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  • 1
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    Currents and turbulence in and near mid-latitude sporadic
    Copernicus GmbH ; 1997
    In:  Annales Geophysicae Vol. 15, No. 6 ( 1997), p. 767-
    Details
    In: Annales Geophysicae, Copernicus GmbH, Vol. 15, No. 6 ( 1997), p. 767-
    Type of Medium: Online Resource
    ISSN: 0992-7689
    URL: Article
    DOI: 10.1007/s005850050494
    Language: Unknown
    Publisher: Copernicus GmbH
    Publication Date: 1997
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  • 2
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    50 MHz continuous wave interferometer observations of the unstable mid-latitude E-region ionosphere
    Copernicus GmbH ; 2003
    In:  Annales Geophysicae Vol. 21, No. 7 ( 2003-07-31), p. 1589-1600
    Details
    In: Annales Geophysicae, Copernicus GmbH, Vol. 21, No. 7 ( 2003-07-31), p. 1589-1600
    Abstract: Abstract. In this paper we describe the conversion of SESCAT (Sporadic-E SCATter experiment), a bistatic 50 MHz continuous wave (CW) Doppler radar located on the island of Crete, Greece, to a single (east-west) baseline interferometer. The first results show that SESCAT, which provides high quality Doppler spectra and excellent temporal resolution, has its measurement capabilities enhanced significantly when operated as an interferometer, as it can also study short-term dynamics of localized scattering regions within mid-latitude sporadic E-layers. The interferometric observations reveal that the aspect sensitive area viewed by the radar often contains a few zonally located backscatter regions, presumably blobs or patches of unstable metallic ion plasma, which drift across the radar field-of-view with the neutral wind. On average, these active regions of backscatter have mean zonal scales ranging from a few kilometers to several tens of kilometers and drift with westward speeds from ~ 20 m/s to 100 m/s, and occasionally up to 150 m/s. The cross-spectral analysis shows that mid-latitude type 1 echoes occur much more frequently than has been previously assumed and they originate in single and rather localized areas of elevated electric fields. On the other hand, typical bursts of type 2 echoes are often found to result from two adjacent regions in azimuth undergoing the same bulk motion westwards but producing scatter of opposite Doppler polarity, a fact that contradicts the notion of isotropic turbulence to which type 2 echoes are attributed. Finally, quasi-periodic (QP) echoes are observed simply to be due to sequential unstable plasma patches or blobs which traverse across the radar field-of-view, sometimes in a wave-like fashion.Key words. Ionosphere (ionospheric irregularities; mid-latitude ionosphere; plasma waves and instabilities)
    Type of Medium: Online Resource
    ISSN: 1432-0576
    URL: Article
    DOI: 10.5194/angeo-21-1589-2003
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2003
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  • 3
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    Mid-latitude 〈i〉E〈/i〉-region bulk motions inferred from digital ionosonde and HF radar measurements
    Copernicus GmbH ; 2004
    In:  Annales Geophysicae Vol. 22, No. 11 ( 2004-11-29), p. 3789-3798
    Details
    In: Annales Geophysicae, Copernicus GmbH, Vol. 22, No. 11 ( 2004-11-29), p. 3789-3798
    Abstract: Abstract. In the mid-latitude E-region there is now evidence suggesting that neutral winds play a significant role in driving the local plasma instabilities and electrodynamics inside sporadicE layers. Neutral winds can be inferred from coherent radar backscatter measurements of the range-/azimuth-time-intensity (RTI/ATI) striations of quasi-periodic (QP) echoes, or from radar interferometer/imaging observations. In addition, neutral winds in the E-region can be estimated from angle-of-arrival ionosonde measurements of sporadic-E layers. In the present paper we analyse concurrent ionosonde and HF coherent backscatter observations obtained when a Canadian Advanced Digital Ionosonde (CADI) was operated under a portion of the field-of-view of the Valensole high frequency (HF) radar. The Valensole radar, a mid-latitude radar located in the south of France with a large azimuthal scanning capability of 82° (24° E to 58° W), was used to deduce zonal bulk motions of QP echoing regions using ATI analysis. The CADI was used to measure angle-of-arrival information in two orthogonal horizontal directions and thus derive the motion of sporadic-E patches drifting with the neutral wind. This paper compares the neutral wind drifts of the unstable sporadic-E patches as determined by the two instruments. The CADI measurements show a predominantly westward aligned motion, but the measured zonal drifts are underestimated relative to those observed with the Valensole radar.
