| 1. |
|
|
| 2. |
- Brenning, Nils, et al.
(författare)
-
Interpretation of the Electric Fields Measured in an Ionospheric Critical Ionization Velocity Experiment
- 1991
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 96, s. 9719-9733
-
Tidskriftsartikel (refereegranskat)abstract
- This paper deals with the quasi-dc electric fields measured in the CRIT I ionospheric release experiment, which was launched from Wallops Island on May 13, 1986. The purpose of the experiment was to study the critical ionization velocity (CIV) mechanism in the ionosphere. Two identical barium shaped charges were fired from distances of 1.99 km and 4.34 km towards a main payload, which made full three-dimensional measurements of the electric field inside the streams. There was also a subpayload separated from the main payload by a couple of kilometers along the magnetic field. The relevance of earlier proposed mechanisms for electron heating in CIV is investigated in the light of the CRIT I results. It is concluded that both the “homogeneous” and the “ionizing front” models probably apply, but in different parts of the stream. It is also possible that electrons are directly accelerated by a magnetic-field-aligned component of the electric field; the quasi-dc electric field observed within the streams had a large magnetic-field-aligned component, persisting on the time scale of the passage of the streams. The coupling between the ambient ionosphere and the ionized barium stream in CRIT I was more complicated than is usually assumed in CIV theories, with strong magnetic-field-aligned electric fields and probably current limitation as important processes. One interpretation of the quasi-dc electric field data is that the internal electric fields of the streams were not greatly modified by magnetic-field-aligned currents, i.e., a state was established where the transverse currents were to a first approximation divergence-free. It is argued that this interpretation can explain both a reversal of the strong explosion-directed electric field in burst 1 and the absence of such a reversal in burst 2.
|
|
| 3. |
- Cumnock, Judy, et al.
(författare)
-
Response of the Ionospheric Convection Pattern to a Rotation of the Interplanetary Magnetic Field on January 14, 1988
- 1992
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 97, s. 19449-19460
-
Tidskriftsartikel (refereegranskat)abstract
- Ionospheric convection signatures observed over the polar regions are provided by the DMSP F8 satellite. We consider five passes over the southern summer hemisphere during a time when the z component of the interplanetary magnetic field was stable and positive and the y component changed slowly from positive to negative. Large-scale regions of sunward flow are observed at very high latitudes consistent with a strong z component. When B(y) and B(z) are positive, but B(y) is greater than B(z), strong evidence exists for dayside merging in a manner similar to that expected when B(z) is negative. This signature is diminished as B(y) decreases and becomes smaller than B(z) resulting in a four-cell convection pattern displaced toward the sunward side of the dawn-dusk meridian. In this case the sign of B(y) affects the relative sizes of the two highest-latitude cells. In the southern hemisphere the dusk side high-latitude cell is dominant for B(y) positive and the dawnside high-latitude cell is dominant for B(y) negative. The relative importance of possible electric field sources in the low-latitude boundary layer, the dayside cusp, and the lobe all need to be considered to adequately explain the observed evolution of the convection pattern.
|
|
| 4. |
- DUBOULOZ, N, et al.
(författare)
-
DETAILED ANALYSIS OF BROAD-BAND ELECTROSTATIC NOISE IN THE DAYSIDE AURORAL-ZONE
- 1991
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - SWEDISH INST SPACE PHYS,S-75590 UPPSALA,SWEDEN. ROYAL INST TECHNOL,S-10044 STOCKHOLM 70,SWEDEN.. - 0148-0227. ; 96:A3, s. 3565-3579
-
Tidskriftsartikel (refereegranskat)abstract
- The great number of bursts of broadband electrostatic noise (BEN) recorded during crossings of the dayside auroral zone by the Viking satellite enables their statistical study. The angular distribution of BEN with respect to the Earth's magnetic field is shown to be most likely isotropic, implying that it cannot consist of a unique linear plasma mode. Most of the bursts evidence a power law spectrum from the lower hybrid and ion plasma frequencies up to frequencies sometimes much higher than the electron plasma frequency, suggesting the presence of nonlinear effects. This is confirmed by their high intensity, and by the correlation between their amplitude and their frequency extension. BEN emissions are associated with ion conical distributions and with field-aligned electron beams. Although most of the power is concentrated at very low frequencies and around the lower hybrid and ion plasma frequencies, electron acoustic and beam mode waves may contribute to the high-frequency extension of BEN. The most intense BEN emissions are also correlated with sharp cold plasma density gradients and probably involve drift instabilities. Strong quasi-static perpendicular electric fields, which induce high-speed plasma flows, are also measured, so that the Doppler effect may contribute to the broadening of the BEN spectrum.
|
|
| 5. |
- Elgered, Gunnar, 1955, et al.
