| 1. |
- Blomberg, Lars G., et al.
(författare)
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High-Latitude Convection Patterns For Various Large-Scale Field-Aligned Current Configurations
- 1991
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 18:4, s. 717-720
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Tidskriftsartikel (refereegranskat)abstract
- The large-scale field-aligned current system for persistent northward interplanetary magnetic field (IMF) is typically different from that for persistent southward IMF. One characteristic difference is that for northward IMF there is often a large-scale field-aligned current system poleward of the main auroral oval. This current system (the NBZ current) typically occupies a large fraction of the region poleward of the region 1 and 2 currents. The present paper models the high-latitude convection as a function of the large-scale field-aligned currents. In particular, a possible evolution of the convection pattern as the current system changes from a typical configuration for southward IMF to a configuration representing northward IMF (or vice versa) is presented. Depending on additional assumptions, for example about the y-component of the IMF, the convection pattern could either turn directly from a two-cell type to a four-cell type, or a three-cell type pattern could show up as an intermediate state. An interesting although rather surprising result of this study is that different ways of balancing the NBZ currents has a minor influence on the large-scale convection pattern.
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| 2. |
- Boardsen, Scott A., et al.
(författare)
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Observations of Kelvin-Helmholtz waves along the dusk-side boundary of Mercury's magnetosphere during MESSENGER's third flyby
- 2010
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 37
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Tidskriftsartikel (refereegranskat)abstract
- During the third MESSENGER flyby of Mercury on 29 September 2009, 15 crossings of the dusk-side magnetopause were observed in the magnetic field data over a 2-min period, during which the spacecraft traveled a distance of 0.2 R-M (where R-M is Mercury's radius). The quasi-periodic nature of the magnetic field variations during the crossings, the characteristic time separations of similar to 16 s between pairs of crossings, and the variations of the magnetopause normal directions indicate that the signals are likely the signature of surface waves highly steepened at their leading edge that arose from the Kelvin-Helmholtz instability. At Earth, the Kelvin-Helmholtz instability is believed to lead to the turbulent transport of solar wind plasma into Earth's plasma sheet. This solar wind entry mechanism could also be important at Mercury. Citation: Boardsen, S. A., T. Sundberg, J. A. Slavin, B. J. Anderson, H. Korth, S. C. Solomon, and L. G. Blomberg (2010), Observations of Kelvin-Helmholtz waves along the dusk-side boundary of Mercury's magnetosphere during MESSENGER's third flyby, Geophys. Res. Lett., 37, L12101, doi: 10.1029/2010GL043606.
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| 3. |
- Boehm, M. H., et al.
(författare)
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Observations of an upward-directed electron beam with the perpendicular temperature of the cold ionosphere
- 1995
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 22, s. 2103-2106
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Tidskriftsartikel (refereegranskat)abstract
- The Freja TESP electron spectrometer has repeatedly observed similar to 100 eV - 1 keV upward-directed, anti-field-aligned electron beams near 1700 km altitude in the auroral zone. A particularly intense event, at energies up to 2 keV, is described. The beam perpendicular temperature T perpendicular to(e)), was as low as 0.1-0.2 eV at 100-200 eV parallel energy. The 10-15 s period of upward fluxes was coincident with a low density (similar to 10 cm(-3)) period and a similar to 5 keV ion conic. Strong low frequency waves and the lack of any downward motion in the simultaneously observed ion conic suggest a strong element of wave acceleration, while electric field and ion loss cone measurements provide limited evidence of potential acceleration to a fraction of the observed energies.
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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
- Kullen, A, et al.
(författare)
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The influence of IMF B-y on the mapping between the Earth's magnetotail and its ionosphere
- 1996
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 23:18, s. 2561-2564
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Tidskriftsartikel (refereegranskat)abstract
- It is well known that the dawn-dusk component of the interplanetary magnetic field (IMF) creates asymmetries in the magnetotail. The main effects are a weak nonuniform penetration of IMF B-y into the tail and a rotation of the whole tail current sheet. In this study we modify the Tsyganenko (1989) model to simulate the IMF B-y induced tail effects. By field line mapping we examine how different regions of the magnetosphere and ionosphere are influenced by the IMF By tail effects. The plasma sheet is shown to rotate around the central axis of the magnetotail. The mapping from the plasma sheet to the ionosphere for nonzero IMF B-y shows a change in the geometry of the auroral oval. The whole oval rotates, a local bulge occurs near midnight and the thickness of the oval is different in the morning and the evening sector. Simple geometrical considerations show that the tilting of the field lines caused by IMF B-y leads to a rotation of the oval, whereas the rotation of the tail current sheet is reponsible for the thickness variations along the oval. These results fit very well with the observed auroral distribution for an IMF with a dawnward or duskward direction.
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| 8. |
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| 9. |
- Marklund, Göran, et al.
(författare)
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On intense diverging electric fields associated with black aurora
- 1994
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Ingår i: Geophysical Research Letters. - 0094-8276. ; 21:17, s. 1859-1862
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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.
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| 10. |
- Mursula, Kalevi, et al.
(författare)
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Dispersive Pc1 bursts observed by Freja
- 1994
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 21:17, s. 1851-1854
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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.
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