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  • 1
    Electronic Resource
    Electronic Resource
    Energy deposition in the E and F regions of the high-latitude ionosphere
    Amsterdam : Elsevier
    Advances in Space Research 12 (1992), S. 137-146 
    Details
    ISSN: 0273-1177
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0273-1177(92)90047-2
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  • 2
    Electronic Resource
    Electronic Resource
    Electron transport and energy degradation in the ionosphere: evaluation of the numerical solution, comparison with laboratory experiments and auroral observations (1994)
    Springer
    Annales geophysicae 12 (1994), S. 1039-1051 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Auroral electron transport calculations are a critical part of auroral models. We evaluate a numerical solution to the transport and energy degradation problem. The numerical solution is verified by reproducing simplified problems to which analytic solutions exist, internal self-consistency tests, comparison with laboratory experiments of electron beams penetrating a collision chamber, and by comparison with auroral observations, particularly the emission ratio of the N2 second positive to N+2 first negative emissions. Our numerical solutions agree with range measurements in collision chambers. The calculated N22P to N+21N emission ratio is independent of the spectral characteristics of the incident electrons, and agrees with the value observed in aurora. Using different sets of energy loss cross sections and different functions to describe the energy distribution of secondary electrons that emerge from ionization collisions, we discuss the uncertainties of the solutions to the electron transport equation resulting from the uncertainties of these input parameters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-994-1039-7
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  • 3
    Electronic Resource
    Electronic Resource
    Effect of suprathermal electrons on the intensity and Doppler frequency of electron plasma lines (1999)
    Springer
    Annales geophysicae 17 (1999), S. 903-912 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (electric fields and currents; solar radiation and cosmic ray effects)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract 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 geometry, to two Langmuir waves travelling towards and away from the radar. In the daytime ionosphere, the presence of a small percentage of photoelectrons produced by the solar EUV of the total electron population can excite or damp these Langmuir waves above the thermal equilibrium, resulting in an enhancement of the intensity of the lines above the thermal level. The presence of photo-electrons also modifies the dielectric response function of the plasma from the Maxwellian and thus influences the Doppler frequency of the plasma lines. In this paper, we present a high time-resolution plasma-line data set collected on the EISCAT VHF radar. The analysed data are compared with a model that includes the effect of a suprathermal electron population calculated by a transport code. By comparing the intensity of the analysed plasma lines data to our model, we show that two sharp peaks in the electron suprathermal distribution in the energy range 20–30 eV causes an increased Landau damping around 24.25 eV and 26.25 eV. We have identified these two sharp peaks as the effect of the photoionisation of N2 and O by the intense flux of monochromatic HeII radiation of wavelength 30.378 nm (40.812 eV) created in the chromospheric network and coronal holes. Furthermore, we see that what would have been interpreted as a mean Doppler drift veloCity for a Maxwellian plasma is actually a shift of the Doppler frequency of the plasma lines due to suprathermal electrons.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-999-0903-x
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  • 4
    Electronic Resource
    Electronic Resource
    Meridional thermospheric neutral wind at high latitude over a full solar cycle (1998)
    Springer
    Annales geophysicae 16 (1998), S. 1400-1409 
    Details
    ISSN: 0992-7689
    Keywords: Auroral ionosphere ; Thermospheri dynamics ; EISCAT
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract EISCAT radar experiments over a full solar cycle between January 1984 and March 1995 have been used to construct meridional neutral wind patterns in the ionospheric F region. For locally geomagnetically quiet periods the neutral winds have been binned according to season, solar activity, and universal time. The diurnal and seasonal behaviors and the effect of the solar flux are described. An empirical model of the meridional neutral wind for the high latitudes at eight altitudes in the ionospheric F region over a full solar cycle is presented. Results are compared with other recent empirical models.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-998-1400-3
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  • 5
    Electronic Resource
    Electronic Resource
    Enhanced incoherent scatter plasma lines (1997)
    Springer
    Annales geophysicae 14 (1997), S. 1462-1472 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Detailed model calculations of auroral secondary and photoelectron distributions for varying conditions have been used to calculate the theoretical enhancement of incoherent scatter plasma lines. These calculations are compared with EISCAT UHF radar measurements of enhanced plasma lines from both the E and F regions, and published EISCAT VHP radar measurements. The agreement between the calculated and observed plasma line enhancements is good. The enhancement from the superthermal distribution can explain even the very strong enhancements observed in the auroral E region during aurora, as previously shown by Kirk-wood et al. The model calculations are used to predict the range of conditions when enhanced plasma lines will be seen with the existing high-latitude incoherent scatter radars, including the new EISCAT Svalbard radar. It is found that the detailed structure, i.e. the gradients in the suprathermal distribution, are most important for the plasma line enhancement. The level of superthermal flux affects the enhancement only in the region of low phase energy where the number of thermal electrons is comparable to the number of suprathermal electrons and in the region of high phase energy where the suprathermal fluxes fall to such low levels that their effect becomes small compared to the collision term. To facilitate the use of the predictions for the different radars, the expected signal-to-noise ratios (SNRs) for typical plasma line enhancements have been calculated. It is found that the high-frequency radars (Søndre Strømfjord, EISCAT UHF) should observe the highest SNR, but only for rather high plasma frequencies. The VHP radars (EISCAT VHP and Svalbard) will detect enhanced plasma lines over a wider range of frequencies, but with lower SNR.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-996-1462-z
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  • 6
    Electronic Resource
    Electronic Resource
    Time-frequency tools of signal processing for EISCAT data analysis (1997)
    Springer
