1,721,036 research outputs found
Resonance scattering by auroral N+ 2 : steady state theory and observations from Svalbard
No full textCo-existing structures from high and low energy precipitation in fine scale aurora
No full textHigh resolution multi-monochromatic 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 0.1 Hz with a 3 Hz modulation, and non-pulsating co-existing 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
ASK keograms and videos for study on variations in energy, flux and brightness of pulsating aurora measured at high time resolution
No full textDataset for "Variations in energy, flux, and brightness of pulsating aurora measured at high time resolution", Dahlgren, H., et al., Ann. Geophys., 2017.
Dataset consists of 2 videos from the Auroral Structure and Kinetics (ASK) instrument, operated by the University of Southampton, UK and Royal Institute of Technology, Stockholm. Also included are "keograms" from 22 October 2006 and 19 January 2007. See the included README.txt file for more information.</span
Simultaneous imaging of aurora on small scale in OI (777.4 nm) and N21P to estimate energy and flux of precipitation
No full textSimultaneous images of the aurora in three emissions, N21P (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 Troms\o, (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 7 mW/m2. 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/m2. 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.<br/
Using spectral characteristics to interpret auroral imaging in the 731.9 nm O+ line
No full textSimultaneous 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A spectrographic study of the aurora and the relation to solar wind pressure pulses (Paper presented at 35th COSPAR Scientific Assembly, Paris, France, 18-25 Jul 2004)
No full textObservation of O+ (4P-4D0) lines in electron aurora over Svalbard
No full textThis work reports on observations of O+ lines in aurora over Svalbard, Norway. The Spectrographic Imaging Facility measures auroral spectra in three wavelength intervals (H, N+2 1N(0,2) and N+2 1N(1,3)). The oxygen ion 4P-4D0 multiplet (4639–4696 A° ) is blended with the N+2 1N(1,3) band. It is found that in electron aurora, the brightness of this multiplet, is on average, about 0.1 of the N+2 1N(0,2) total brightness. A joint optical and incoherent scatter radar study of an electron aurora event shows that the ratio is enhanced when the ionisation in the upper E-layer (140–190 km) is significant with respect to the E-layer peak below 130 km. Rayed arcs were observed on one such occasion, whereas on other occasions the auroral intensity was below the threshold of the imager. A one-dimensional electron transport model is used to estimate the cross section for production of the multiplet in electron collisions, yielding 0.18×10-18 cm2
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