| 1. |
- Belova, E., et al.
(författare)
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The dynamical background of polar mesosphere winter echoes from simultaneous EISCAT and ESRAD observations
- 2005
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Ingår i: Annales Geophysicae. - : European Geosciences Union. - 0992-7689 .- 1432-0576. ; 23:4, s. 1239-1247
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Tidskriftsartikel (refereegranskat)abstract
- On 30 October 2004 during a strong solar proton event, layers of enhanced backscatter from altitudes between 55 and 75km have been observed by both ESRAD (52MHz) and the EISCAT VHF (224MHz) radars. These echoes have earlier been termed Polar Mesosphere Winter Echoes, PMWE. After considering the morphology of the layers and their relation to observed atmospheric waves, we conclude that the radars have likely seen the same phenomenon even though the radars' scattering volumes are located about 220km apart and that the most long-lasting layer is likely associated with wind-shear in an inertio-gravity wave. An ion-chemistry model is used to determine parameters necessary to relate wind-shear induced turbulent energy dissipation rates to radar backscatter. The model is verified by comparison with electron density profiles measured by the EISCAT VHF radar. Observed radar signal strengths are found to be 2-3 orders of magnitude stronger than the maximum which can be expected from neutral turbulence alone, assuming that previously published results relating radar signal scatter to turbulence parameters, and turbulence parameters to wind shear, are correct. The possibility remains that some additional or alternative mechanism may be involved in producing PMWE, such as layers of charged dust/smoke particles or large cluster ions.
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| 2. |
- Ekeberg, Jonas, 1978-, et al.
(författare)
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Incoherent scatter ion line enhancements and auroral arc-induced Kelvin-Helmholtz turbulence
- 2015
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - Oxford : Pergamon Press. - 1364-6826 .- 1879-1824. ; 122, s. 119-128
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Tidskriftsartikel (refereegranskat)abstract
- We present two cases of incoherent-scatter ion line enhancements in conjunction with auroral arcs drifting through the radar beam. The up- and downshifted ion line shoulders as well as the spectral region between them are enhanced equally and simultaneously. The power enhancements are one order of magnitude above the thermal level and are concentrated in less than 15 km wide altitude ranges at the ionospheric F region peak. The auroral arc passages are preceded by significantly enhanced ion temperatures in the E region, which are shown to generate high velocity shears. We use a Hall MHD model of velocity shears perpendicular to the geomagnetic field and show that a Kelvin-Helmholtz instability will grow for the two presented cases. We assess the possibility that the subsequently generated low frequency turbulence can explain the observed spectrally uniform ion line power enhancements.
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| 3. |
- Ekeberg, Jonas, 1978-, et al.
(författare)
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Ion-acoustic solitary waves and spectrally uniform scattering cross section enhancements
- 2010
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 28:6, s. 1299-1306
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Tidskriftsartikel (refereegranskat)abstract
- Spectra measured by incoherent scatter radars are formed predominantly by scattering of the incident signal off ion-acoustic and Langmuir waves in the ionosphere. Occasionally, the upshifted and/or downshifted lines produced by the ion-acoustic waves are enhanced well above thermal levels and referred to as naturally enhanced ion-acoustic lines. In this paper, we study another kind of enhancement, which is spectrally uniform over the whole ion-line, i.e. the up- and downshifted shoulder and the spectral valley in between. Based on observations made with the EISCAT Svalbard radar (ESR) facility, we investigate the transient and spectrally uniform power enhancements, which can be explained by ion-acoustic solitary waves. We use a theory of nonlinear waves in a magnetized plasma to determine the properties of such waves and evaluate their effects on scattered signals measured by ESR. We suggest a new mechanism that can explain backscattered power enhancements by one order of magnitude above the thermal level and show that it is consistent with observations.
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| 4. |
- Ekeberg, Jonas, 1978-
(författare)
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Solitary waves and enhanced incoherent scatter ion lines
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis addresses solitary waves and their significance for auroral particle acceleration, coronal heating and incoherent scatter radar spectra. Solitary waves are formed due to a balance of nonlinear and dispersive effects. There are several nonlinearities present in ideal magnetohydrodynamics (MHD) and dispersion can be introduced by including theHall termin the generalised Ohm’s law. The resulting system of equations comprise the classical ideal MHD waves, whistlers, drift waves and solitarywave solutions. The latter reside in distinct regions of the phase space spanned by the speed and the angle (to the magnetic field) of the propagating wave. Within each region, qualitatively similar solitary structures are found. In the limit of neglected electron intertia, the solitary wave solutions are confined to two regions of slow and fast waves, respectively. The slow (fast) structures are associated with density compressions (rarefactions) and positive (negative) electric potentials. Such negative potentials are shown to accelerate electrons in the auroral region (solar corona) to tens (hundreds) of keV. The positive electric potentials could accelerate solar wind ions to velocities of 300–800 km/s. The structure widths perpendicular to themagnetic field are in the Earth’s magnetosphere (solar corona) of the order of 1–100 km (m). This thesis also addresses a type of incoherent scatter radar spectra, where the ion line exhibits a spectrally uniform power enhancement with the up- and downshifted shoulder and the spectral region in between enhanced simultaneously and equally. The power enhancements are one order of magnitude above the thermal level and are often localised to an altitude range of less than 20 km at or close to the ionospheric F region peak. The observations are well-described by a model of ion-acoustic solitary waves propagating transversely across the radar beam. Two cases of localised ion line enhancements are shown to occur in conjunction with auroral arcs drifting through the radar beam. The arc passages are associated with large gradients in ion temperature, which are shown to generate sufficiently high velocity shears to give rise to growing Kelvin-Helmholtz (K-H) instabilities. The observed ion line enhancements are interpreted in the light of the low-frequency turbulence associated with these instabilities.
