| 1. |
- Feldstein, Y. I., et al.
(författare)
-
Auroral electrojets and 3D currents in the ionosphere-magnetosphere system
- 2006
-
Ingår i: “Physics of Auroral Phenomena”, Proc. XXIX Annual Seminar, Apatity. - : Kola Science Centre, Russian Academy of Science. ; , s. 25-30
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- There are shortly described results of the analysis of variations in the location and intensity of the auroral electrojets during magnetic storms and substorms using a numerical method for estimating the equivalent ionospheric currents based on data from meridian chains of magnetic observatories. It is shown that the westward electrojet adjoins to the polar electrojet located at cusp latitudes in the dayside sector. The association of electrojets with the field-aligned currents (FACs), namely Region 1 FAC and Region 2 FAC is considered. During intense disturbances a Region 3 FAC (accompanied with diffuse electron precipitation from the plasma sheet boundary layer) with the downward current was identified. The analysis of observational data is summarized in terms of 2D time-latitude distribution of electrojets at ionospheric altitudes. The magnetic field sawtooth variations generated during the storm main and early recovery phases are also discussed. To follow 3D currents in the magnetosphereionosphere system a clarified view of interrelated 3D currents and magnetospheric plasma domains is presented.
|
|
| 2. |
- Feldstein, Y. I., et al.
(författare)
-
Auroral electrojets and boundaries of plasma domains in the magnetosphere during magnetically disturbed intervals
- 2006
-
Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 24:8, s. 2243-2276
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate variations in the location and intensity of the auroral electrojets during magnetic storms and substorms using a numerical method for estimating the equivalent ionospheric currents based on data from meridian chains of magnetic observatories. Special attention was paid to the complex structure of the electrojets and their interrelationship with diffuse and discrete particle precipitation and field-aligned currents in the dusk sector. During magnetospheric substorms the eastward electrojet (EE) location in the evening sector changes with local time from cusp latitudes (Phi similar to 77 degrees) during early afternoon to latitudes of diffuse auroral precipitation (Phi similar to 65 degrees) equatorward of the auroral oval before midnight. During the main phase of an intense magnetic storm the eastward currents in the noon-early evening sector adjoin to the cusp at Phi similar to 65 degrees and in the pre-midnight sector are located at subauroral latitude Phi similar to 57 degrees. The westward electrojet (WE) is located along the auroral oval from evening through night to the morning sector and adjoins to the polar electrojet (PE) located at cusp latitudes in the day-side sector. The integrated values of the eastward (westward) equivalent ionospheric current during the intense substorm are similar to 0.5 MA (similar to 1.5 MA), whereas they are 0.7 MA (3.0 MA) during the storm main phase maximum. The latitudes of auroral particle precipitation in the dusk sector are identical with those of both electrojets. The EE in the evening sector is accompanied by particle precipitation mainly from the Alfven layer but also from the near-Earth part of the central plasma sheet. In the lower-latitude part of the EE the field-aligned currents (FACs) flow into the ionosphere (Region 2 FAC), and at its higher-latitude part the FACs flow out of the ionosphere (Region 1 FAC). During intense disturbances, in addition to the Region 2 FAC and the Region 1 FAC, a Region 3 FAC with the downward current was identified. This FAC is accompanied by diffuse electron precipitation from the plasma sheet boundary layer. Actually, the triple system of FAC is observed in the evening sector and, as a consequence, the WE and the EE overlap. The WE in the evening sector comprises only the high-latitude periphery of the plasma precipitation region and corresponds to the Hall current between the Region 1 FAC and Region 3 FAC. During the September 1998 magnetic storm, two velocity bursts (similar to 2-4 km/s) in the magnetospheric convection were observed at the latitudes of particle precipitation from the central plasma sheet and at subauroral latitudes near the ionospheric trough. These kind of bursts are known as subauroral polarization streams (SAPS). In the evening sector the Alfven layer equatorial boundary for precipitating ions is located more equatorward than that for electrons. This may favour northward electric field generation between these boundaries and may cause high speed westward ions drift visualized as SAPS. Meanwhile, high speed ion drifts cover a wider range of latitudes than the distance between the equatorward boundaries of ions and electrons precipitation. To summarize the results obtained a new scheme of 3-D currents in the magnetosphere-ionosphere system and a clarified view of interrelated 3-D currents and magnetospheric plasma domains are proposed.
|
|
| 3. |
- Ronneteg, Sabina, et al.