(författare)
-
Geodesy by Radio Interferometry: Water Vapor Radiometry for Estimation of the Wet Delay
- 1991
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 96:B4, s. 6541-6555
-
Tidskriftsartikel (refereegranskat)abstract
- An important source of error in very-long-baseline interferometry (VLBI) estimates of baseline length is unmodeled variations of the refractivity of the neutral atmosphere along the propagation path of the radio signals. We present and discuss the method of using data from a water vapor radiometer (WVR) to correct for the propagation delay caused by atmospheric water vapor, the major cause of these variations. Data from different WVRs are compared with estimated propagation delays obtained by Kalman filtering of the VLBI data themselves. The consequences of using either WVR data or Kalman filtering to correct for atmospheric propagation delay at the Onsala VLBI site are investigated by studying the repeatability of estimated baseline lengths from Onsala to several other sites. The lengths of the baselines range from 919 to 7941 km. The repeatability obtained for baseline length estimates shows that the methods of water vapor radiometry and Kalman filtering offer comparable accuracies when applied to VLBI observations obtained in the climate of the Swedish west coast. For the most frequently measured baseline in this study, the use of WVR data yielded a 13% smaller weighted-root-mean-square (WRMS) scatter of the baseline length estimates compared to the use of a Kalman filter. It is also clear that the “best” minimum elevation angle for VLBI observations depends on the accuracy of the determinations of the total propagation delay to be used, since the error in this delay increases with increasing air mass. For use of WVR data along with accurate determinations of total surface pressure, the best minimum is about 20 degrees; for use of a model for the wet delay based on the humidity and temperature at the ground, the best minimum is about 35 degrees.
|
|
| 6. |
- HULTQVIST, B, et al.
(författare)
-
ON THE UPWARD ACCELERATION OF ELECTRONS AND IONS BY LOW-FREQUENCY ELECTRIC-FIELD FLUCTUATIONS OBSERVED BY VIKING
- 1991
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : American Geophysical Union (AGU). - 0148-0227. ; 96:A7, s. 11609-11615
-
Tidskriftsartikel (refereegranskat)abstract
- The dependence of the generation of upward electron beams in the to side ionosphere on the amplitude of the slow electric field fluctuations and the magnitude of the dc potential difference in the acceleration region is found to agree with the model involving the fluctuating electric field component presented in 1988 by Hultqvist. The ratio between the amplitude of the electric field fluctuations, E(f), determined by the dipole electric field experiment, and the dc acceleration voltage V(acc), given by the energy of the upflowing ion beam, is much higher on average for satellite spins showing an upward electron beam than for the spins without. It is also shown that low plasma density favors the generation of electron beams.
|
|
| 7. |
- Knipp, D.J., et al.
(författare)
-
Ionospheric Convection Response to Slow, Strong Variations in a Northward Interplanetary Magnetic Field: A Case Study for January 14, 1988
- 1993
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 98, s. 19273-19292
-
Tidskriftsartikel (refereegranskat)abstract
- We analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high-latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground-based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF B(y) and B(z) components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon-to-morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern (winter) hemisphere became weak and disordered with a dawn-to-dusk potential drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interpret this behavior in terms of a recently proposed merging model for northward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over the dayside magnetosphere, producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF B(x) component present at this time could have contributed to the observed hemispheric asymmetry. Reverse convection in the summer hemisphere broke down rapidly after the ratio \textbackslashB(y)/B(z)\textbackslash exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn-to-dusk potential drop was established in both hemispheres when the magnitude of B(y) exceeded that of B(z), with potential drops of the order of 100 kV, even while B(z) remained northward. The later transition to southward B(z) produced a gradual intensification of the convection, but a greater qualitative change occurred at the transition through \textbackslashB(y)/B(z)\textbackslash = 1 than at the transition through B(z) = 0. The various convection patterns we derive under northward IMF conditions illustrate all possibilities previously discussed in the literature: nearly single-cell and multicell, distorted and symmetric, ordered and unordered, and sunward and antisunward.
|
|
| 8. |
|
|
| 9. |
|
|
| 10. |
- Lönnqvist, Håkan, et al.
(författare)
-
Generation of VLF saucer emissions observed by the Viking satellite
- 1993
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 98, s. 13565-13574
-
Tidskriftsartikel (refereegranskat)abstract
- Simultaneous observations by the Viking satellite of electric and magnetic fields as well as charged particles have been used to investigate V-shaped wave phenomena. The intensity of these VLF and ELF emissions is V-shaped when shown in a frequency versus time plot. Simultaneous observations of V-shaped so-called VLF saucer emissions, particles and field-aligned currents strongly suggest, for the first time, that upgoing electrons with energies less than a few hundred electron volts can generate these waves. Broadband waves observed inside the saucer generation region, form frequencies much less than the ion cyclotron frequency up to the plasma frequency, may also be generated by these electrons. Viking observations of VLF saucers at altitudes between 4000 km and 13,500 km show that these emissions occur at higher altitudes tha discussed in previous reports. The generation regions seem to be more extended at these higher altitudes than what has been reported at lower altitudes by other observations.
|
|