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| 5. |
- Ekeberg, Jonas, 1978-, et al.
(författare)
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Soliton-induced spectrally uniform ion line power enhancements at the ionospheric F region peak
- 2012
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Ingår i: Earth Planets and Space. - Tokyo : Terra scientific publishing company. - 1343-8832 .- 1880-5981. ; 64:7, s. 605-611
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Tidskriftsartikel (refereegranskat)abstract
- We present European incoherent scatter (EISCAT) observations of spectrally uniform ion line power enhancements (SUIPE), where the up- and downshifted shoulder and the spectral valley between them are enhanced simultaneously and equally. We have identified 48 cases of this type of ion line enhancements in data from the EISCAT Svalbard radar taken during the International Polar Year (extending from March 2007 to the end of February 2008). The SUIPEs are observed at altitudes between 210 km and 280 km with a standard deviation of 9% of the average occurrence height 230 km. The power enhancements are one order of magnitude above the thermal level. The SUIPEs occur at the ionospheric F region peak with 85% of the cases located within 10 km of the peak. The occurrence shows a clear preference for magnetically disturbed conditions, with the likelihood of occurrence increasing with increasing K index. A majority of the events occur in the magnetic evening to pre-midnight sector.
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| 6. |
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| 7. |
- Stasiewicz, Krzysztof, et al.
(författare)
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Dispersive MHD waves and alfvenons in charge non-neutral plasmas
- 2008
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Ingår i: Nonlinear processes in geophysics. - 1023-5809 .- 1607-7946. ; 15:4, s. 681-693
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Tidskriftsartikel (refereegranskat)abstract
- Dispersive properties of linear and nonlinear MHD waves, including shear, kinetic, electron inertial Alfven, and slow and fast magnetosonic waves are analyzed using both analytical expansions and a novel technique of dispersion diagrams. The analysis is extended to explicitly include space charge effects in non-neutral plasmas. Nonlinear soliton solutions, here called alfvenons, are found to represent either convergent or divergent electric field structures with electric potentials and spatial dimensions similar to those observed by satellites in auroral regions. Similar solitary structures are postulated to be created in the solar corona, where fast alfvenons can provide acceleration of electrons to hundreds of keV during flares. Slow alfvenons driven by chromospheric convection produce positive potentials that can account for the acceleration of solar wind ions to 300-800 km/s. New results are discussed in the context of observations and other theoretical models for nonlinear Alfven waves in space plasmas.
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| 8. |
- Stasiewicz, Kristof, et al.
(författare)
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Electric potentials and energy fluxes available for particle acceleration by alfvenons in the solar corona
- 2008
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 0004-637X .- 1538-4357. ; 680:2, s. L153-L156
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Tidskriftsartikel (refereegranskat)abstract
- We show that solitary wave solutions of the one- and two-fluid MHD equations, here called alfvenons, represent two types of electric field structures with negative or positive potentials that can explain the acceleration of particles in the solar corona. Negative potentials are created self-consistently by fast alfvenons and can reach hundreds of kilovolts, which could accelerate the electrons that produce X-ray emissions during flares. Slow alfvenons create positive potential structures of a few kV that accelerate solar wind ions. These alfvenons can be powered by irregular plasma flows in the photosphere and chromosphere, as well as by time-varying magnetic fields during reconnection events at the tops of coronal loops. Similar alfvenon structures are observed in the terrestrial magnetosphere, where they accelerate particles related to aurorae.
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| 9. |
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| 10. |
- Tsirikoglou, Apostolia, et al.
(författare)
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S(wi)SS: A flexible and robust sub-surface scattering shader
- 2014
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Ingår i: Proceedings of SIGRAD 2014.
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Konferensbidrag (refereegranskat)abstract
- S(wi)SS is a new, flexible artist friendly multi-layered sub-surface scattering shader that simulates accurately subsurface scattering for a large range of translucent materials. It is a physically motivated multi-layered approach where the sub-surface scattering effect is generated using one to three layers. It enables seamless mixing of the classical dipole, the better dipole and the quantized diffusion reflectance model in the sub-surface scattering layers, and additionally provides the scattering coming of front and back illumination, as well as all the BSDFcomponents, in separate render channels enabling the artist to either use them physically accurately or tweak them independently during compositing to produce the desired result. To demonstrate the usefulness of our approach, we show a set of high quality rendering results from different user scenarios.
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