| 1. |
- Knudsen, D. J., et al.
(författare)
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Sub-kilometer thermal plasma structure near 1750 km altitude in the polar cusp/cleft
- 1994
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Ingår i: Geophysical Research Letters. - 0094-8276. ; 21:17, s. 1907-1910
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Tidskriftsartikel (refereegranskat)abstract
- We present Freja Cold Plasma Analyzer (CPA) measurements from an encounter with the low altitude (approx.1750 km) polar cusp during which the CPA measured 2-D images of the thermal (0-16 eV) particle distributions at 1.2 s time resolution, and simultaneously made rapid estimates (600/s) of integrated thermal particle flux into the instrument. The high resolution data show bursty ion flux enhancements of the order of tens of percent on time scales of tens of ms, or alternatively, hundreds of m spatial scales. The flux of electrons from 0-16 eV also varied by tens of percent and on temporal/spatial scales comparable to those in the ion cases. There is some evidence that the thermal particle flux variations are associated with intense low-frequency electromagnetic fluctuations with temporal/spatial scales identical to those seen by the CPA (tens of ms. hundreds of m).
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| 2. |
- Erlandson, R. E., et al.
(författare)
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Freja observations of electromagnetic ion cyclotron ELF waves and transverse oxygen ion acceleration on auroral field lines
- 1994
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Ingår i: Geophysical Research Letters. - 0094-8276. ; 21:17, s. 1855-1858
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Tidskriftsartikel (refereegranskat)abstract
- Extremely low-frequency (ELF) magnetic and electric field plasma wave emissions were recorded on 2 October 1993 on auroral field lines by the Magnetic Field Experiment during Freja orbit 4770. The ELF wave frequencies were below the local oxygen gyrofrequency (25 Hz) and between the helium and proton gyrofrequencies (100 to 400 Hz). The ELF waves, interpreted as electromagnetic ion cyclotron (EMIC) waves, were observed in a region of inverted-v-type electron precipitation. The EMIC waves were correlated over time with auroral and lower energy (≈ 100 eV) electrons, which are both possible sources of free energy, and also with transversely accelerated oxygen ions. The waves above the helium gyrofrequency were more closely correlated with the transverse oxygen ion acceleration than the waves below the oxygen gyrofrequency. These observations are consistent with a scenario in which electron beams generate EMIC waves, which then produce transverse oxygen ion acceleration through a gyroresonant interaction.
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| 3. |
- Feldstein, Y. I., et al.
(författare)
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Electromagnetic weather at 100 km altitude on 3 August 1986
- 1994
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 21, s. 2095-2098
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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.
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| 4. |
- Lönnqvist, Håkan, et al.
(författare)
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Generation of VLF saucer emissions observed by the Viking satellite
- 1993
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 98, s. 13565-13574
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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.
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| 5. |
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| 6. |
- Marklund, Göran, et al.
(författare)
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The double probe electric field experiment on Freja : Experiment description and first results
- 1994
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Ingår i: Space Science Reviews. - 0038-6308. ; 70:3-4, s. 483-508
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Tidskriftsartikel (refereegranskat)abstract
- A description is given of the Freja double-probe electric field instrument. Its capability to perform high-resolution measurements of the aurora and its fine-structure as well as collect information on sub-auroral and low-latitude phenomena is illustrated by selected results from the first six months of operation. The instrument is highly flexible and possible to operate in a number of different modes. It is also equipped with a 4-Megabyte burst memory for high data sampling rate and temporary storage of data. It has been fully operational since October 1992, and delivers data from ≈22 hr day-1 including about 5-6 auroral crossings of the northern and southern auroral ionosphere. New and important information on the auroral fine structure and electrodynamics is obtained by means of burst resolution data (6144 samples s-1) and normal resolution data (768 samples s-1). Common burst data collection triggered by the electric field event detector has turned out to be very useful for the selection of scientifically interesting events. This is illustrated by high-resolution data of a pair of extremely intense and narrow electric field structures (1 V m-1) which are associated with a total absence of precipitating particles, depletions of the thermal plasma and with an intense wave activity. The low inclination of the Freja orbit provides a new perspective for studying largescale phenomena associated with east-west gradients as is exemplified by electric field data from a satellite crossing over north-south oriented auroral structures presumably resulting from rotational distortions of east-west aligned auroral arcs. The different plasma regimes encountered by Freja are continuously monitored by means of current sweeps applied to the probes and by the satellite potential. In addition, overview data (8 samples s-1) are collected from full orbits and stored in the on-board memory and have proved to be extremely valuable, providing new information on global electric field phenomena at subauroral and lower latitudes, such as the intense poleward electric fields and Pc-1 observations that have been made near the plasmapause during substorm activity. © 1994 Kluwer Academic Publishers.
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| 7. |
- Nordebo, S, et al.
(författare)
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Noise Reduction Using an Adaptive Microphone Array for Speech Recognition in a Car
- 1993
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Konferensbidrag (refereegranskat)abstract
- TIGS paper describes an evaluation of an adaptive microphone array with respect to speech recognition performauce in a tar. The microphone array is compared KO two conventional microphones of different rypes. The speech recognition device is aimed to be a part of a man/maehine-interface between the driver and tar information services.
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| 8. |
- Parker, JL, et al.
(författare)
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Plasma modification of mica
- 1990
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Ingår i: Journal of Colloid and Interface Science. - 0021-9797 .- 1095-7103. ; 134, s. 449-458
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Tidskriftsartikel (refereegranskat)abstract
- When mica is exposed to a low temperature nonpolymer-forming plasma the surface structure is altered by a combination of sputtering and chemical reaction with reactive species in the plasma. The chemical composition of the surface is altered without a significant increase in surface roughness under optimal conditions. The effect of the plasma process parameters (exposure time, power, flow rate, and pressure) on the extent of surface modification has been investigated by means of ESCA and contact angle measurements. After exposure to water vapor plasma the mica surface becomes reactive to silanation with chlorosilanes in the gas phase. The durability of mica surfaces modified by plasma treatment and subsequent reaction with some chlorosilanes has been investigated. The forces between water plasmatreated surfaces after silanation have been measured using a surface force apparatus.
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| 9. |
- Persson, M. A. L., et al.
(författare)
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Near-Earth substorm onset - a coordinated study
- 1994
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 21:17, s. 1875-1878
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Tidskriftsartikel (refereegranskat)abstract
- We present simultaneous satellite and ground-based measurements of a substorm. Throughout the initial substorm expansion, southward drifting arcs are observed poleward of the expanding substorm aurora, indicating two independent systems of particle precipitation. Freja passes the brightening onset arc in the topside ionosphere near the moment of the substorm onset, observing an Alfven wave, field aligned current and oxygen ion outflow. The substorm onset occurs at low magnetospheric L-shells, near the poleward edge of the region of trapped particles. The location and time for the substorm injection are confirmed by geostationary spacecraft together with magnetometers, all-sky cameras and radar on the ground. We believe that the substorm onset may be triggered by modification of the oxygen content of the inner magnetosphere during the growth-phase caused by ionospheric ion outflow.
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