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  • Copernicus  (15)
  • 1
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    Determination of the vertical electron-density profile in ionospheric tomography: experimental results (1997)
    Copernicus
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    Publikationsdatum: 1997-06-30
    Beschreibung: The reconstruction of the vertical electron-density profile is a fundamental problem in ionospheric tomography. Lack of near-horizontal ray paths limits the information available on the vertical profile, so that the resultant image of electron density is biased in a horizontal sense. The vertical profile is of great importance as it affects the authenticity of the entire tomographic image. A new method is described whereby the vertical profile is selected using relative total-electron-content measurements. The new reconstruction process has been developed from modelling studies. A range of background ionospheres, representing many possible peak heights, scale heights and electron densities are formed from a Chapman profile on the bottomside with a range of topside profiles. The iterative reconstruction process is performed on all of these background ionospheres and a numerical selection criterion employed to select the final image. The resulting tomographic images show excellent agreement in electron density when compared with independent verification provided by the EISCAT radar.
    Print ISSN: 0992-7689
    Digitale ISSN: 1432-0576
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Copernicus im Namen von European Geosciences Union.
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  • 2
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    Imaging of structures in the high-latitude ionosphere: model comparisons (1998)
    Copernicus
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    Publikationsdatum: 1998-08-31
    Beschreibung: The tomographic reconstruction technique generates a two-dimensional latitude versus height electron density distribution from sets of slant total electron content measurements (TEC) along ray paths between beacon satellites and ground-based radio receivers. In this note, the technique is applied to TEC values obtained from data simulated by the Sheffield/UCL/SEL Coupled Thermosphere/Ionosphere/Model (CTIM). A comparison of the resulting reconstructed image with the 'input' modelled data allows for verification of the reconstruction technique. All the features of the high-latitude ionosphere in the model data are reproduced well in the tomographic image. Reconstructed vertical TEC values follow closely the modelled values, with the F-layer maximum density (NmF2) agreeing generally within about 10%. The method has also been able successfully to reproduce underlying auroral-E ionisation over a restricted latitudinal range in part of the image. The height of the F2 peak is generally in agreement to within about the vertical image resolution (25 km).Key words. Ionosphere (modelling and forecasting; polar ionosphere) · Radio Science (instruments and techniques)
    Print ISSN: 0992-7689
    Digitale ISSN: 1432-0576
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Copernicus im Namen von European Geosciences Union.
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  • 3
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    On the possible role of cusp/cleft precipitation in the formation of polar-cap patches (1999)
    Copernicus
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    Publikationsdatum: 1999-10-31
    Beschreibung: The work describes experimental observations of enhancements in the electron density of the ionospheric F-region created by cusp/cleft particle precipitation at the dayside entry to the polar-cap convection flow. Measurements by meridian scanning photometer and all-sky camera of optical red-line emissions from aurora are used to identify latitudinally narrow bands of soft-particle precipitation responsible for structured enhancements in electron density determined from images obtained by radio tomography. Two examples are discussed in which the electron density features with size scales and magnitudes commensurate with those of patches are shown to be formed by precipitation at the entry region to the anti-sunward flow. In one case the spectrum of the incoming particles results in ionisation being created, for the most part below 250 km, so that the patch will persist only for minutes after convecting away from the auroral source region. However in a second example, at a time when the plasma density of the solar wind was particularly high, a substantial part of the particle-induced enhancement formed above 250 km. It is suggested that, with the reduced recombination loss in the upper F-region, this structure will retain form as a patch during passage in the anti-sunward flow across the polar cap.Key words. Ionosphere (ionospheric irregularities; particle precipitation; polar ionosphere)
    Print ISSN: 0992-7689
    Digitale ISSN: 1432-0576
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Copernicus im Namen von European Geosciences Union.
