| 1. |
- Aikio, A T, et al.
(författare)
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On the origin of the high-altitude electric field fluctuations in the auroral zone
- 1996
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A12, s. 27157-27170
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Tidskriftsartikel (refereegranskat)abstract
- Intense fluctuations in the electric field at high altitudes in the auroral zone are frequently measured by the Viking satellite. We have made an analysis of the origin of electric and magnetic fluctuations in the frequency range of 0.1 - 1 Hz by assuming four different sources for the signals: (I) spatial structures, (2) spatial structures with a parallel potential drop below the satellite, (3) traveling; shear Alfven waves, and (4) interfering shear Alfven waves. We will shaw that these different sources of the signals may produce similar amplitude ratios and phase differences between the perpendicular electric and magnetic fields. Since the different sources have different frequency dependencies, this can be used as an additional test if the signals are broadband. In other cases, additional information is needed, for example, satellite particle measurements or ground; magnetic measurements. The ideas presented in the theory were tested for one Viking eveningside pass over Scandinavia, where ground-based magnetometer and EISCAT radar measurements were available. The magnetic conditions were active during this pass and several interfering shear Alfven waves were found. Also, a spatial structure with a parallel potential drop below the satellite was identified. The magnitude of the 10-km-wide potential drop was at least 2 kV and the upward field-aligned current 26 mu A m(-2) (value mapped to the ionospheric level). The held-aligned conductance was estimated as 1.3 - 2.2x10(-8) S m(-2).
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| 2. |
- Cumnock, J. A., et al.
(författare)
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Interplanetary magnetic field control of theta aurora development
- 2002
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 107:A7
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Tidskriftsartikel (refereegranskat)abstract
- [1] We ascertain the influence of the B-y component of the interplanetary magnetic field (IMF) on theta aurora evolution. During most cases where a transpolar arc is observed to move across the polar region, and form a theta aurora, there are brief (minutes) southward excursions of IMF B-z, however northward IMF is required prior to theta aurora formation. Observations show that theta aurora can form during strictly northward IMF with its motion consistent with a change in sign of IMF B-y. It is important to note that since transpolar arcs can persist for 20-30 min after the IMF turns southward, errors will occur in assigning instantaneous IMF conditions to snapshots'' of particular auroral patterns. We consider the entire evolution of the theta aurora and the changing IMF conditions. The influence of IMF B-y is best illustrated by examples which occur during steady northward IMF as compared to times when the IMF is northward on average. We show examples, provided by the Polar UV imager, when the IMF is steady northward. For one case, DMSP F13 and F14 provide in situ measurements of precipitating particles, ionospheric plasma flows and ion density. This unique data set enables us to analyze in detail the evolution of a theta aurora, in one case crossing the entire polar region. No sign change in B-z is needed for theta aurora formation.
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| 3. |
- Eriksson, Stefan, et al.
(författare)
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Comparing a spherical harmonic model of the global electric field distribution with Astrid-2 observations
- 2002
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 107:A11
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Tidskriftsartikel (refereegranskat)abstract
- [1] Electric field measurements provided by the double probe instrument on the Astrid-2 satellite are compared with the empirical Weimer electric field model for all magnetic local times, except between 11 and 13 MLT, and poleward of 55degrees corrected geomagnetic latitude (CGLat). We focus the model evaluation on its ability to predict the latitudinal locations of the convection reversal boundaries for two-cell convection patterns and to estimate the magnitude of the electric field above 55degrees CGLat. A total number of 780 polar cap passes are employed from the Northern Hemisphere between January and July 1999. The measured average electric field magnitude in the dawn-dusk meridian plane above 55degrees CGLat is generally 25% larger than the predicted field independent of the interplanetary magnetic field (IMF) direction. The model shows a better correspondence with the observed electric field for southward IMF than for northward IMF, with most cases centered around B-z = -1.5 nT and r = 0.88. However, the agreement for northward IMF is promising, and a few examples are shown to corroborate this fact. The observed and predicted convection reversal boundary locations along the satellite track for southward IMF are on the average found 2-3degrees CGLat apart in the dawn-dusk meridian plane but may be as far apart as 9degrees CGLat. An initial investigation of the relative timing of a 20-min averaging window for the IMF along the 20-25 min polar cap crossing suggests that a time-dependent transfer function may be found that applies a higher weight to the input solar wind data early in the pass and a lower weight later in the pass for an IMF window that corresponds to the first half of the crossing and the opposite weight versus time dependence for an IMF window corresponding to the last half of the crossing.