    Annales geophysicae 14 (1997), S. 1513-1525 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We demonstrate the usefulness of some signal-processing tools for the EISCAT data analysis. These tools are somewhat less classical than the familiar periodogram, squared modulus of the Fourier transform, and therefore not as commonly used in our community. The first is a stationary analysis, “Thomson’s estimate” of the power spectrum. The other two belong to time-frequency analysis: the short-time Fourier transform with the spectrogram, and the wavelet analysis via the scalogram. Because of the highly non-stationary character of our geophysical signals, the latter two tools are better suited for this analysis. Their results are compared with both a synthetic signal and EISCAT ion-velocity measurements. We show that they help to discriminate patterns such as gravity waves from noise.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-996-1513-5
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  • 7
    Electronic Resource
    Electronic Resource
    Auroral ionospheric conductivities: a comparison between experiment and modeling, and theoretical f10.7-dependent model for EISCAT and ESR (1997)
    Springer
    Annales geophysicae 14 (1997), S. 1297-1304 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Several authors have published models of auroral conductances in the past. Some of them are based on theoretical approaches; others are the result of observations. The data bases for the latter models range from 8 h to 3 years of experiments. In this paper, we show the results of our own modeling, based on a coupled kinetic/fluid approach. We compare these results to the statistical model based on 3 years of experiment. We then model the auroral ionospheric conductances above EISCAT and the EISCAT-Svalbard Radar for different solar-activity indices. These modeled conductances are fitted with a simple law depending on the f10.7 index and on the solar zenith angle.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-996-1297-7
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  • 8
    Electronic Resource
    Electronic Resource
    Calibration of a numerical ionospheric model with EISCAT observations (1997)
    Springer
    Annales geophysicae 14 (1997), S. 1375-1390 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A set of EISCAT UHF and VHP observations is used for calibrating a coupled fluid-kinetic model of the ionosphere. The data gathered in the period 1200−2400 UT on 24 March 1995 had various intervals of interest for such a calibration. The magnetospheric activity was very low during the afternoon, allowing for a proper examination of a case of quiet ionospheric conditions. The radars entered the auroral oval just after 1900 UT: a series of dynamic events probably associated with rapidly moving auroral arcs was observed until after 2200 UT. No attempts were made to model the dynamical behaviour during the 1900–2200 UT period. In contrast, the period 2200–2400 UT was characterised by quite steady precipitation: this latter period was then chosen for calibrating the model during precipitation events. The adjustment of the model on the four primary parameters observed by the radars (namely the electron concentration and temperature and the ion temperature and velocity) needed external inputs (solar fluxes and magnetic activity index) and the adjustments of a neutral atmospheric model in order to reach a good agreement. It is shown that for the quiet ionosphere, only slight adjustments of the neutral atmosphere models are needed. In contrast, adjusting the observations during the precipitation event requires strong departures from the model, both for the atomic oxygen and hydrogen. However, it is argued that this could well be the result of inadequately representing the vibrational states of N2 during precipitation events, and that these factors have to be considered only as ad hoc corrections.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-996-1375-x
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  • 9
    Electronic Resource
    Electronic Resource
    Electron velocity distribution function in a plasma with temperature gradient and in the presence of suprathermal electrons: application to incoherent-scatter plasma lines (1998)
    Springer
    Annales geophysicae 16 (1998), S. 1226-1240 
    Details
    ISSN: 0992-7689
    Keywords: Non-Maxwellian electron velocity distribution ; Incoherent scatter plasma lines ; EISCAT ; Dielectric response function
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract 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 the ambient electrons is modelled by a near-Maxwellian distribution function in presence of a temperature gradient and a potential electric field. The velocity distribution function of the suprathermal electrons is derived from a numerical model of the angular energy flux spectrum obtained by solving the transport equation of electrons. The numerical method used to calculate the plasma dispersion function and the reduced velocity distribution is described. The numerical code is used with simulated data to evaluate the Doppler frequency asymmetry between the up- and downshifted plasma lines of the incoherent-scatter plasma lines at different wave vectors. It is shown that the observed Doppler asymmetry is more dependent on deviation from the Maxwellian through the thermal part for high-frequency radars, while for low-frequency radars the Doppler asymmetry depends more on the presence of a suprathermal population. It is also seen that the full evaluation of the plasma dispersion function gives larger Doppler asymmetry than the heat flow approximation for Langmuir waves with phase velocity about three to six times the mean thermal velocity. For such waves the moment expansion of the dispersion function is not fully valid and the full calculation of the dispersion function is needed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-998-1226-z
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  • 10
    Electronic Resource
    Electronic Resource
    A numerical model of the ionosphere, including the E-region above EISCAT (1996)
    Springer
    Annales geophysicae 14 (1996), S. 191-200 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract It has been previously demonstrated that a two-ion (O+ and H+) 8-moment time-dependent fluid model was able to reproduce correctly the ionospheric structure in the altitude range probed by the EISCAT-VHF radar. In the present study, the model is extended down to the E-region where molecular ion chemistry (NO+ and O+2, essentially) prevails over transport; EISCAT-UHF observations confirmed previous theoretical predictions that during events of intense E×B induced convection drifts, molecular ions (mainly NO+) predominate over O+ ions up to altitudes of 300 km. In addition to this extension of the model down to the E-region, the ionization and heating resulting from both solar insolation and particle precipitation is now taken into account in a consistent manner through a complete kinetic transport code. The effects of E×B induced convection drifts on the E- and F-region are presented: the balance between O+ and NO+ ions is drastically affected; the electric field acts to deplete the O+ ion concentration. The [NO+]/[O+] transition altitude varies from 190 km to 320 km as the perpendicular electric field increases from 0 to 100 mV m−1. An interesting additional by-product of the model is that it also predicts the presence of a noticeable fraction of N+ ions in the topside ionosphere in good agreement with Retarding Ion Mass Spectrometer measurements onboard Dynamic Explorer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-996-0191-7
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