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- Marklund, Göran, et al.
(författare)
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On the occurrence and characteristics of intense low-altitude electric fields observed by Freja
- 1995
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 13, s. 704-712
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution measurements by the double probe electric field instrument on the Freja satellite are presented. The observations show that extremely intense (up to 1 V m-1) and fine-structured (< 1 km) electric fields exist at auroral latitudes within the altitude regime explored by Freja (up to 1700 km). The intense field events typically occur within the early morning sector of the auroral oval (01-07 MLT) during times of geomagnetic activity. In contrast to the observations within the auroral acceleration region characterized by intense converging electric fields associated with electron precipitation, upward ion beams and upward field-aligned currents, the intense electric fields observed by Freja are often found to be diverging and located within regions of downward field-aligned currents outside the electron aurora. Moreover, the intense fields are observed in conjunction with precipitating and transversely energized ions of energies 0.5-1 keV and may play an important role in the ion heating. The observations suggest that the intense electric field events are associated with small-scale low-conductivity ionospheric regions void of auroral emissions such as east-west aligned dark filaments or vortex streets of black auroral curls located between or adjacent to auroral arcs within the morningside diffuse auroral region. We suggest that these intense fields also exist at ionospheric altitudes although no such observations have yet been made. This is possible since the height-integrated conductivity associated with the dark filaments may be as low as 0.1 S or less. In addition, Freja electric field data collected outside the auroral region are discussed with particular emphasis on subauroral electric fields which are observed within the 19-01 MLT sector between the equatorward edge of the auroral oval and the inner edge of the ring current.
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- Nakamura, R, et al.
(författare)
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Substorm observations in the early morning sector with Equator-S and Geotail
- 1999
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Ingår i: Annales Geophysicae. - : SPRINGER VERLAG. - 0992-7689 .- 1432-0576. ; 17:12, s. 1602-1610
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Tidskriftsartikel (refereegranskat)abstract
- Data from Equator-S and Geotail are used to study the dynamics of the plasma sheet observed during a substorm with multiple intensifications on 25 April 1998, when both spacecraft were located in the early morning sector (03-04 MLT) at a radial distance o
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- Réchou, A., et al.
(författare)
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Mountain wave motions determined by the Esrange MST radar
- 1999
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 17, s. 957-970
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Tidskriftsartikel (refereegranskat)abstract
- Abstract. A European campaign of ground-based radar, lidar and optical measurements was carried out during the winter of 1996/1997 (28 December-2 February) to study lee waves in the northern part of Scandinavia. The participants operated ozone lidars, backscatter lidars and MST radars at ALOMAR/Andoya and Esrange/Kiruna, and an ALIS imaging system in Kiruna. The Andoya site was generally windward of the Scandinavian mountains, the Kiruna site on the leeward side. The goal of the experiment was to examine the influence of lee waves on the formation of Polar Stratospheric Clouds (PSCs). This paper studies the radar data from MST-radar ESRAD located at Esrange [68.°N, 21.°E], i.e. in the lee of the mountains. We present three cases where strong lee waves were observed: in one case they propagated upwards to the lower stratosphere and in the other two cases they were trapped or absorbed in the troposphere. We examine the local waves and the direction and strength of the local wind using the radar, the synoptic meteorological situation using weather maps (European Meteorological Bulletin) and the synoptic stratospheric temperatures using ECMWF data. We observed that waves propagate up to the stratosphere during frontal passages. When anticyclonic ridges are present, the propagation to the stratosphere is very weak. This is due to trapping of the waves at or below the tropopause. We also show that the radar data alone can be used to characterise the different weather conditions for the three cases studied (through the variation of the height of the tropopause). The synoptic stratospheric temperatures in the three cases were similar, and were above the expected threshold for PSC formation. Lidar and visual observation of PSCs and nacreous clouds, respectively, showed that these were present only in the case when the lee waves propagated up to the lower stratosphere.Key words. Atmospheric composition and structure (aerosols and particles) · Electromagnetic (wave propa- gation) · Meteorology and atmospheric dynamics (mesoscale meteorology)
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- Eglitis, P, et al.
(författare)
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New techniques for auroral irregularity studies with COSCAT
- 1998
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Ingår i: ANNALES GEOPHYSICAE-ATMOSPHERES HYDROSPHERES AND SPACE SCIENCES. - : SPRINGER VERLAG. - 0992-7689. ; 16:10, s. 1241-1250
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The COSCAT system enables the detection of E-region auroral backscatter with the EISCAT remote receivers at magnetic aspect angles close to 90 degrees. This is achieved by utilising a low-power transmitter stationed in Oulu, Finland. Many important observ
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- Janhunen, P, et al.
(författare)
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How does the U-shaped potential close above the acceleration region? A study using Polar data
- 1999
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Ingår i: ANNALES GEOPHYSICAE-ATMOSPHERES HYDROSPHERES AND SPACE SCIENCES. - : SPRINGER VERLAG. - 0992-7689. ; 17:10, s. 1276-1283
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We present a statistical study. of Polar electric field observations using auroral oval passes over Scandinavia above the acceleration region. We are especially interested in seeing whether we can find large perpendicular electric fields associated with a
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