| 1. |
- Figueiredo, Sonia, et al.
(författare)
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Temporal and spatial evolution of discrete auroral arcs as seen by Cluster
- 2005
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 23:7, s. 2531-2557
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Tidskriftsartikel (refereegranskat)abstract
- Two event studies are presented in this paper where intense convergent electric fields, with mapped intensities up to 1350 mV/m, are measured in the auroral upward current region by the Cluster spacecraft, at altitudes between 3 and 5 Earth radii. Both events are from May 2003, Southern Hemisphere, with equatorward crossings by the Cluster spacecraft of the pre-midnight auroral oval. Event 1 occurs during the end of the recovery phase of a strong substorm. A system of auroral arcs associated with convergent electric field structures, with a maximum perpendicular potential drop of about similar to 10 kV, and upflowing field-aligned currents with densities of 3 mu A/m(2) (mapped to the ionosphere), was detected at the boundary between the Plasma Sheet Boundary Layer (PSBL) and the Plasma Sheet (PS). The auroral arc structures evolve in shape and in magnitude on a timescale of tens of minutes, merging, broadening and intensifying, until finally fading away after about 50 min. Throughout this time, both the PS region and the auroral arc structure in its poleward part remain relatively fixed in space, reflecting the rather quiet auroral conditions during the end of the substorm. The auroral upward acceleration region is shown for this event to extend beyond 3.9 Earth radii altitude. Event 2 occurs during a more active period associated with the expansion phase of a moderate substorm. Images from the Defense Meteorological Satellite Program (DMSP) F13 spacecraft show that the Cluster spacecraft crossed the horn region of a surge-type aurora. Conjugated with the Cluster spacecraft crossing above the surge horn, the South Pole All Sky Imager recorded the motion and the temporal evolution of an east-west aligned auroral arc, 30 to 50 km wide. Intense electric field variations are measured by the Cluster spacecraft when crossing above the auroral arc structure, collocated with the density gradient at the PS poleward boundary, and coupled to intense upflowing field-aligned currents with mapped densities of up to 20 mu A/m(2). The surge horn consists of multiple arc structures which later merge into one structure and intensify at the PS poleward boundary. The surge horn and the associated PS region moved poleward with a velocity at the ionospheric level of 0.5 km/s, following the large-scale poleward expansion of the auroral oval associated with the substorm expansion phase.
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| 2. |
- Johansson, Tommy, et al.
(författare)
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A statistical study of intense electric fields at 4-7 R-E geocentric distance using Cluster
- 2005
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 23:7, s. 2579-2588
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Tidskriftsartikel (refereegranskat)abstract
- Intense high-latitude electric fields (> 150 mV/m mapped to ionospheric altitude) at 4-7 R-E geocentric distance have been investigated in a statistical study, using data from the Cluster satellites. The orbit of the Cluster satellites limits the data collection at these altitudes to high latitudes, including the poleward part of the auroral oval. The occurrence and distribution of the selected events have been used to characterize the intense electric fields and to investigate their dependance on parameters such as MLT, CGLat, altitude, and also K-p. Peaks in the local time distribution are found in the evening to morning sectors but also in the noon sector, corresponding to cusp events. The electric field intensities decrease with increasing latitude in the region investigated (above 60 CGLat). A dependence on geomagnetic activity is indicated since the probability of finding an event increases up to K-p=5-6. The scales sizes are in the range up to 10 km (mapped to ionospheric altitude) with a maximum around 4-5 km, consistent with earlier findings at lower altitudes and Cluster event studies. The magnitudes of the electric fields are inversely proportional to the scale sizes. The type of electric field structure (convergent or divergent) is consistent with the FAC direction for a subset of events with electric field intensities in the range 500-1000 mV/m and with clear bipolar signatures. The FAC directions are also consistent with the Region 1 and NBZ current systems, the latter of which prevail only during northward IMF conditions. For scale sizes less than 2 km the majority of the events were divergent electric field structures. Both converging and diverging electric fields were found throughout the investigated altitude range (4-7 RE geocentric distance).
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| 3. |
- Johansson, Tommy, et al.
(författare)
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Intense high-altitude auroral electric fields : temporal and spatial characteristics
- 2004
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 22:7, s. 2485-2495
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Tidskriftsartikel (refereegranskat)abstract
- Cluster electric field, magnetic field, and energetic electron data are analyzed for two events of intense auroral electric field variations, both encountered in the Plasma Sheet Boundary Layer (PSBL), in the evening local time sector, and at approximately 5 R-E geocentric distance. The most intense electric fields (peaking at 450 and 1600 mV/m, respectively) were found to be quasi-static, unipolar, relatively stable on the time scale of at least half a minute, and associated with moving downward FAC sheets (peaking at similar to10 muA/m(2)), downward Poynting flux (peaking at similar to35 mW/m(2)), and upward electron beams with characteristic energies consistent with the perpendicular potentials (all values being mapped to 1 R-E geocentric distance). For these two events in the return current region, quasi-static electric field structures and associated FACs were found to dominate the upward acceleration of electrons, as well as the energy transport between the ionosphere and the magnetosphere, although Alfven waves clearly also contributed to these processes.
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| 4. |
- Johansson, Tommy, et al.
