| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- Dahlgren, Hanna, 1980-, et al.
(författare)
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Morphology and dynamics of aurora at fine scale : first results for the ASK instrument
- 2008
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 26:5, s. 1041-1048
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Tidskriftsartikel (refereegranskat)abstract
- The ASK instrument (Auroral Structure and Kinetics) is a narrow field auroral imager, providing simultaneous images of aurora in three different spectral bands at multiple frames per second resolution. The three emission species studied are O-2(+) (5620 angstrom), O+ (7319 angstrom) and O (7774 angstrom). ASK was installed and operated for the first time in an observational campaign on Svalbard, from December 2005 to March 2006. The measurements were supported by data from the Spectrographic Imaging Facility (SIF). The relation between the morphology and dynamics of the visible aurora and its spectral characteristics is studied for selected events from this period. In these events it is found that dynamic aurora is coupled to high energy electron precipitation. By studying the O-2(+)/O intensity ratio we find that some auroral filaments are caused by higher energy precipitation within regions of lower energy precipitation, whereas other filaments are the result of a higher particle flux compared to the surroundings.
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| 6. |
- Dahlgren, Hanna, 1980-, et al.
(författare)
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Using spectral characteristics to interpret auroral imaging in the 731.9 nm 0+ line
- 2008
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 26:7, s. 1905-1917
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Tidskriftsartikel (refereegranskat)abstract
- Simultaneous observations were made of dynamic aurora during substorm activity on 26 January 2006 with three high spatial and temporal resolution instruments: the ASK (Auroral Structure and Kinetics) instrument, SIF (Spectrographic Imaging Facility) and ESR (EISCAT Svalbard Radar), all located on Svalbard (78° N, 16.2° E). One of the narrow field of view ASK cameras is designed to detect O+ ion emission at 731.9 nm. From the spectrographic data we have been able to determine the amount of contaminating N2 and OH emission detected in the same filter. This is of great importance to further studies using the ASK instrument, when the O+ ion emission will be used to detect flows and afterglows in active aurora. The ratio of O+ to N2 emission is dependent on the energy spectra of electron precipitation, and was found to be related to changes in the morphology of the small-scale aurora. The ESR measured height profiles of electron densities, which allowed estimates to be made of the energy spectrum of the precipitation during the events studied with optical data from ASK and SIF. It was found that the higher energy precipitation corresponded to discrete and dynamic features, including curls, and low energy precipitation corresponded to auroral signatures that were dominated by rays. The evolution of these changes on time scales of seconds is of importance to theories of auroral acceleration mechanisms.
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| 7. |
- Ivchenko, Nickolay V., et al.
(författare)
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Multispectral observations of auroral rays and curls
- 2005
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 32:18
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Tidskriftsartikel (refereegranskat)abstract
- Two cases of discrete aurora are presented, in which auroral curls and auroral rays, respectively, were seen. The aurora was imaged by two spatially separated imagers with a long-pass filter ( mainly sensitive to N-2 and N-2(+) emissions), and another imager with a narrow- band 7325 angstrom filter ( sensitive to forbidden O+ doublet). Also, spectra of the aurora were recorded. Using the multispectral imaging and spectra we find that the curls occurred in aurora caused by precipitation of energetic electrons with a lack of low-energy population, while in the rays both high and low energy precipitation were present simultaneously. These findings are confirmed by the altitude determination from triangulation.
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| 8. |
- Jokiaho, O., et al.
(författare)
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Resonance scattering by auroral N-2(+) : steady state theory and observations from Svalbard
- 2009
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 27:9, s. 3465-3478
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Tidskriftsartikel (refereegranskat)abstract
- Studies of auroral energy input at high latitudes often depend on observations of emissions from the first negative band of ionised nitrogen. However, these emissions are affected by solar resonance scattering, which makes photometric and spectrographic measurements difficult to interpret. This work is a statistical study from Longyear-byen, Svalbard, Norway, during the solar minimum between January and March 2007, providing a good coverage in shadow height position and precipitation conditions. The High Throughput Imaging Echelle Spectrograph (HiTIES) measured three bands of N-2(+) 1N (0,1), (1,2) and (2,3), and one N-2 2P band (0,3) in the magnetic zenith. The brightness ratios of the N-2(+) bands are compared with a theoretical treatment with excellent results. Balance equations for all important vibrational levels of the three lowest electronic states of the N-2(+) molecule are solved for steady-state, and the results combined with ion chemistry modelling. Brightnesses of the (0,1), (1,2) and (2,3) bands of N-2(+) 1N are calculated for a range of auroral electron energies, and different values of shadow heights. It is shown that in sunlit aurora, the brightness of the (0,1) band is enhanced, with the scattered contribution increasing with decreasing energy of precipitation (10-fold enhancements for energies of 100 eV). The higher vibrational bands are enhanced even more significantly. In sunlit aurora the observed 1N (1,2)/(0,1) and (2,3)/(0,1) ratios increase as a function of decreasing precipitation energy, as predicted by theory. In non-sunlit aurora the N-2(+) species have a constant proportionality to neutral N-2. The ratio of 2P(0,3)/ 1N(0,1) in the morning hours shows a pronounced decrease, indicating enhancement of N-2(+) 1N emission. Finally we study the relationship of all emissions and their ratios to rotational temperatures. A clear effect is observed on rotational development of the bands. It is possible that greatly enhanced rotational temperatures may be a signature of ion upflows.
