| 1. |
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| 2. |
- Marklund, Göran, et al.
(författare)
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Observations of the electric field fine structure associated with the westward traveling surge and large-scale auroral spirals
- 1998
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 103:A3, s. 4125-4144
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Tidskriftsartikel (refereegranskat)abstract
- The characteristics of the fine scale electric field associated with the westward traveling surge and large-scale auroral spirals and surges are investigated using high-resolution electric field, magnetic field, particle and UV imager observations from four eveningside auroral oval crossings by the Freja satellite. Three of the crossings were associated with signatures of auroral substorms and one crossing went directly through the head of a surge close in time and space to substorm onset. Three passes were adjacent to auroral spiral formations, one poleward of and one equatorward of such forms and one through the multiple arc region near the front of an extended region of auroral activity. The ambient electric field was found to intensify in the direction toward the spiral head (or the center of the auroral activity region) over a region comparable to the size of the visible auroral forms. These results confirm previous findings that the spiral or surge head is associated with negative space charge and an intense upward field-aligned current. The fourth pass, directly through the surge head reveals a very complicated structure of the surge region. Narrowly structured, intense (up to 700 mV/m) and mostly converging electric fields associated with intense electron precipitation (of both high and medium energies) and balanced field-aligned currents (up to 30 μA/m2) are seen near the edge of the surge head and adjacent to auroral structures in the wake. These narrow regions are embedded within more extended regions of intense high-energy electron precipitation but very weak electric fields and field-aligned currents. According to some existing models of the surge, a pronounced westward electric field component and a southward polarisation electric field is expected within the entire high-conductivity region but evidence in support of this was not found in the data. Rather, these suggest that a significant part of the upward surge current is closed by distributed downward field-aligned currents from the near surroundings. The Freja electric field is typically seen to intensify at the edges of or in-between bright auroral structures and to decrease within the arcs similar to what is observed in the ionosphere. The surge electric field is, however, much more intense than previously observed or anticipated at these altitudes with characteristics rather similar to those observed in the auroral acceleration region. Since the particle data indicate that most of the acceleration takes place above Freja altitudes, it seems as if Freja traversed the lower part of the auroral acceleration region associated with the surge.
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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- Stasiewicz, K, et al.
(författare)
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Cavity resonators and Alfven resonance cones observed on Freja
- 1997
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 102:A2, s. 2565-2575
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Tidskriftsartikel (refereegranskat)abstract
- Multiresolution wavelet analysis of magnetic field, electric field, and plasma density records taken on Freja during strong auroral events shows evidence for cavity Alfven resonators in the topside ionosphere. The cavity (or transverse) resonators consist of standing perpendicular wave modes which are trapped inside plasma cavities of different perpendicular scales. The smallest size cavities have perpendicular widths comparable to the electron skin depth, lambda(s) = 2 pi c/omega(pe), and are presumably associated with the resonance cones of Alfven waves launched by a magnetospheric source. The Alfven resonance cones (ARCs) carry intense field-aligned currents, support strong parallel electric fields and represent discharge and heating channels for auroral particles. We have made a detailed analysis of the electromagnetic properties of two singular auroral structures associated with ARCs. Field-aligned currents at the resonance structures reach intensities of 100-300 rho Am-2 in the upward and downward directions and are carried mainly by cold ionospheric plasma in both directions. The parallel electric field of ARCs is observed at amplitudes up to 100 mV/m, which is 2 orders of magnitude larger than expected for the unbounded Alfven waves. Field-aligned electron beams accelerated inside ARCs are observed to drive Langmuir waves with parallel electric field occasionally exceeding 1 V/m. One of the analyzed ARC structures has electromagnetic and particle properties characteristic of ''black aurora'' with electric field diverging from the center of the cavity; the other has converging electric field.
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| 7. |
- Stasiewicz, K, et al.
(författare)
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Cavity resonators and Alfven resonance cones observed on Freja
- 1997
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. ; 102:A2, s. 2565-2575
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Tidskriftsartikel (refereegranskat)abstract
- Multiresolution wavelet analysis of magnetic field, electric field, and plasma density records taken on Freja during strong auroral events shows evidence for cavity Alfven resonators in the topside ionosphere. The cavity (or transverse) resonators consist
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| 8. |
- Wahlund, JE, et al.
(författare)
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Broadband ELF plasma emission during auroral energization 1. Slow ion acoustic waves
- 1998
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. ; 103:A3, s. 4343-4375
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- High-resolution measurements by the Freja spacecraft of broadband extremely low frequency (BB-ELF) emission from de up to the lower hybrid frequency (a few kHz) are reported from regions of transverse ion acceleration (TAI) and broad-energy suprathermal e
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