| 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. |
- 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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| 5. |
- 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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| 6. |
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| 7. |
- 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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| 8. |
- 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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| 9. |
- Sundberg, Torbjörn, et al.
(författare)
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Properties of the Boundary Layer Potential for Northward Interplanetary Magnetic Field
- 2009
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 36:11, s. L11104-
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Tidskriftsartikel (refereegranskat)abstract
- We present a method for estimating the portion of the ionospheric high-latitude potential that maps to the magnetospheric boundary layer during steady northward IMF and global ionospheric 4-cell convection patterns associated with lobe reconnection, together with the results of a statistical study based on DMSP F13 data from 1996-2004. In comparison with a previous study for steady southward IMF by Sundberg et al. [2008], the results show significantly larger boundary layer potentials, with a mean value of 10 kV for the 271 events studied, corresponding to roughly 30-35% of the potential generated by the solar wind interaction. In a statistical analysis, the boundary layer potential is also shown to depend significantly on viscous parameters such as the solar wind velocity, density and pressure.
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| 10. |
- Sundberg, Torbjörn, et al.
(författare)
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Statistical analysis of the sources of the cross-polar potential for southward IMF, based on particle precipitation characteristics
- 2008
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:8, s. L08103-
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Tidskriftsartikel (refereegranskat)abstract
- There are several proposed physical processes which may contribute to the cross-polar potential and thus drive ionospheric convection around the polar caps. It is generally believed that magnetic reconnection is the dominant process, however dynamos such as viscous interaction and impulsive penetration are other possible contributors. A comprehensive statistical study has been conducted using data from the DMSP F13 satellite for passages along the northern hemisphere dawn-dusk meridian, with focus on typical two-cell convection patterns during times of steady southward IMF conditions. The results show that the low-latitude dynamo (viscous interaction or reconnection in the LLBL) on average accounts for only 1–2 kV of the total potential drop, values lower than those previously predicted. At rare occasions this dynamo can be a significant source of energy, however, contributing to more than 20 kV of the cross-polar potential.
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