| 1. |
- Feldstein, Y. I., et al.
(författare)
-
Electromagnetic weather at 100 km altitude on 3 August 1986
- 1994
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 21, s. 2095-2098
-
Tidskriftsartikel (refereegranskat)abstract
- The electromagnetic weather at high altitudes above the Earth’s surface is determined by the transport of ionospheric plasma, which in turn is governed by the magnitude as well as the direction of the electric and magnetic fields. Different models [Levitin et al., 1984; Friis-Christensen et al., 1985; Mishin, 1990] have been proposed that allow an estimation of the electromagnetic parameters of the upper atmosphere, given a knowledge of the magnitude and orientation of the interplanetary magnetic field. Here we use one such model to estimate the global convection pattern and its temporal evolution during a pass of the Swedish satellite Viking over the northern polar cap. The model predictions are shown to agree well with the electric and magnetic fields measured along the satellite trajectory. The good agreement implies that the model can be used to reconstruct, with reasonable confidence, the large-scale distribution of electric and magnetic fields and their time-variation in the entire auroral ionosphere.
|
|
| 2. |
- KLETZING, CA, et al.
(författare)
-
Electric-fields derived from electron-drift measurements
- 1994
-
Ingår i: GEOPHYSICAL RESEARCH LETTERS. - : American Geophysical Union (AGU). - 0094-8276. ; 21:17, s. 1863-1866
-
Tidskriftsartikel (refereegranskat)abstract
- The first observations of electric fields derived from electron E x B drift measurements aboard the Freja spacecraft are presented. The instrument injects a weak beam of 3 keV electrons and measures the displacement of the returning electrons after one gyroperiod. After removing effects due to beam-detector geometry and applying an empirical calibration based upon comparison with the computed v x B electric field induced by the spacecraft motion, good agreement is found when the electron drift measurements are compared with the electric field components measured by the double probe experiment. Examples are presented in which moderately large electric fields are observed near the edges of or adjacent to electron precipitation regions with little or no electric field within.
|
|
| 3. |
- Lühr, H, et al.
(författare)
-
Fine-structure of field-aligned currente sheets deduced from spacecraft and ground-based observations - initial Freja results
- 1994
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 21, s. 1883-1886
-
Tidskriftsartikel (refereegranskat)abstract
- Due to its special orbit-over large segments tangential to the auroral oval and at altitudes around 1600 km-FREJA provides measurements from a sparsely sampled region. Here we will introduce initial results from the magnetic field experiment. For one event occurring about an hour after midnight we investigate the fine structure of the field-aligned currents associated with a westward electrojet. From simultaneous ground-based observations we deduce the orientation and the spatial and temporal variations of the electrojet. Space-borne magnetic field measurements provide estimates of the field-aligned current density. We deduce a lower limit of 1.75 km for the thickness of field-aligned current filaments. The comparison between electric and magnetic field recordings on FREJA suggests a poor correlation of these quantities for this event.
|
|
| 4. |
- MALINGRE, M, et al.
(författare)
-
SPORADIC ELECTROMAGNETIC EMISSIONS IN THE AKR FREQUENCY-RANGE ASSOCIATED WITH ELECTROSTATIC PLASMA TURBULENCE
- 1992
-
Ingår i: GEOPHYSICAL RESEARCH LETTERS. - : American Geophysical Union (AGU). - 0094-8276. ; 19:13, s. 1339-1342
-
Tidskriftsartikel (refereegranskat)abstract
- We present heretofore unreported Viking observations of sporadic AKR bursts occurring well above the electron gyrofrequency together with spiky electrostatic emissions recorded just above the lower hybrid frequency, which exhibit a high degree of correlation in time. Both types of waves show simultaneous large amplitude variations of the order of 30-40 dB on time scales of a few hundreds of ms. Possible interpretations which may account for the observed correlation are investigated. Arguments are given which suggest that such events could be the signature of small scale non-linear structures in the plasma.
|
|
| 5. |
- Marklund, Göran, et al.