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| 4. |
- Eriksson, Stefan, et al.
(författare)
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Lobe cell convection and field-aligned currents poleward of the region 1 current system
- 2002
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 107:A8
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Tidskriftsartikel (refereegranskat)abstract
- [1] We present a case and statistical study of plasma convection in the Northern Hemisphere during summer conditions using electric field, magnetic field, and particle data taken during dawn-dusk directed orbits of the FAST satellite. To our knowledge, this set provides the most comprehensive combination of data as yet presented in support of lobe cell convection from an ionospheric perspective this far from the noon sector. In particular, we study the current systems and convection patterns for all passes in July 1997 that show evidence for six large-scale field-aligned currents (FACs) rather than the usual system of four FACs associated with the region 1/region 2 current systems. A total of 71 passes out of 232 in the study had the extra pair of FACs. The extra pair of FACs in 30 of the 71 cases lies either on the dawnside or on the duskside of the noon-midnight meridian, and their position is strongly correlated with the polarity of the IMF By (negative and positive, respectively). This is consistent with the IMF dependence of a three-cell convection pattern of coexisting merging, viscous, and lobe-type convection cells. The occurrence of the asymmetric FAC pair was also strongly linked to conditions of IMF |B-y/B-z | > 1. The extra pair of FACs in these cases was clearly associated with the lobe cell of the three-cell convection system. The remaining 41 cases had the pair of FACs straddling the noon-midnight meridian. The extra pair of FACs was often (20 cases out of 30) observed at magnetic local times more than three hours away from noon, rather than being confined to regions near noon and the typical location of the cusp. Such a current system consisting of a pair of FACs poleward of the nearest region 1 current is consistent with the IMF B-y-dependent global MHD model developed by Ogino et al. [1986] for southward IMF conditions, as well as with other magnetospheric and ionospheric convection models that include the effects of merging occuring simultaneously at both low-latitude dayside and high-latitude lobe and flank magnetopause reconnection sites. Finally, the presence of the additional FACs and three-cell convection well away from noon show that the entire dayside ionosphere is affected by IMF-dependent processes, rather than only a limited region around noon.
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| 5. |
- Feldstein, Y I, et al.
(författare)
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Electromagnetic characteristics of the high-latitude ionosphere during the various phases of magnetic substorms
- 1996
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A9, s. 19921-19936
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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.
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| 6. |
- Kullen, A., et al.
(författare)
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Solar wind dependence of the occurrence and motion of polar auroral arcs : A statistical study
- 2002
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 107:A11
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Tidskriftsartikel (refereegranskat)abstract
- [1] Polar UV images from a 3-month period in winter 1998-1999 are used for a statistical study of polar arcs. The study covers all auroral arcs that are located poleward of the northern auroral oval, and which are detectable by the UV imager. The arcs are examined with respect to their spatial and temporal behavior as well as to a possible connection to solar wind parameters using ACE satellite data. It is found that the majority of polar arcs appear during northward IMF, strong IMF magnitude, and high solar wind speed. A modified Akasofu-Perreault epsilon parameter with a cosine function instead of a sine function (nuB(2) cos(4) (theta/2)(l(0)(2)/mu(0))) combines these results. It correlates well with the occurrence frequency of polar arcs for long timescales. The location of polar arcs is strongly dependent on the sign of the IMF B-y component. Static polar arcs occur in the Northern Hemisphere on the dawn (dusk) side of the oval for negative (positive) IMF B-y, whereas poleward-moving arcs separate from the opposite side of the oval, and then move in the direction of IMF B-y. All polar arcs are sorted into five different categories according to their spatial structure and evolution: oval-aligned, bending, moving, midnight, and multiple arcs. Each polar arc type occurs for a characteristic combination of solar wind parameters. IMF clock angle changes seem to have a strong influence on what type of arc occurs. Oval-aligned arcs appear mainly during steady IMF, bending arcs after an IMF B-z sign change, and moving arcs after an IMF B-y sign change. For the rare midnight and multiple arc events, no characteristic IMF clock angle dependence has been found. The different types of clear polar arcs are discussed in the context of existing observational studies and transpolar arc models.
