| 1. |
- Berthomier, M., et al.
(författare)
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Alfven : magnetosphere-ionosphere connection explorers
- 2012
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Ingår i: Experimental astronomy. - Dordrecht : Springer. - 0922-6435 .- 1572-9508. ; 33:2-3, s. 445-489
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Tidskriftsartikel (refereegranskat)abstract
- The aurorae are dynamic, luminous displays that grace the night skies of Earth's high latitude regions. The solar wind emanating from the Sun is their ultimate energy source, but the chain of plasma physical processes leading to auroral displays is complex. The special conditions at the interface between the solar wind-driven magnetosphere and the ionospheric environment at the top of Earth's atmosphere play a central role. In this Auroral Acceleration Region (AAR) persistent electric fields directed along the magnetic field accelerate magnetospheric electrons to the high energies needed to excite luminosity when they hit the atmosphere. The "ideal magnetohydrodynamics" description of space plasmas which is useful in much of the magnetosphere cannot be used to understand the AAR. The AAR has been studied by a small number of single spacecraft missions which revealed an environment rich in wave-particle interactions, plasma turbulence, and nonlinear acceleration processes, acting on a variety of spatio-temporal scales. The pioneering 4-spacecraft Cluster magnetospheric research mission is now fortuitously visiting the AAR, but its particle instruments are too slow to allow resolve many of the key plasma physics phenomena. The Alfv,n concept is designed specifically to take the next step in studying the aurora, by making the crucial high-time resolution, multi-scale measurements in the AAR, needed to address the key science questions of auroral plasma physics. The new knowledge that the mission will produce will find application in studies of the Sun, the processes that accelerate the solar wind and that produce aurora on other planets.
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| 2. |
- Whiter, D. K., et al.
(författare)
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Relative brightness of the O+(2D-2P) doublets in low-energy aurorae
- 2014
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 797:1, s. 64-
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Tidskriftsartikel (refereegranskat)abstract
- The ratio of the emission line doublets from O+ at 732.0 nm (I-732) and 733.0 nm (I-733) has been measured in auroral conditions of low-energy electron precipitation from Svalbard (78.degrees 20 north, 15.degrees 83 east). Accurate determination of R = I-732/I-733 provides a powerful method for separating the density of the O+ P-2(1/2,3/2)o levels in modeling of the emissions from the doublets. A total of 383 spectra were included from the winter of 2003-2004. The value obtained is R = I-732/I-733 = 1.38 +/- 0.02, which is higher than theoretical values for thermal equilibrium in fully ionized plasma, but is lower than reported measurements by other authors in similar auroral conditions. The continuity equations for the densities of the two levels are solved for different conditions, in order to estimate the possible variations of R. The results suggest that the production of ions in the two levels from O(P-3(1)) and O(P-3(2)) does not follow the statistical weights, unlike astrophysical calculations for plasmas in nebulae. The physics of auroral impact ionization may account for this difference, and therefore for the raised value of R. In addition, the auroral solution of the densities of the ions, and thus of the value of R, is sensitive to the temperature of the neutral atmosphere. Although the present work is a statistical study, it shows that it is necessary to determine whether there are significant variations in the ratio resulting from non-equilibrium conditions, from auroral energy deposition, large electric fields, and changes in temperature and composition.
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| 3. |
- Dahlgren, Hanna, et al.
(författare)
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Energy and flux variations across thin auroral arcs
- 2011
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 29:10, s. 1699-1712
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Tidskriftsartikel (refereegranskat)abstract
- Two discrete auroral arc filaments, with widths of less than 1 km, have been analysed using multi-station, multi-monochromatic optical observations from small and medium field-of-view imagers and the EISCAT radar. The energy and flux of the precipitating electrons, volume emission rates and local electric fields in the ionosphere have been determined at high temporal (up to 30 Hz) and spatial (down to tens of metres) resolution. A new time-dependent inversion model is used to derive energy spectra from EISCAT electron density profiles. The energy and flux are also derived independently from optical emissions combined with ion-chemistry modelling, and a good agreement is found. A robust method to obtain detailed 2-D maps of the average energy and number flux of small scale aurora is presented. The arcs are stretched in the north-south direction, and the lowest energies are found on the western, leading edges of the arcs. The large ionospheric electric fields (250 mV m(-1)) found from tristatic radar measurements are evidence of strong currents associated with the region close to the optical arcs. The different data sets indicate that the arcs appear on the boundaries between regions with different average energy of diffuse precipitation, caused by pitch-angle scattering. The two thin arcs on these boundaries are found to be related to an increase in number flux (and thus increased energy flux) without an increase in energy.