(författare)
-
On intense diverging electric fields associated with black aurora
- 1994
-
Ingår i: Geophysical Research Letters. - 0094-8276. ; 21:17, s. 1859-1862
-
Tidskriftsartikel (refereegranskat)abstract
- Results are presented from the double-probe electric field instrument on the Freja satellite with particular focus on the fine-structured and dynamic plasma of the upper auroral ionosphere. The high-resolution measurements show frequently occuring intense and irregular fine-scale electric fields similar to those observed at higher altitudes by, for example, the S3-3 and Viking satellites. Whereas the high-altitude fields tend to be directly related to the auroral fine-structure this is not always the case for the low-altitude fields as illustrated by high-resolution data of a pair of very intense (≈ 1 V/m), narrow electric field structures in the post-midnight sector in the large-scale downward field-aligned current region. The structures are found to be associated with an excess of positive space charge (diverging electric fields), dropouts of precipitating electrons as well as depletions of thermal plasma, and significant wave activity. Combined with the scale-size of the structures (≈ 1 km) and the spacing between them (≈ 5 km) these observations suggest that the intense electric fields are related to east-west aligned vortex street structures of black aurora, similar to auroral curls but with opposite sense of rotation (clockwise seen antiparallel to B) and a total absence of auroral emissions. The detection of these structures was made possible by the relatively low inclination of the Freja orbit, which at times is almost tangential to the auroral oval. Thus, the Freja orbit provides a new perspective for studying many nightside auroral phenomena both at smaller scales (vortices) and at larger scales as exemplified by observations of north-south oriented auroral structures caused by rotational arc distortions.
|
|
| 6. |
- Mursula, Kalevi, et al.
(författare)
-
Dispersive Pc1 bursts observed by Freja
- 1994
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 21:17, s. 1851-1854
-
Tidskriftsartikel (refereegranskat)abstract
- Electric field measurements by the Freja double probe sensor are used to study equatorially generated ion cyclotron waves, also called Pc1 pulsations. We have examined the global occurrence and spectral properties of these waves in the upper ionosphere during 12-hour period on Nov. 18, 1992, when a long chain of structured Pc1 waves (pearls) was observed on ground. In agreement with ground observations, Pc1 waves were found to occur as short bursts of 10-25 s in early morning to postnoon MLT sector. Most Pc1 activity was detected within a small latitudinal range, extending from 60-degrees CGMlat at dawn to 630 CGMlat at noon. The latitudinal width of the source was only about 0.5-degrees CGMlat. Observations give evidence for a plasmapause connected source region that was several hours wide in MLT and active during many hours. One burst displayed a fully developed classical dispersive Pc1 pearl, now detected for the first time above the ionosphere. In all studied Pc1 events, two spectral maxima (bands) were observed. The longer Pc1 wave bursts showed evidence for a small time delay between the lower and upper frequency bands, unveiling a new dispersive phenomenon.
|
|
| 7. |
- VOGELSANG, H, et al.
(författare)
-
An ionospheric travelling convection vortex event observed by ground‐based magnetometers and by Viking
- 1993
-
Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 20:21, s. 2343-2346
-
Tidskriftsartikel (refereegranskat)abstract
- A transient ionospheric travelling convection vortex (ITCV) event was recorded by the EISCAT magnetometer cross in northern Scandinavia on April 21, 1986 around 8:40 MLT. Simultaneously, the near-conjugate satellite VIKING observed strong magnetic and electric field variations on closed magnetic field lines. The signatures seen by the spacecraft are consistent with a passage through two oppositely directed field-aligned current tubes. The total upward and downward currents turned out to be matched within a region of approximately 1000 km in the ionosphere. Because of the close spatial and temporal proximity of both observations we interpret the field-aligned currents as being associated with the ITCV. This is the first time that in-situ measurements of the currents connecting the ITCV to the source region close to the magnetopause have been published. Both the current density and the dimensions of the current tubes could be estimated here.
|
|
