Artikler, rapporter og annet (fysikk og teknologi): Nye registreringer

Viser treff 1041-1060 av 1062

    • An interferometer experiment to explore the aspect angle dependence of stimulated electromagnetic emission spectra. 

      Rietveld, Michael T.; Isham, B.; Hagfors, T.; Belyey, Vasyl; La Hoz, Cesar; Khudukon, B.; Brekke, Asgeir; Yurik, R.Yu; Tereshchenko, E.D.; Grill, M.; Heinselman, C. (Journal article; Peer reviewed; Tidsskriftartikkel, 2005-01-31)
      When the Earth's ionosphere is irradiated by a radiofrequency (RF) electromagnetic wave of sufficiently high power density and tuned to match a natural E- or F-region plasma frequency, ionospheric magnetoionic wave modes may be excited and may generate RF electromagnetic sideband waves via nonlinear interactions. These secondary emissions, which may then escape from the ionosphere, have been termed ...

    • Monthly mean climatology of the prevailing winds and tides in the Arctic mesosphere/lower thermosphere. 

      Hall, Chris; Portnyagin, Y.I.; Solovjova, T.V.; Makarov, N.A.; Merzlyakov, E.G.; Manson, A.H.; Meek, C.E.; Hocking, W.; Pancheva, D.; Hoffmann, P.; Singer, W.; Murayama, Y.; Igarashi, K.; Forbes, J.M.; Palo, S.; Nozawa, S.; Mitchell, N. (Journal article; Peer reviewed; Tidsskriftartikkel, 2004-11-03)
      The Arctic MLT wind regime parameters measured at the ground-based network of MF and meteor radar stations (Andenes 69° N, Tromsø 70° N, Esrange 68° N, Dixon 73.5° N, Poker Flat 65° N and Resolute Bay 75° N) are discussed and compared with those observed in the mid-latitudes. The network of the ground-based MF and meteor radars for measuring winds in the Arctic upper mesosphere and lower thermosphere ...

    • Mesopause dynamics from the scandinavian triangle of radars within the PSMOS-DATAR Project. 

      Hall, Chris; Manson, A.H.; Meek, C.E.; Nozawa, S.; Mitchell, N.J.; Pancheva, D.; Singer, W.; Hoffmann, P. (Journal article; Peer reviewed; Tidsskriftartikkel, 2004-01-01)
      The "Scandinavian Triangle" is a unique trio of radars within the DATAR Project (Dynamics and Temperatures from the Arctic MLT (60–97km) region): Andenes MF radar (69°N, 16°E); Tromsø MF radar (70°N, 19°E) and Esrange "Meteor" radar (68°N, 21°E). The radar-spacings range from 125-270km, making it unique for studies of wind variability associated with small-scale waves, comparisons of large-scale ...

    • On the influence of neutral turbulence on ambipolar diffusivities deduced from meteor trail expansion. 

      Hall, Chris (Journal article; Peer reviewed; Tidsskriftartikkel, 2002)
      By measuring fading times of radar echoes from underdense meteor trails, it is possible to deduce the ambipolar diffusivities of the ions responsible for these radar echoes. It could be anticipated that these diffusivities increase monotonically with height akin to neutral viscosity. In practice, this is not always the case. Here, we investigate the capability of neutral turbulence to affect ...

    • Seasonal variations of the semi-diurnal and diurnal tides in the MLT: multi-year MF radar observations from 2–70 N, modelled tides (GSWM, CMAM) 

      Hall, Chris; Manson, A.H.; Meek, C.; Hagan, M.; Koshyk, J.; Franke, S.; Fritts, D.; Hocking, W.; Igarashi, K.; MacDougall, J.; Riggin, D.; Vincent, R. (Journal article; Peer reviewed; Tidsskriftartikkel, 2002)
      In an earlier paper (Manson et al., 1999a) tidal data (1990–1997) from six Medium Frequency Radars (MFR) were compared with the Global Scale Wave Model (GSWM, original 1995 version). The radars are located between the equator and high northern latitudes: Christmas Island (2° N), Hawaii (22° N), Urbana (40° N), London (43° N), Saskatoon (52° N) and Tromsø (70° N). Common harmonic analysis was applied, ...

    • Irregular structures of the F layer at high latitudes during ionospheric heating. 