    Type of Medium: Online Resource
    ISSN: 1432-0576
    URL: Article
    DOI: 10.5194/angeo-22-3789-2004
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2004
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  • 4
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    E
    Copernicus GmbH ; 1997
    In:  Annales Geophysicae Vol. 15, No. 7 ( 1997), p. 918-
    Details
    In: Annales Geophysicae, Copernicus GmbH, Vol. 15, No. 7 ( 1997), p. 918-
    Type of Medium: Online Resource
    ISSN: 0992-7689
    URL: Article
    DOI: 10.1007/s005850050510
    Language: Unknown
    Publisher: Copernicus GmbH
    Publication Date: 1997
    detail.hit.zdb_id: 1458425-6
    SSG: 16,13
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  • 5
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    〈i〉E〈/i〉-region mid-latitude decametre irregularities observed at four radar frequencies. Experiment and first results
    Copernicus GmbH ; 1997
    In:  Annales Geophysicae Vol. 15, No. 7 ( 1997-07-31), p. 918-924
    Details
    In: Annales Geophysicae, Copernicus GmbH, Vol. 15, No. 7 ( 1997-07-31), p. 918-924
    Abstract: Abstract. In this paper an experiment designed for multifrequency azimuthal Doppler-spectrum investigations of decametre-scale plasma irregularities in the mid-latitude E region is introduced and some preliminary results are presented. The observations were made with the high-frequency Valensole radar in the south of France. The radar operated in a multifrequency mode that allowed simultaneous measurements of Doppler spectra at the four frequencies of 9.23, 11.03, 12.71 and 16.09 MHz, which correspond to scatter from field-aligned irregularities with wavelengths of 16.2, 13.6, 11.8 and 9.3 m, respectively. In addition, a digital ionosonde was operating beneath a small part of the radar viewing region. The data show that lower-frequency echoes are stronger, more frequent and more spatially extended than higher-frequency ones, in general agreement with theory and rocket measurements. On the other hand, the preliminary analysis shows no pronounced differences of the Doppler spectrum with radar frequency. Some trends in the spectral moments do exist, however, which can be understood as the result of temporal and/or spatial mixing of backscatter from directly generated (primary) plasma waves by the gradient drift instability and secondary low-velocity waves. Finally, a close relation of mid-latitude coherent backscatter with patchy sporadic E-layers is present in the data, but the details of this relation remain unresolved.
    Type of Medium: Online Resource
    ISSN: 1432-0576
    URL: Article
    DOI: 10.1007/s00585-997-0918-0
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 1997
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  • 6
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    Currents and turbulence in and near mid-latitude sporadic 〈i〉E〈/i〉-layers caused by strong acoustic impulses
    Copernicus GmbH ; 1997
    In:  Annales Geophysicae Vol. 15, No. 6 ( 1997-06-30), p. 767-773
    Details
    In: Annales Geophysicae, Copernicus GmbH, Vol. 15, No. 6 ( 1997-06-30), p. 767-773
    Abstract: Abstract. The generation of Hall and field-aligned currents in and in the vicinity of nighttime mid-latitude sporadic E-layers moving under the action of strong acoustic impulses of seismic, anthropogenic, or meteorological nature is considered in a model presented in this paper. The influence of the electrical polarization fields caused by charges at the horizontal edges of the sporadic layers and the finite conductivity of the external circuits are also taken into account. The theoretical model is applicable for ionospheric altitudes between 95 and 130 km. The estimates show that under certain conditions in a system with two sporadic E-layers, one of which is the current generator and the other is situated in the external circuit, the Farley-Buneman instability could be generated. On the other hand, observations show that Farley-Buneman waves are likely responsible for the infrequent echoes of mid-latitude 50-MHz backscatter with Doppler velocities near 300 m s–1. The possibility exists that the proposed current-generator model is at the origin of the observed mid-latitude Farley-Buneman waves.
