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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
    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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  • 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
    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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  • 5
    Electronic Resource
    Electronic Resource
    Proton-electron precipitation effects on the electron production and density above EISCAT (Tromsø) and ESR (1998)
    Springer
    Annales geophysicae 16 (1998), S. 1299-1307 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (auroral ionosphere; ionosphere-magnetosphere interactions; particle precipitation).
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The suprathermal particles, electrons and protons, coming from the Sun and precipitating into the high-latitude atmosphere are an energy source for the Earth’s ionosphere. They interact with the ambient thermal gas through inelastic and elastic collisions. Most of the physical quantities perturbed by the precipitation, such as the electron production rate, may be evaluated by solving the stationary Boltzmann transport equation, which yields the particle fluxes as a function of altitude, energy, and pitch angle. This equation has been solved for the three different suprathermal species (electrons, protons and hydrogen atoms). We first compare the results of our theoretical code to a coordinated DMSP/EISCAT experiment, and to another approach. Then, we show the effects that pure proton precipitation may have on the ionosphere, through primary and secondary ionization. Finally, we compare the effects of proton precipitation and electron precipitation in some selected cases above EISCAT (Tromsó) and ESR.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-998-1299-8
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  • 6
    Electronic Resource
    Electronic Resource
    Proton transport model in the ionosphere. 2. Influence of magnetic mirroring and collisions on the angular redistribution in a proton beam (1998)
    Springer
    Annales geophysicae 16 (1998), S. 1308-1321 
    Details
    ISSN: 0992-7689
    Keywords: Auroral ionosphere ; Particle precipitation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We investigate the influence of magnetic mirroring and elastic and inelastic scattering on the angular redistribution in a proton/hydrogen beam by using a transport code in comparison with observations. H-emission Doppler profiles viewed in the magnetic zenith exhibit a red-shifted component which is indicative of upward fluxes. In order to determine the origin of this red shift, we evaluate the influence of two angular redistribution sources which are included in our proton/hydrogen transport model. Even though it generates an upward flux, the redistribution due to magnetic mirroring effect is not sufficient to explain the red shift. On the other hand, the collisional angular scattering induces a much more significant red shift in the lower atmosphere. The red shift due to collisions is produced by 〈 1 -keV protons and is so small as to require an instrumental bandwidth 〈0.2nm. This explains the absence of measured upward proton/hydrogen fluxes in the Proton I rocket data because no useable data concerning protons 〈 1 keV are available. At the same time, our model agrees with measured ground-based H-emission Doppler profiles and suggests that previously reported red shift observations were due mostly to instrumental bandwidth broadening of the profile. Our results suggest that Doppler profile measurements with higher spectral resolution may enable us to quantify better the angular scattering in proton aurora.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-998-1308-y
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  • 7
    Electronic Resource
    Electronic Resource
    Effects of atmospheric oscillations on the field-aligned ion motions in the polar F-region (2000)
    Springer
    Annales geophysicae 18 (2000), S. 1154-1163 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (Ionosphere–atmosphere interactions) ; Meteorology and atmospheric dynamics (thermospheric dynamics; waves and tides)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The field-aligned neutral oscillations in the F-region (altitudes between 165 and 275 km) were compared using data obtained simultaneously with two independent instruments: the European Incoherent Scatter (EISCAT) UHF radar and a scanning Fabry-Perot interferometer (FPI). During the night of February 8, 1997, simultaneous observations with these instruments were conducted at Tromsø, Norway. Theoretically, the field-aligned neutral wind velocity can be obtained from the field-aligned ion velocity and by diffusion and ambipolar diffusion velocities. We thus derived field-aligned neutral wind velocities from the plasma velocities in EISCAT radar data. They were compared with those observed with the FPI (λ=630.0 nm), which are assumed to be weighted height averages of the actual neutral wind. The weighting function is the normalized height dependent emission rate. We used two model weighting functions to derive the neutral wind from EISCAT data. One was that the neutral wind velocity observed with the FPI is velocity integrated over the entire emission layer and multiplied by the theoretical normalized emission rate. The other was that the neutral wind velocity observed with the FPI corresponds to the velocity only around an altitude where the emission rate has a peak. Differences between the two methods were identified, but not completely clarified. However, the neutral wind velocities from both instruments had peak-to-peak correspondences at oscillation periods of about 10–40 min, shorter than that for the momentum transfer from ions to neutrals, but longer than from neutrals to ions. The synchronizing motions in the neutral wind velocities suggest that the momentum transfer from neutrals to ions was thought to be dominant for the observed field-aligned oscillations rather than the transfer from ions to neutrals. It is concluded that during the observation, the plasma oscillations observed with the EISCAT radar at different altitudes in the F-region are thought to be due to the motion of neutrals.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-000-1154-z
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  • 8
    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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  • 9
    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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  • 10
    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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