      Tereshchenko, E.D.; Rietveld, Michael T.; Brekke, Asgeir; Kozlova, M.O.; Evstafiev, O.V.; Khudukon, B.Z.; Nygren, T. (Journal article; Peer reviewed; Tidsskriftartikkel, 2000)
      Results on heating the ionospheric F region above Tromsø, Norway are presented. The ionosphere was monitored by satellite tomography and amplitude scintillation methods as well as the EISCAT incoherent scatter radar. No effect of heating was observed in the daytime. In the evening and in the pre-midnight sector, noticeable tilts of the F region were observed during heating periods. The tilts overlapped ...

    • Comparison of the orientation of small-scale electron density irregularities and F region plasma flow direction. 

      Rietveld, Michael T.; Tereshchenko, E.D.; Brekke, Asgeir; Khudukon, B.Z.; Kozlova, M.O.; Evstafiev, O.V.; Nygren, T. (Journal article; Tidsskriftartikkel; Peer reviewed, 2000)
      Results are shown from an experimental campaign where satellite scintillation was observed at three sites at high latitudes and, simultaneously, the F region plasma flow was measured by the nearby EISCAT incoherent scatter radar. The anisotropy parameters of field-aligned irregularities are determined from amplitude scintillation using a method based on the variance of the relative logarithmic ...

    • Geomagnetic control of polar mesosphere summer echoes. 

      Bremer, J.; Hansen, T.L.; Hoffmann, P. (Journal article; Tidsskriftartikkel, 2000)
      Using observations with the ALOMAR SOUSY radar near Andenes (69.3°N, 16.0°E) from 1994 until 1997 polar mesosphere summer echoes (PMSE) have been investigated in dependence on geomagnetic K indices derived at the Auroral Observa- tory Tromsù (69.66°N, 18.94°E). During night-time and morning hours a significant correlation between the signal-to-noise ratio (SNR) of the radar results and ...

    • Effect of suprathermal electrons on the intensity and Doppler frequency of electron plasma lines. 

      Guio, Patrick; Lilensten, J. (Journal article; Tidsskriftartikkel, 1999)
      In an incoherent scattering radar experiment, the spectral measurement of the so-called up- and downshifted electron plasma lines provides information about their intensity and their Doppler frequency. These two spectral lines correspond, in the backscatter geom- etry, to two Langmuir waves travelling towards and away from the radar. In the daytime ionosphere, the presence of a small percentage ...

    • Spectral characteristics of spring arctic mesosphere dynamics. 

      Hall, Chris; Manson, A.H.; Meek, C.E. (Journal article; Tidsskriftartikkel, 1998)
      The spring of 1997 has represented a stable period of operation for the joint University of Tromsù / University of Saskatchewan MF radar, being between refurbishment and upgrades. We examine the horizontal winds from the February to June inclusive and also include estimates of energy dissipation rates derived from signal fading times and presented as upper limits on the turbulent energy ...

    • Electron velocity distribution function in a plasma with temperature gradient and in the presence of suprathermal electrons: application to incoherent-scatter plasma lines. 

      Guio, Patrick; Lilensten, J.; Kofman, W.; Bjørnå, Noralv (Journal article; Tidsskriftartikkel, 1998-10-31)
      The plasma dispersion function and the reduced velocity distribution function are calculated numerically for any arbitrary velocity distribution function with cylindrical symmetry along the magnetic field. The electron velocity distribution is separated into two distributions representing the distribution of the ambient electrons and the suprathermal electrons. The velocity distribution function of ...

    • Enhancement of stratospheric aerosols after solar proton event. 

      Shumilov, O.I.; Henriksen, K.; Kasatkina, E.A.; Vashenyuk, E.V. (Journal article; Tidsskriftartikkel, 1996)
      The lidar measurements at Verhnetulomski observatory (68.6¡N, 31.8¡E) at Kola peninsula detected a considerable increase of stratospheric aerosol concentration after the solar proton event of GLE (ground level event) type on the 16/02/84. This increase was located at precisely the same altitude range where the energetic solar protons lost their energy in the atmosphere. The aerosol layer ...

    • The electron energy distribution during HF pumping, a picture painted with all colors. 

      Rietveld, Michael T.; Gustavsson, B.; Sergienko, T.; Grydeland, Tom; La Hoz, Cesar; Kosch, M.J.; Brändström, B.U.E.; Leyser, T.B.; Isham, B.; Gallop, P.; Aso, T.; Ejiri, M.; Steen, A.; Kaila, K.; Jussila, J.; Holma, H. (Journal article; Peer reviewed; Tidsskriftartikkel, 2005-07-28)
      The shape of the electron energy distribution has long been a central question in the field of highfrequency radio-induced optical emission experiments. This report presents estimates of the electron energy distribution function, fe(E), from 0 to 60 eV, based on optical multiwavelength (6300, 5577, 8446, 4278 A° ) data and 930-MHz incoherent scatter radar measurements of ion temperature, electron ...

