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- Ohtani, S, et al.
(författare)
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Altitudinal comparison of dayside field-aligned current signatures by viking and DMSP-F7 : Intermediate-scale field-aligned current systems
- 1996
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A7, s. 15297-15310
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Tidskriftsartikel (refereegranskat)abstract
- Dayside large-scale and intermediate-scale field-aligned current (FAG) signatures are examined with multi-instrument measurements from Viking and DMSP-F7 at magnetic conjunctions. The present paper reports four such conjunction events, with an emphasis on an event that occurred on October 13, 1986. In these four events both Viking and DMSP-F7 crossed prenoon FAC systems approximately along meridians. The altitude of DMSP-F7 was 835 km, whereas that of Viking ranged from 8500 to 12,000 km. The electric to magnetic field ratio measured by Viking indicates that intermediate-scale FAC systems, as well as large-scale FAC systems, are often quasi-stationary, This is also supported by the comparison between the Viking and DMSP-F7 magnetic measurements. The only obvious exception is the equatorward part of the October 13 event, in which the Viking and DMSP-F7 measurements are better explained in terms of Alfven waves. In two other events the Viking signature projected to the DMSP-F7 altitude is significantly more structured than the DMSP-F7 signature, although the electric to magnetic field ratio observed by Viking suggests that the associated FACs were quasistationary. This apparent discrepancy is possibly ascribed to the fact that Viking stays longer in FAC systems and therefore has more chance to observe temporal changes in FACs. However, such temporal effects must operate longer than the Alfven transit time so that FAC systems become quasi-stationary. Although the generation mechanism(s) of intermediate-scale FAC systems remains an open question, possibilities include a localized shear of plasma convection and a localized merging between the solar wind and magnetospheric field lines.
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- Ohtani, S, et al.
(författare)
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Response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field
- 1997
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - 0148-0227. ; 102:A10, s. 22247-22260
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Tidskriftsartikel (refereegranskat)abstract
- The response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field (IMF) is examined with multi-instrument data from the Viking and DMSP-F7 satellites as well as ground magnetometer data. The event selected, which occurred on October 5, 1986, was previously identified as a high-latitude dayside form by Murphree and Elphinstone [1988]. IMF B-Y was positive during most of the Viking auroral UV observation, whereas IMF B-Z made a transient southward excursion, followed by a sharp increase to +8 nT. The results are summarized as follows: (1) There were two auroral belts extending westward from the early afternoon sector. The equatorward belt was persistent throughout the event and was embedded in the CPS/BPS precipitation region, whereas the poleward belt was in the open field line region and was associated with northward IMF B-Z. (2) A new auroral spot emerged in the afternoon sector just poleward of the previously active region, delayed 10 min from the sudden increase in IMF B-Z. This time lag is ascribed to the travel time of the new IMF orientation from the subsolar point to a solar wind-magnetosphere interaction site tailward of the dayside cusp, as well as to the response time of the dayside auroral acceleration process. (3) The new auroral activity expanded both eastward and westward during the first few minutes and then expanded primarily westward across the noon meridian, forming the poleward belt. The speed of the westward extension in the second phase was comparable to the speed of the ionospheric convection. (4) The dayside auroral process has a finite decay constant, which is inferred to be at least 10 min. (5) The response time of the midday ionospheric convection to IMF variations is estimated to be a few to several minutes.
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- Ohtani, S, et al.
(författare)
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Simultaneous prenoon and postnoon observations of 3 field-aligned current systems from Viking and DMSP F7
- 1995
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 100:A1, s. 119-136
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Tidskriftsartikel (refereegranskat)abstract
- The spatial structure of dayside large-scale field-aligned current (FAG) systems is examined by using Viking and DMSP-F7 data. We focus on four events in which the satellites simultaneously observed postnoon and prenoon three FAC systems: the region 2, the region 1, and the mantle (referred to as midday region 0) systems, from equatorward to poleward. These events provide the most solid evidence to date that the midday region 0 system is a separate and unique FAC system, and is not an extension of the region 1 system from other local times. The events are examined comprehensively by making use of a multi-instrument data set, which includes magnetic field, particle flux, electric field, auroral UV image data from the satellites, and the Sondrestrom convection data. The results are summarized as follows: (1) Region 2 currents flow mostly in the CPS precipitation region, often overlapping with the BPS at their poleward edge. (2) The region 1 system is located in the core part of the auroral oval and is confined in a relatively narrow range in latitude which includes the convection reversal. The low-latitude boundary layer, possibly including the outer part of the plasma sheet, and the external cusp are the major source regions of dayside region 1 currents. (3) Midday region 0 currents flow on open field lines and are collocated with the shear of antisunward convection flows with velocities decreasing poleward. On the basis of these results we support the view that both prenoon and postnoon current systems consist of the three-sheet structure when the distortion of the convection pattern associated with interplanetary magnetic field (IMF) B-y is small and both morningside and eveningside convection cells are crescent-shaped. We also propose that the midday region 0 and a part of the region 1 systems are closely coupled to the same source.
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- Potemra, T A, et al.
(författare)
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A survey of Pc 5 pulsations in the dayside high-latitude regions observed by Viking
- 1996
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A11, s. 24801-24813
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Tidskriftsartikel (refereegranskat)abstract
- We have examined magnetic and electric field observations acquired with the polar orbiting Viking satellite near its 3 R(E) apogee and at local times from dawn through noon to dusk. Our objective was to determine the statistical properties of Pc 5 (150-600 s period) pulsations at different dayside local times and their relationship to large-scale Birkeland currents. The Viking data examined in detail here were acquired from 41 orbits during the period from March 26, 1986, to July 19, 1986. Pulsations could be readily identified on prenoon orbits as evidenced in a series of 10 consecutive orbits in one case and six consecutive orbits in another. The waveforms in the postnoon hours are more complicated than in the morning, and this made it more difficult to identify pulsations in this region. The simultaneous electric and magnetic field measurements from Viking were used to distinguish the magnetic variations associated with large-scale Birkeland currents from those associated with waves. These observations also made it possible to derive values for Pedersen and wave conductivities and Poynting fluxes for the waves. The average wave period for the 40 cases studied here is 4.8 min (frequency of 3.5 mHz), the average magnetic field amplitude is 30 nT, the average electric field amplitude (for the 11 cases when the electric field observations were available) is 7 mV/m, and the average incident Poynting flux is 85 mu W/m(2). The principal characteristics of the waves include the following: (1) From morning to noon, the magnetic field pulsations are polarized primarily in the east-west direction, and the associated electric field oscillations lag by 60 degrees-80 degrees and are polarized close to the north-south direction. Consequently, these pulsations are interpreted as the fundamental modes of resonant toroidal magnetic field oscillations. (2) In the afternoon hours, the magnetic field pulsations have a strong north-south component and include higher-frequency components (possibly the second harmonic). (3) In both the morning and afternoon hours, the pulsations occur 2 degrees-5 degrees equatorward of the interface between the counterflowing region 1 and region 2 Birkeland currents. (4) The average wave periods increase from similar to 3 min near 0800 magnetic local time (MLT) to similar to 5 min near noon. (5) The average wave amplitudes range from similar to 40 nT near 0900-1000 MLT to similar to 25 nT near 1300-1600 MLT.
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