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  • 4
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    EISCAT verification in the development of ionospheric tomography (1996)
    Copernicus
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    Publikationsdatum: 1996-12-31
    Beschreibung: This paper highlights the important role played by the EISCAT radar for verification in the development of tomographic techniques to produce images of ionospheric electron density. A brief review is given of some of the stages in the application of tomographic reconstruction techniques to the ionosphere. Results are presented to illustrate the effectiveness of the method in imaging ionospheric structures at high latitudes. In addition, the results include the first tomographic image of the ionosphere for a region extending from mid-latitudes over mainland Scandinavia to high latitudes above Svalbard.
    Print ISSN: 0992-7689
    Digitale ISSN: 1432-0576
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Copernicus im Namen von European Geosciences Union.
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  • 5
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    The spatial structure of the dayside ionospheric trough (1998)
    Copernicus
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    Publikationsdatum: 1998-10-31
    Beschreibung: Tomographic imaging provides a powerful technique for obtaining images of the spatial distribution of ionospheric electron density at polar latitudes. The method, which involves monitoring radio transmissions from the Navy Navigation Satellite System at a meridional chain of ground receivers, has particular potential for complementing temporal measurements by other observing techniques such as the EISCAT incoherent-scatter radar facility. Tomographic reconstructions are presented here from a two-week campaign in November 1995 that show large-scale structuring of the polar ionosphere. Measurements by the EISCAT radar confirm the authenticity of the technique and provide additional information of the plasma electron and ion temperatures. The dayside trough, persistently observed at high latitudes during a geomagnetically quiet period but migrating to lower latitudes with increasing activity, is discussed in relationship to the pattern of the polar-cap convection.Key words. Ionosphere-magnetosphere interactions · Polar ionosphere · Radio science · Ionospheric propagation
    Print ISSN: 0992-7689
    Digitale ISSN: 1432-0576
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Copernicus im Namen von European Geosciences Union.
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  • 6
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    Probing discrete auroral arcs by ionospheric tomography (1998)
    Copernicus
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    Publikationsdatum: 1998-05-31
    Beschreibung: Optical observations of 557.7 nm and 630.0 nm emissions from discrete auroral arcs in the post-noon sector have been related to localised field-aligned enhancements in the spatial distribution of E- and F-layer electron density respectively seen in images reconstructed by ionospheric tomography. Results from two case studies are presented in which meridian scanning photometer and all-sky camera observations on Svalbard have been compared to electron-density structures found by tomographic inversion of measurements made by reception of radio signals at a chain of four stations at high latitude. The F-layer features are long-lived and show exact correspondence to the red-line emissions. Transient arcs in green-line intensity result in E-region structures that are resolved in one case, but not in another where the dynamic auroral forms are separated by less than one degree of latitude. The signature of an inverted-V precipitation event is clearly evident in one example.Key words. Ionosphere (Auroral ionosphere) · Magnetospheric physics (Auroral phenomena) · Radio science (Ionospheric physics)
    Print ISSN: 0992-7689
    Digitale ISSN: 1432-0576
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Copernicus im Namen von European Geosciences Union.
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  • 7
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    EISCAT Svalbard radar observations of ionospheric signatures of magnetopause reconnection during a changing IMF Bz polarity (2002)
    Copernicus
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    Publikationsdatum: 2002-04-30
    Beschreibung: Observations by the EISCAT Svalbard radar are presented that show the response of the spatial structure of the ionosphere in the dayside cusp region to a rotational trend in the IMF clock angle. Over a period of one hour, the clock angle increased from about 45° to some 150°, moving the likely location of the magnetopause reconnection site from the high-latitude lobe to near the equatorial plane. Increased topside electron temperatures measured by the ESR identified footprints of the reconnection process. Temporal changes in the spatial distribution of the temperature reflected the change from lobe to equatorial reconnection. Discrete spatial enhancements in ion temperature were found resulting from ion-neutral frictional heating in the fast flows where it was likely that field lines were being convected from the reconnection locations. The corresponding electron density structuring is interpreted in terms of the particle precipitation, field-aligned currents and convection flows driven by the IMF.Key words. Ionosphere (ionosphere – magnetosphere interactions; plasma temperature and density; polar ionosphere)
    Print ISSN: 0992-7689
    Digitale ISSN: 1432-0576
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Copernicus im Namen von European Geosciences Union.