    • Software Radar signal processing. 

      Lind, F.D.; Grydeland, Tom; Holt, J.M.; Erickson, P.J. (Journal article; Tidsskriftartikkel; Peer reviewed, 2005-01-31)
      Software infrastructure is a growing part of modern radio science systems. As part of developing a generic infrastructure for implementing Software Radar systems, we have developed a set of reusable signal processing components. These components are generic softwarebased implementations for use on general purpose computing systems. The components allow for the implementation of signal processing ...

    • Dynamic rayed aurora and enhanced ion-acoustic radar echoes 

      Blixt, Erik Mårten; Ivchenko, N.; Hagfors, T.; Grydeland, Tom; Trondsen, T.S.; La Hoz, Cesar; Lanchester, B.S.; Løvhaug, Unni Pia (Journal article; Tidsskriftartikkel; Peer reviewed, 2005-01-31)
      The generation mechanism for naturally enhanced ion-acoustic echoes is still debated. One important issue is how these enhancements are related to auroral activity. All events of enhanced ion-acoustic echoes observed simultaneously with the EISCAT Svalbard Radar (ESR) and with high-resolution narrow field-of-view auroral imagers have been collected and studied. Characteristic of all the ...

    • MIDAS-W: a workstation-based incoherent scatter radar data aquisition system. 

      Holt, J.M.; Grydeland, Tom; Erickson, P.J.; Gorczyca, A.M. (Journal article; Tidsskriftartikkel, 2000)
      The Millstone Hill Incoherent Scatter Data Acquisition System (MIDAS) is based on an abstract model of an incoherent scatter radar. This model is implemented in a hierarchical software system, which serves to isolate hardware and low-level software implementation details from higher levels of the system. Inherent in this is the idea that implementation details can easily be changed in response to ...

    • Phase calibration of the EISCAT Svalbard Radar interferometer using optical satellite signatures. 

      Grydeland, Tom; Sullivan, J.M.; Blixt, Erik Mårten; Ivchenko, N.; Lockwood, M.; Lanchester, B.S. (Journal article; Peer reviewed; Tidsskriftartikkel, 2006-09-20)
      The link between natural ion-line enhancements in radar spectra and auroral activity has been the subject of recent studies but conclusions have been limited by the spatial and temporal resolution previously available. The next challenge is to use shorter sub-second integration times in combination with interferometric programmes to resolve spatial structure within the main radar beam, and so ...

    • An imaging interferometry capability for the EISCAT Svalbard Radar. 

      Grydeland, Tom; Chau, J.L.; La Hoz, C.; Brekke, Asgeir (Journal article; Peer reviewed; Tidsskriftartikkel, 2005-01-31)
      Interferometric imaging (aperture synthesis imaging) is a technique used by radio astronomers to achieve angular resolution that far surpasses what is possible with a single large aperture. A similar technique has been used for radar imaging studies of equatorial ionospheric phenomena at the Jicamarca Radio Observatory. We present plans for adding an interferometric imaging capability to the ...

    • A procedure to correct the effects of a relative delay between the quadrature components of radar signals at base band. 

      Westman, A.; Grydeland, Tom; Belyey, Vasyl; La Hoz, Cesar (Journal article; Peer reviewed; Tidsskriftartikkel, 2005-01-31)
      The real and imaginary parts of baseband signals are obtained from a real narrow–band signal by quadrature mixing, i.e. by mixing with cosine and sine signals at the narrow band’s selected center frequency. We address the consequences of a delay between the outputs of the quadrature mixer, which arise when digital samples of the quadrature baseband signals are not synchronised, i.e. when ...

    • Interferometric radar observations of filamented structures due to plasma instabilities and their relation to dynamic auroral rays. 

      Grydeland, Tom; Hagfors, T.; Trondsen, T.S.; Blixt, Erik Mårten; Løvhaug, Unni Pia; La Hoz, Cesar (Journal article; Peer reviewed; Tidsskriftartikkel, 2004-04-02)
      Several explanations have been proposed for Naturally Enhanced ion-acoustic Echoes observed at midand high-latitude Incoherent Scatter observatories. A decisive measure for distinguishing between these explanations is whether or not simultaneously observed up- and downshifted enhancement occur simultaneously, or if they are the result of temporal and/or spatial averaging. The EISCAT Svalbard ...