherent SCATer) Scientific Association has provided versatile incoherent scatter (IS) radar facilities on the mainland of northern Scandinavia (the EISCAT UHF and VHF radar systems) and on Svalbard (the electronically scanning radar ESR (EISCAT Svalbard Radar) for studies of the high-latitude ionised upper atmosphere (the ionosphere). The mainland radars were constructed about 30years ago, based on technological solutions of that time. The science drivers of today, however, require a more flexible instrument, which allows measurements to be made from the troposphere to the topside ionosphere and gives the measured parameters in three dimensions, not just along a single radar beam. The possibility for continuous operation is also an essential feature. To facilitatefuture science work with a world-leading IS radar facility, planning of a new radar system started first with an EU-funded Design Study (2005-2009) and has continued with a follow-up EU FP7 EISCAT_3D Preparatory Phase project (2010-2014). The radar facility will be realised by using phased arrays, and a key aspect is the use of advanced software and data processing techniques. This type of software radar will act as a pathfinder for other facilities worldwide. The new radar facility will enable the EISCAT_3D science community to address new, significant science questions as well as to serve society, which is increasingly dependent on space-based technology and issues related to space weather. The location of the radar within the auroral oval and at the edge of the stratospheric polar vortex is also ideal for studies of the long-term variability in the atmosphere and global change. This paper is a summary of the EISCAT_3D science case, which was prepared as part of the EU-funded Preparatory Phase project for the new facility. Three science working groups, drawn from the EISCAT user community, participated in preparing this document. In addition to these working group members, who are listed as authors, thanks are due to many others in the EISCAT scientific community for useful contributions, discussions, and support.
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| 21. |
- Nordblad, Erik, 1981-
(author)
-
Opening New Radio Windows and Bending Twisted Beams
- 2011
-
Doctoral thesis (other academic/artistic)abstract
- 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.
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| 22. |
- Nordblad, Erik, et al.
(author)
-
Ray tracing analysis of L mode pumping of the ionosphere, with implications for the magnetic zenith effect
- 2010
-
In: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 28:9, s. 1749-1759
-
Journal article (peer-reviewed)abstract
- 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.
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| 23. |
- Nordblad, Erik, 1981-, et al.
(author)
-
Transverse and Lateral Shifts of the Centre of Gravity of a Refracted Nonparaxial Bessel Beam Carrying Spin and Orbital Angular Momentum
-
Journal article (peer-reviewed)abstract
- 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.
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| 24. |
- Norin, Lars, et al.
(author)
-
Ionospheric plasma density irregularities measured by stimulated electromagnetic emission
- 2008
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A9, s. A09314-
-
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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| 25. |
- Norin, Lars, et al.
(author)
-
Unprecedentedly strong and narrow electromagnetic emissions stimulated by high-frequency radio waves in the ionosphere
- 2009
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:6, s. 065003-
-
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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