| 1. |
- Brändström, U., et al.
(författare)
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Results from the intercalibration of optical low light calibration sources 2011
- 2012
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Ingår i: Geoscientific Instrumentation, Methods and Data Systems. - : Copernicus GmbH. - 2193-0856 .- 2193-0864. ; 1:1, s. 43-51
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Tidskriftsartikel (refereegranskat)abstract
- Following the 38th Annual European Meeting onAtmospheric Studies by Optical Methods in Siuntio in Finland,an intercalibration workshop for optical low light calibrationsources was held in Sodankyl¨a, Finland. The mainpurpose of this workshop was to provide a comparable scalefor absolute measurements of aurora and airglow. All sourcesbrought to the intercalibration workshop were compared tothe Fritz Peak reference source using the Lindau CalibrationPhotometer built by Wilhelm Barke and Hans Lauche in1984. The results were compared to several earlier intercalibrationworkshops. It was found that most sources were fairlystable over time, with errors in the range of 5–25 %. To furthervalidate the results, two sources were also intercalibratedat UNIS, Longyearbyen, Svalbard. Preliminary analysis indicatesagreement with the intercalibration in Sodankyl¨a withinabout 15–25 %.
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| 2. |
- Frey, H. U., et al.
(författare)
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Small and meso-scale properties of a substorm onset auroral arc
- 2010
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 115, s. A10209-
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Tidskriftsartikel (refereegranskat)abstract
- We present small and meso-scale properties of a substorm onset arc observed simultaneously by the Reimei and THEMIS satellites together with ground-based observations by the THEMIS GBO system. The optical observations revealed the slow equatorward motion of the growth-phase arc and the development of a much brighter onset arc poleward of it. Both arcs showed the typical particle signature of electrostatic acceleration in an inverted-V structure together with a strong Alfven wave acceleration signature at the poleward edge of the onset arc. Two THEMIS spacecraft encountered earthward flow bursts around the times the expanding optical aurora reached their magnetic footprints in the ionosphere. The particle and field measurements allowed for the reconstruction of the field-aligned current system and the determination of plasma properties in the auroral source region. Auroral arc properties were extracted from the optical and particle measurements and were used to compare measured values to theoretical predictions of the electrodynamic model for the generation of auroral arcs. Good agreement could be reached for the meso-scale arc properties. A qualitative analysis of the internal structuring of the bright onset arc suggests the operation of the tearing instability which provides a 'rope-like' appearance due to advection of the current in the sheared flow across the arc. We also note that for the observed parameters ionospheric conductivity gradients due to electron precipitation will be unstable to the feedback instability in the ionospheric Alfven resonator that can drive structuring in luminosity over the range of scales observed.
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| 3. |
- 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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