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| 9. |
- Jokiaho, O., et al.
(författare)
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Rotational temperature of N-2(+) (0,2) ions from spectrographic measurements used to infer the energy of precipitation in different auroral forms and compared with radar measurements
- 2008
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 26:4, s. 853-866
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Tidskriftsartikel (refereegranskat)abstract
- High resolution spectral data are used to estimate neutral temperatures at auroral heights. The data are from the High Throughput Imaging Echelle Spectrograph (HiTIES) which forms part of the Spectrographic Imaging Facility (SIF), located at Longyearbyen, Svalbard in Norway. The platform also contains photometers and a narrow angle auroral imager. Quantum molecular spectroscopy is used for modelling N-2(+) 1NG (0,2), which serves as a diagnostic tool for neutral temperature and emission height variations. The theoretical spectra are convolved with the instrument function and fitted to measured rotational transition lines as a function of temperature. Measurements were made in the magnetic zenith, and along a meridian slit centred on the magnetic zenith. In the results described, the high spectral resolution of the data (0.08 nm) allows an error analysis to be performed more thoroughly than previous findings, with particular attention paid to the correct subtraction of background, and to precise wavelength calibration. Supporting measurements were made with the Svalbard Eiscat Radar (ESR). Estimates were made from both optical and radar observations of the average energy of precipitating electrons in different types of aurora. These provide confirmation that the spectral results are in agreement with the variations observed in radar profiles. In rayed aurora the neutral temperature was highest (800 K) and the energy lowest (1 keV). In a bright curling arc, the temperature at the lower border was about 550 K, corresponding to energies of 2 keV. The radar and modelling results confirm that these average values are a lower limit for an estimation of the characteristic energy. In each event the energy distribution is clearly made up of more than one spectral shape. This work emphasises the need for high time resolution as well as high spectral resolution. The present work is the first to provide rotational temperatures using a method which pays particular attention to errors in measurement and fitting, and background subtraction.
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| 10. |
- Lanchester, B. S., et al.
(författare)
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Simultaneous imaging of aurora on small scale in OI (777.4 nm) and N(2)1P to estimate energy and flux of precipitation
- 2009
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 27:7, s. 2881-2891
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Tidskriftsartikel (refereegranskat)abstract
- Simultaneous images of the aurora in three emissions, N(2)1P (673.0 nm), OII (732.0 nm) and OI (777.4 nm), have been analysed; the ratio of atomic oxygen to molecular nitrogen has been used to provide estimates of the changes in energy and flux of precipitation within scale sizes of 100 m, and with temporal resolution of 32 frames per second. The choice of filters for the imagers is discussed, with particular emphasis on the choice of the atomic oxygen line at 777.4 nm as one of the three emissions measured. The optical measurements have been combined with radar measurements and compared with the results of an auroral model, hence showing that the ratio of emission rates OI/N2 can be used to estimate the energy within the smallest auroral structures. In the event chosen, measurements were made from mainland Norway, near Tromso, (69.6 N, 19.2 E). The peak energies of precipitation were between 1-15 keV. In a narrow curling arc, it was found that the arc filaments resulted from energies in excess of 10 keV and fluxes of approximately 7mW/m(2). These filaments of the order of 100 m in width were embedded in a region of lower energies (about 5-10 keV) and fluxes of about 3 mW/m(2). The modelling results show that the method promises to be most powerful for detecting low energy precipitation, more prevalent at the higher latitudes of Svalbard where the multispectral imager, known as ASK, is now installed.
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