| 1. |
- Andrews, David J., et al.
(författare)
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MARSIS Observations of Field-Aligned Irregularities and Ducted Radio Propagation in the Martian Ionosphere
- 2018
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 123:8, s. 6251-6263
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Artificial field-aligned irregularities in the nightside auroral ionosphere
- 2006
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Ingår i: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 38:11, s. 2503-2510
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Heater-induced phenomena in a coupled ionosphere-magnetosphere system
- 2006
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Ingår i: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 38:11, s. 2495-2502
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Numerical study of upper hybrid to Z-mode leakage during electromagnetic pumping of groups of striations in the ionosphere
- 2015
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 33:8, s. 1019-1030
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Electron gyroharmonic effects in ionization and electron acceleration during high-frequency pumping in the ionosphere
- 2006
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 97:19, s. 195002-
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Excitation of localized rotating waves in plasma density cavities by scattering of fast magnetosonic waves
- 2004
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 92:25 pt.1, s. 255002-
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Tidskriftsartikel (refereegranskat)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-
(författare)
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Interaction between Electromagnetic Waves and Localized Plasma Oscillations
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
(författare)
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Electromagnetic interaction of localized upper hybrid oscillations in a system of density depletions
- 2006
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Ingår i: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 38:11, s. 2516-2517
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Enhanced nonlinear interaction of powerful electromagnetic waves with ionospheric plasma near the second electron gyroharmonic
- 2013
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 20:5, s. 052904-
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Tidskriftsartikel (refereegranskat)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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