    Type of Medium: Online Resource
    ISSN: 1432-0576
    URL: Article
    DOI: 10.1007/s00585-997-0767-x
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 1997
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  • 7
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    Auroral E-region electron density height profile modificationby electric field driven vertical plasma transport:some evidence in EISCAT CP-1 data statistics
    Copernicus GmbH ; 2004
    In:  Annales Geophysicae Vol. 22, No. 3 ( 2004-03-19), p. 901-910
    Details
    In: Annales Geophysicae, Copernicus GmbH, Vol. 22, No. 3 ( 2004-03-19), p. 901-910
    Abstract: Abstract. A model developed several years ago by Huuskonen et al. (1984) predicted that vertical transport of ions in the nocturnal auroral E-region ionosphere can shift the electron density profiles in altitude during times of sufficiently large electric fields. If the vertical plasma transport effect was to operate over a sufficiently long enough time, then the real height of the E-region electron maximum should be shifted some km upwards (downwards) in the eastward (westward) auroral electrojet, respectively, when the electric field is strong, exceeding, say, 50 mV/m. Motivated by these predictions and the lack of any experimental verification so far, we made use of the large database of the European Incoherent Scatter (EISCAT) radar to investigate if the anticipated vertical plasma transport is at work in the auroral E-region ionosphere and thus to test the Huuskonen et al. (1984) model. For this purpose a new type of EISCAT data display was developed which enabled us to order a large number of electron density height profiles, collected over 16 years of EISCAT operation, according to the electric field magnitude and direction as measured at the same time at the radar's magnetic field line in the F-region. Our analysis shows some signatures in tune with a vertical plasma transport in the auroral E-region of the type predicted by the Huuskonen et al. model. The evidence brought forward is, however, not unambiguous and requires more rigorous analysis. Key words. Ionosphere (auroral ionosphere; plasma convection; electric fields and currents)
    Type of Medium: Online Resource
    ISSN: 1432-0576
    URL: Article
    DOI: 10.5194/angeo-22-901-2004
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2004
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  • 8
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    Seasonal variability and descent of mid-latitude sporadic E layers at Arecibo
    Copernicus GmbH ; 2009
    In:  Annales Geophysicae Vol. 27, No. 3 ( 2009-03-02), p. 923-931
    Details
    In: Annales Geophysicae, Copernicus GmbH, Vol. 27, No. 3 ( 2009-03-02), p. 923-931
    Abstract: Abstract. Sporadic E layers (Es) follow regular daily patterns in variability and altitude descent, which are determined primarily by the vertical tidal wind shears in the lower thermosphere. In the present study a large set of sporadic E layer incoherent scatter radar (ISR) measurements are analyzed. These were made at Arecibo (Geog. Lat. ~18° N; Magnetic Dip ~50°) over many years with ISR runs lasting from several hours to several days, covering evenly all seasons. A new methodology is applied, in which both weak and strong layers are clearly traced by using the vertical electron density gradient as a function of altitude and time. Taking a time base equal to the 24-h local day, statistics were obtained on the seasonal behavior of the diurnal and semidiurnal tidal variability and altitude descent patterns of sporadic E at Arecibo. The diurnal tide, most likely the S(1,1) tide with a vertical wavelength around 25 km, controls fully the formation and descent of the metallic Es layers at low altitudes below 110 km. At higher altitudes, there are two prevailing layers formed presumably by vertical wind shears associated mainly with semidiurnal tides. These include: 1) a daytime layer starting at ~130 km around midday and descending down to 105 km by local midnight, and 2) a less frequent and weaker nighttime layer which starts prior to midnight at ~130 km, descending downwards at somewhat faster rate to reach 110 km by sunrise. The diurnal and semidiurnal-like pattern prevails, with some differences, in all seasons. The differences in occurrence, strength and descending speeds between the daytime and nighttime upper layers are not well understood from the present data alone and require further study.