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| 7. |
- Lönnqvist, Håkan, et al.
(författare)
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Generation of VLF saucer emissions observed by the Viking satellite
- 1993
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 98, s. 13565-13574
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Tidskriftsartikel (refereegranskat)abstract
- Simultaneous observations by the Viking satellite of electric and magnetic fields as well as charged particles have been used to investigate V-shaped wave phenomena. The intensity of these VLF and ELF emissions is V-shaped when shown in a frequency versus time plot. Simultaneous observations of V-shaped so-called VLF saucer emissions, particles and field-aligned currents strongly suggest, for the first time, that upgoing electrons with energies less than a few hundred electron volts can generate these waves. Broadband waves observed inside the saucer generation region, form frequencies much less than the ion cyclotron frequency up to the plasma frequency, may also be generated by these electrons. Viking observations of VLF saucers at altitudes between 4000 km and 13,500 km show that these emissions occur at higher altitudes tha discussed in previous reports. The generation regions seem to be more extended at these higher altitudes than what has been reported at lower altitudes by other observations.
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| 8. |
- 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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| 9. |
- Marklund, Göran, et al.
(författare)
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On the electrodynamic state of the auroral ionosphere during northward interplanetary magnetic field - a transpolar arc case study
- 1991
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 96:A6, s. 9567-9578
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Tidskriftsartikel (refereegranskat)abstract
- The ionospheric electrodynamical state has been reconstructed for a transpolar arc event during northward interplanetary magnetic field conditions. An extensive set of observations by Viking and other satellites and by ground-based radars has been used to provide realistic model input data or to verify the modeling results. The resulting convection pattern is found to be consistent with the Viking electric field and intimately linked to the prevalent auroral distribution. It is characterized by a large evening cell, well extended across noon and split up by two separated potential minima, and a minor crescent-shaped morning cell. The convection signatures are found to vary a lot along the transpolar arc depending on the relative role of the arc-associated convection and the ambient convection. The transpolar arc is generally embedded in antisunward convective flow except near the connection points with the auroral oval, where sunward flow exists in localized regions.
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| 10. |
- Marklund, Göran, et al.
(författare)
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On the influence of localized electric fields and field-aligned currents associated with polar arcs on the global potential distribution
- 1991
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 96:A8, s. 13977-13983
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Tidskriftsartikel (refereegranskat)abstract
- The influence of localized field-aligned current, associated with intense transpolar arcs mostly occuring during periods of northward interplanetary magnetic field (IMF), on the global electrodynamics has been investigated using a numerical simulation model. Idealized field-aligned current distributions representing both the region 1/2 system of the auroral oval and the transpolar arc as well as a corresponding ionospheric conductivity distribution are fed into the model to calculate the potential distributions. The transpolar arc has been represented by a few alternative field-aligned current distributions which are different in the way the downward return currents are distributed in the ionosphere. For the case with a single, upward current sheet the potential pattern assumes a form similar to that typical for IMF B(y) positive conditions, namely a large dusk cell with sunward drift reaching very high latitudes and a crescent-shaped dawn cell. If the conductivity of the main auroral oval is comparable to that of the polar arc the dusk cell will have two local potential minima and thus a region of weak antisunward convection in between. For the cases with two equal but oppositely directed current sheets the potential patterns are very similar to the symmetrical two-cell reference pattern associated with solely the region 1/2 system with an exception for the immediate vicinity of the theta aurora. Depending on the direction of the polar arc current sheets the dawn-dusk electric field will either be reversed (or weakened) or intensified at the location of the transpolar arc. The presence of a reversal depend, however, not only on the relative magnitude between the polar arc currents and those of the region 1/2 system but also on the characteristics of the acceleration region and of the conductivity distribution associated with the polar arc. Comparisons are made between the model results and Viking electric field data for a number of polar arc crossings to reveal the most common electrodynamical signatures of these auroral phenomena.
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