(författare)
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On the profile of intense high-altitude auroral electric fields at magnetospheric boundaries
- 2006
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 24:6, s. 1713-1723
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Tidskriftsartikel (refereegranskat)abstract
- The profile of intense high-altitude electric fields on auroral field lines has been studied using Cluster data. A total of 41 events with mapped electric field magnitudes in the range between 0.5-1 V/m were examined, 27 of which were co-located with a plasma boundary, defined by gradients in particle flux, plasma density and plasma temperature. Monopolar electric field profiles were observed in 11 and bipolar electric field profiles in 16 of these boundary-associated electric field events. Of the monopolar fields, all but one occurred at the polar cap boundary in the late evening and midnight sectors, and the electric fields were typically directed equatorward, whereas the bipolar fields all occurred at plasma boundaries clearly within the plasma sheet. These results support the prediction by Marklund et al. (2004), that the electric field profile depends on whether plasma populations, able to support intense field-aligned currents and closure by Pedersen currents, exist on both sides, or one side only, of the boundary.
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| 5. |
- Johansson, Tommy, et al.
(författare)
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Scale sizes of intense auroral electric fields observed by Cluster
- 2007
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 25:11, s. 2413-2425
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Tidskriftsartikel (refereegranskat)abstract
- The scale sizes of intense (>0.15 V/m, mapped to the ionosphere), high-altitude (4-7 R-E geocentric distance) auroral electric fields (measured by the Cluster EFW instrument) have been determined in a statistical study. Monopolar and bipolar electric fields, and converging and diverging events, are separated. The relations between the scale size, the intensity and the potential variation are investigated. The electric field scale sizes are further compared with the scale sizes and widths of the associated field-aligned currents (FACs). The influence of, or relation between, other parameters (proton gyroradius, plasma density gradients, and geomagnetic activity), and the electric field scale sizes are considered. The median scale sizes of these auroral electric field structures are found to be similar to the median scale sizes of the associated FACs and the density gradients (all in the range 4.2-.9 km) but not to the median proton gyroradius or the proton inertial scale length at these times and locations (22-30km). (The scales are mapped to the ionospheric altitude for reference.) The electric field scale sizes during summer months and high geomagnetic activity (K-p>3) are typically 2-3 km, smaller than the typical 4-5 km scale sizes during winter months and low geomagnetic activity (K-p <= 3), indicating a dependence on ionospheric conductivity.
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| 6. |
- Karlsson, Tomas, et al.
(författare)
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Separating spatial and temporal variations in auroral electric and magnetic fields by Cluster multipoint measurements
- 2004
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 22:7, s. 2463-2472
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Tidskriftsartikel (refereegranskat)abstract
- Cluster multipoint measurements of the electric and magnetic fields from a crossing of auroral field lines at an attitude of 4 R-E are used to show that it is possible to resolve the ambiguity of temporal versus spatial variations in the fields. We show that the largest electric fields (of the order of 300 mV/m when mapped down to the ionosphere) are of a quasi-static nature, unipolar, associated with upward electron beams, stable on a time scale of at least half a minute, and located in two regions of downward current. We conclude that they are the high-altitude analogues of the intense return current/black auroral electric field structures observed at lower altitudes by Freja and FAST. In between these structures there are temporal fluctuations, which are shown to likely be downward travelling Alfven waves. The periods of these waves are 20-40 s, which is not consistent with periods associated with either the Alfvenic ionospheric resonator, typical field line resonances or substorm onset related Pi2 oscillations. The multipoint measurements enable us to estimate a lower limit to the perpendicular wavelength of the Alfven waves to be of the order of 120 kin, which suggests that the perpendicular wavelength is similar to the dimension of the region between the two quasi-static structures. This might indicate that the Alfven waves are ducted within a wave guide, where the quasi-static structures are associated with the gradients making up this waveguide.
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| 7. |
- Liléo, Sonia, et al.
(författare)
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Magnetosphere-ionosphere coupling during periods of extended high auroral activity : A case study
- 2008
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 26, s. 583-591
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Tidskriftsartikel (refereegranskat)abstract
- Results are presented from a case study of a plasma boundary crossing by the Cluster spacecraft during an extended period of high auroral activity. The boundary between the magnetotail lobe region of the Southern Hemisphere and the plasma sheet boundary layer, was characterized by intense electric and magnetic field variations, structured upward accelerated ion beams, narrow-scale large field-aligned Poynting fluxes directed upward away from the ionosphere, and a relatively sharp plasma density gradient. The observations are shown to be consistent with the concept of a multi-layered boundary with temporal and/or spatial variations in the different layers. H+ and O+ ion beams are seen to be accelerated upwards both by means of a field-aligned electric field and by magnetic pumping caused by large-amplitude and low-frequency electric field fluctuations. The peak energy of the ion beams may here be used as a diagnostic tool for the temporal evolution of the spatial structures, since the temporal changes occur on a time-scale shorter than the times-of-flight of the detected ion species. The case study also shows the boundary region to be mainly characterized by a coupling of the detected potential structures to the low ionosphere during the extended period of high auroral activity, as indicated by the intense field-aligned Poynting fluxes directed upward away from the ionosphere.
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