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| 4. |
- Whiter, D. K., et al.
(författare)
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Small-scale structures in flickering aurora
- 2008
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 35:23
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Tidskriftsartikel (refereegranskat)abstract
- A state-of-the-art multispectral imager has been used to study the flickering component of an auroral event at high spatial (40 m) and temporal (32 fps) resolution. Scale sizes for the flickering patches were found to be regularly smaller than 1 km. The typical flickering frequency observed was in the range 6-8 Hz, although flickering patches at both lower and higher frequencies were identified. The flickering structure was correlated with the coincident non-flickering aurora, showing that although there is a temporal relationship between the two, there is no spatial correlation. These results support the theory that flickering structure is caused by interfering dispersive Alfven waves.
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| 5. |
- Whiter, D. K., et al.
(författare)
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Using multispectral optical observations to identify the acceleration mechanism responsible for flickering aurora
- 2010
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 115, s. A12315-
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Tidskriftsartikel (refereegranskat)abstract
- We present an analysis of flickering (2-10 Hz) auroras observed with a state-of-the-art multispectral imaging system, Auroral Structure and Kinetics, located in Tromso, Norway. Short (1-2 s) periods of flickering aurora have been identified in which the frequency of the brightness oscillations decreases or increases smoothly over time. To the authors' knowledge this is the first detailed analysis of such "chirps" in flickering aurora or field-aligned bursts. We have found that the electron precipitation energy is strongly anticorrelated with the flickering frequency during all identified chirps. This result is consistent with the theory that flickering aurora is caused by the resonance interaction between electromagnetic ion cyclotron waves and precipitating electrons and that the wave parallel phase velocity is the primary factor determining the electron acceleration produced by this mechanism. Other current theories known to the authors cannot completely explain our observations.
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| 6. |
- Ashrafi, M., et al.
(författare)
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Modelling of N(2)1P emission rates in aurora using various cross sections for excitation
- 2009
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 27:6, s. 2545-2553
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of N(2)1P auroral emissions from the (4,1) and (5,2) bands have been made at high temporal and spatial resolution in the region of the magnetic zenith. The instrument used was the auroral imager ASK, situated at Ramfjordmoen, Norway (69.6 N, 19.2 E) on 22 October 2006. Measurements from the European Incoherent Scatter Radar (EISCAT) have been combined with the optical measurements, and incorporated into an ionospheric model to obtain height profiles of electron density and emission rates of the N(2)1P bands. The radar data provide essential verification that the energy flux used in the model is correct. One of the most important inputs to the model is the cross section for excitation to the B-3 Pi(g) electronic state, as well as the cross sections to higher states from which cascading into the B state occurs. The balance equations for production and loss of the populations of all levels in each state are solved in order to find the cascade contributions. Several sets of cross sections have been considered, and selected cross sections have been used to construct "emission" cross sections for the observed bands. The resulting brightnesses are compared with those measured by ASK. The importance of specific contributions from cascading is found, with more than 50% of the total brightness resulting from cascading. The cross sections used are found to produce a range of brightnesses well within the uncertainty of both the modelled and measured values.
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| 7. |
- Blixt, E. M., et al.
(författare)
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Dynamic rayed aurora and enhanced ion-acoustic radar echoes
- 2005
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 23:1, s. 3-11
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Tidskriftsartikel (refereegranskat)abstract
- The generation mechanism for naturally enhanced ion-acoustic echoes is still debated. One important issue is how these enhancements are related to auroral activity. All events of enhanced ion-acoustic echoes observed simultaneously with the EISCAT Svalbard Radar (ESR) and with high-resolution narrow field-of-view auroral imagers have been collected and studied. Characteristic of all the events is the appearance of very dynamic rayed aurora, and some of the intrinsic features of these auroral displays are identified. Several of these identified features are directly related to the presence of low energy (10-100 eV) precipitating electrons in addition to the higher energy population producing most of the associated light. The low energy contribution is vital for the formation of the enhanced ion-acoustic echoes. We argue that this type of aurora is sufficient for the generation of naturally enhanced ion-acoustic echoes. In one event two imagers were used to observe the auroral rays simultaneously, one from the radar site and one 7 kin away. The data from these imagers shows that the auroral rays and the strong backscattering filaments (where the enhanced echoes are produced) are located on the same field line, which is in contrast to earlier statements in the litterature that they should be separated.