    Type of Medium: Online Resource
    ISSN: 1432-0576
    URL: Article
    DOI: 10.5194/angeo-27-923-2009
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2009
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  • 9
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    Auroral 〈i〉E〈/i〉-region electron density gradients measured
    Copernicus GmbH ; 2000
    In:  Annales Geophysicae Vol. 18, No. 9 ( 2000-09-30), p. 1172-1181
    Details
    In: Annales Geophysicae, Copernicus GmbH, Vol. 18, No. 9 ( 2000-09-30), p. 1172-1181
    Abstract: Abstract. In the theory of E-region plasma instabilities, the ambient electric field and electron density gradient are both included in the same dispersion relation as the key parameters that provide the energy for the generation and growth of electrostatic plasma waves. While there exist numerous measurements of ionospheric electric fields, there are very few measurements and limited knowledge about the ambient electron density gradients, ∇Ne, in the E-region plasma. In this work, we took advantage of the EISCAT CP1 data base and studied statistically the vertical electron density gradient length, Lz=Ne/(dNe/dz), at auroral E-region heights during both eastward and westward electrojet conditions and different ambient electric field levels. Overall, the prevailing electron density gradients, with Lz ranging from 4 to 7 km, are found to be located below 100 km, but to move steadily up in altitude as the electric field level increases. The steepest density gradients, with Lz possibly less than 3 km, occur near 110 km mostly in the eastward electrojet during times of strong electric fields. The results and their implications are examined and discussed in the frame of the linear gradient drift instability theory. Finally, it would be interesting to test the implications of the present results with a vertical radar interferometer.Key words: Ionosphere (auroral ionosphere; ionospheric irregularities; plasma waves and instabilities)  
    Type of Medium: Online Resource
    ISSN: 1432-0576
    URL: Article
    DOI: 10.1007/s00585-000-1172-x
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2000
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  • 10
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    A model of mid-latitude E-region plasma convergence inside a planetary wave cyclonic vortex
    Copernicus GmbH ; 2002
    In:  Annales Geophysicae Vol. 20, No. 8 ( 2002-08-31), p. 1193-1201
    Details
    In: Annales Geophysicae, Copernicus GmbH, Vol. 20, No. 8 ( 2002-08-31), p. 1193-1201
    Abstract: Abstract. Recently, Shalimov et al. (1999) proposed a new mechanism for large-scale accumulation of long-lived metallic ions in the mid-latitude ionosphere driven by planetary waves in the lower thermosphere. In this mechanism, the combined action of frictional and horizontal magnetic field forces at E-region altitudes causes the plasma to converge and accumulate in large areas of positive neutral wind vorticity within a propagating planetary wave. The present paper provides a theoretical formulation for this mechanism by modelling both horizontal and vertical plasma transport effects within a planetary wave vortex, of cyclonic neutral wind. Non-steady-state numerical solutions of the ion continuity equation show that the proposed accumulation process can enhance the ionization significantly inside the planetary wave vortex but its efficiency depends strongly on altitude, whereas on the other hand, it can be complicated by vertical plasma motions. The latter, which are driven by the same planetary wave wind field under the action of the vertical Lorentz force and meridional wind forcing along the magnetic field lines, can lead to either plasma compressions or depletions, depending on the prevailing wind direction. We conclude that, for shorter times, vertical plasma transport may act constructively to the horizontal gathering process to produce considerable E-region plasma accumulation over large sectors of a planetary wave vortex of cyclonic winds.Key words. Ionosphere (ionosphere-atmosphere interactions; mid-latitude ionosphere; sporadic E-layers) – Meteorology and atmospheric dynamics (waves and tides)
    Type of Medium: Online Resource
    ISSN: 1432-0576
    URL: Article
    DOI: 10.5194/angeo-20-1193-2002
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2002
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