(författare)
-
Longitudinal spin fluctuations of thermal and magnetic properties of TlCo2Se2-xSx
- 2004
-
Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 281:2-3, s. 388-393
-
Tidskriftsartikel (refereegranskat)abstract
- At isovalent substitution of Se for S, the layered chalcogenides TlCo2Se2−xSx (0⩽x⩽2) exhibit a change of magnetic order type, from an antiferromagnetic helix at low S content to a ferromagnet at high S content. The effective magnetic moments peff∼2 μB per Co, as estimated from the magnetic susceptibility at high temperatures, significantly exceed what would be the result of the Co magnetic moments μsat∼0.5 μB in both the ferromagnetic and antiferromagnetic phases. This signifies the crucial role of longitudinal spin fluctuations in the magnetic behaviour of these conducting compounds. The spin fluctuations reveal themselves also in specific heat measurements, leading to a diminution of the expected anomalies at the Curie/Néel temperatures and to an excess of heat capacity at high temperatures as compared to phonon contributions.
|
|
| 4. |
- Feldstein, Y I, et al.
(författare)
-
Electromagnetic characteristics of the high-latitude ionosphere during the various phases of magnetic substorms
- 1996
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A9, s. 19921-19936
-
Tidskriftsartikel (refereegranskat)abstract
- Model calculations of the electrodynamics of the high-latitude ionosphere are compared to measurements made by the Viking satellite during July-August 1986. The model calculations are based on the IZMEM procedure, where the electric field and currents in the ionosphere are given as functions of the interplanetary magnetic field. The events chosen correspond to the growth, the expansion, and the recovery phases of substorms. During the growth and expansion phases the correlation between the model results and the satellite data is rather good. During recovery phase the correlation is not as good. The correlation between modeled and observed quantities suggest that during growth and expansion phase the magnetosphere is mainly directly driven by the solar wind, whereas during recovery phase it is mainly driven by internal processes, i.e., loading-unloading. Best fit is obtained when averaging the measured quantities over a few minutes, which means adjusting the spatial resolution of the measurements to the resolution of the model. Different time delays between the interplanetary magnetic field observations and those of Viking were examined. Best agreement was obtained, not surprisingly, for time delays corresponding to the estimated information transit time from the solar wind spacecraft to the ionosphere.
|
|
| 5. |
- 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.
|
|
| 6. |
|
|
| 7. |
|
|
| 8. |
- Gromova, L. I., et al.
(författare)
-
High-Latitude Ionospheric Convection Patterns Dependent on the IMF Orientation
- 2007
-
Ingår i: <em>“</em>Physics of Auroral Phenomena”, Proc. XXX Annual Seminar, Apatity. - : Kola Science Centre, Russian Academy of Science. ; , s. 64-68
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The IZMEM model provides high-latitude ionospheric plasma convection patterns in both hemispheres as a function of the IMF orientation. Model electric potentials are compared with electric field measurements from the DE2, FAST and DMSP satellites along high-latitude passes of the Northern and Southern hemispheres during IMF Bz < 0 and By < 0 (By >0). It has been shown that the IZMEM model electric potentials are in good agreement with measurements along the satellite passes, which makes the IZMEM global spatial convection patterns for these plausible. For small IMF magnitude ionospheric convection patterns generally consist of two cells with a positive potential cell on the dawn-side and a negative potential cell on the dusk-side. For IMF By<0 (By>0) a positive (negative) potential cell becomes dominant in the northern hemisphere, and oppositely in the southern hemisphere. During Bz > 0 the convection pattern changes from the standard two-cell pattern to a more complicated one. IZMEM shows two additional convection cells in the dayside polar cap, positive (negative) potential cell is present duskward (dawnward) of the noon-midnight meridian, and may cause three-cell or four-cell convection pattern depending on By/Bz ratio.
|
|