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| 8. |
- Dahlgren, Hanna, et al.
(författare)
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Coexisting structures fromhigh- and low-energy precipitation in fine-scale aurora
- 2015
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 42:5, s. 1290-1296
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution multimonochromatic measurements of auroral emissions have revealed the first optical evidence of coexisting small-scale auroral features resulting from separate high- and low-energy populations of precipitating electrons on the same field line. The features exhibit completely separate motion and morphology. From emission ratios and ion chemistry modeling, the average energy and energy flux of the precipitation is estimated. The high-energy precipitation is found to form large pulsating patches of 0.1Hz with a 3Hz modulation, and nonpulsating coexisting discrete auroral filaments. The low-energy precipitation is observed simultaneously on the same field line as discrete filaments with no pulsation. The simultaneous structures do not interact, and they drift with different speeds in different directions. We suggest that the high- and low-energy electron populations are accelerated by separate mechanisms, at different distances from Earth. The small-scale structures could be caused by local instabilities above the ionosphere.
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| 9. |
- Dahlgren, Hanna, et al.
(författare)
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Electrodynamics and energy characteristics of aurora at high resolution by optical methods
- 2016
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 121:6, s. 5966-5974
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Tidskriftsartikel (refereegranskat)abstract
- Technological advances leading to improved sensitivity of optical detectors have revealed that aurora contains a richness of dynamic and thin filamentary structures, but the source of the structured emissions is not fully understood. In addition, high-resolution radar data have indicated that thin auroral arcs can be correlated with highly varying and large electric fields, but the detailed picture of the electrodynamics of auroral filaments is yet incomplete. The Auroral Structure and Kinetics (ASK) instrument is a state-of-the-art ground-based instrument designed to investigate these smallest auroral features at very high spatial and temporal resolution, by using three electron multiplying CCDs in parallel for three different narrow spectral regions. ASK is specifically designed to utilize a new optical technique to determine the ionospheric electric fields. By imaging the long-lived O+ line at 732 nm, the plasma flow in the region can be traced, and since the plasma motion is controlled by the electric field, the field strength and direction can be estimated at unprecedented resolution. The method is a powerful tool to investigate the detailed electrodynamics and current systems around the thin auroral filaments. The two other ASK cameras provide information on the precipitation by imaging prompt emissions, and the emission brightness ratio of the two emissions, together with ion chemistry modeling, is used to give information on the energy and energy flux of the precipitating electrons. In this paper, we discuss these measuring techniques and give a few examples of how they are used to reveal the nature and source of fine-scale structuring in the aurora.
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| 10. |
- Dahlgren, Hanna, et al.
(författare)
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First direct optical observations of plasma flows using afterglow of O+ in discrete aurora
- 2009
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 71:2, s. 228-238
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Tidskriftsartikel (refereegranskat)abstract
- Imaging of active structured aurora in the forbidden O+ ion line at 732.0 nm provides a possibility of direct observation of plasma drifts in the topside ionosphere. The metastable O+ P-2 state has a radiative lifetime of 5 s, so the oxygen ions can be detected after the precipitation creating them has ceased. The decay time of the O+ emission is studied and modelled with a time-dependent electron transport and ion chemistry model. Four examples are given of O+ afterglow observed with the multi-spectral imager, auroral structure and kinetics (ASK), which was located near Tromso, Norway, in 2006. Estimates are given of drift velocities resulting from the analysis of the afterglow motions. Bulk plasma velocities of 340 and 720 m/s directed eastwards were found for two afterglowing arc filaments, corresponding to southward electric fields of 18 and 40 mV/m, respectively.
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