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  • 8
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    Polar patches observed by ESR and their possible origin in the cusp region (2000)
    Copernicus
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    Publikationsdatum: 2000-09-30
    Beschreibung: Observations by the EISCAT Svalbard radar in summer have revealed electron density enhancements in the magnetic noon sector under conditions of IMF Bz southward. The features were identified as possible candidates for polar-cap patches drifting anti-Sunward with the plasma flow. Supporting measurements by the EISCAT mainland radar, the CUTLASS radar and DMSP satellites, in a multi-instrument study, suggested that the origin of the structures lay upstream at lower latitudes, with the modulation in density being attributed to variability in soft-particle precipitation in the cusp region. It is proposed that the variations in precipitation may be linked to changes in the location of the reconnection site at the magnetopause, which in turn results in changes in the energy distribution of the precipitating particles.Key words: Ionosphere (ionosphere-magnetosphere interactions; plasma temperature and density; polar ionosphere)
    Print ISSN: 0992-7689
    Digitale ISSN: 1432-0576
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Copernicus im Namen von European Geosciences Union.
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  • 9
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    Multi-instrument probing of the polar ionosphere under steady northward IMF (2000)
    Copernicus
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    Publikationsdatum: 2000-01-31
    Beschreibung: Observations are presented of the polar ionosphere under steady, northward IMF. The measurements, made by six complementary experimental techniques, including radio tomography, all-sky and meridian scanning photometer optical imaging, incoherent and coherent scatter radars and satellite particle detection, reveal plasma parameters consistent with ionospheric signatures of lobe reconnection. The optical green-line footprint of the reconnection site is seen to lie in the sunward plasma convection of the lobe cells. Downstream in the region of softer precipitation the reverse energy dispersion of the incoming ions can be identified. A steep latitudinal density gradient at the equatorward edge of the precipitation identifies the general location of an adiaroic boundary, separating the open field lines of polar lobe cells from the closed field of viscous-driven cells. Enhancements in plasma density to the south of the gradient are interpreted as ionisation being reconfigured as it is thrust against the boundary by the antisunward flow of the viscous cells near noon. Each of the instruments individually provides valuable information on certain aspects of the ionosphere, but the paper demonstrates that taken together the different experiments complement each other to give a consistent and comprehensive picture of the dayside polar ionosphere..Key words. Ionosphere (polar ionosphere) · Magnetospheric physics (magnetosphere-ionosphere interactions; polar cap phenomena)
    Print ISSN: 0992-7689
    Digitale ISSN: 1432-0576
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Copernicus im Namen von European Geosciences Union.
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  • 10
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    Ionospheric signatures of magnetospheric boundaries in the post-noon sector (2000)
    Copernicus
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    Publikationsdatum: 2000-01-31
    Beschreibung: Spatial structures in ionospheric electron density revealed in a tomographic image have been identified with auroral forms and related to their sources in precipitating particles observed by DMSP satellites. The observations of plasma enhancements relate to discrete auroral arcs seen in the post-noon sector, identified by both red- and green-line emissions measured by a meridional scanning photometer. The features lie within a very narrow latitudinal band on L-shells where the satellite detectors observed electron precipitation classified as from the boundary plasma sheet (BPS). The harder particles are identified with an E-region structure, while further north the precipitation is softer, resulting in a localised F-layer blob and 630.0 nm emissions. A steep gradient in plasma density represent a signature in the ionosphere of the central plasma sheet (CPS)/BPS boundary. A transition to a less-structured F-layer is found on crossing the convection reversal boundary..Key words. Ionosphere (auroral ionosphere; ionosphere-magnetosphere interactions; polar ionosphere)
    Print ISSN: 0992-7689
    Digitale ISSN: 1432-0576
    Thema: Geologie und Paläontologie , Physik
    Publiziert von Copernicus im Namen von European Geosciences Union.
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