1,720,988 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
Modelling N21P contamination in auroral O+ emissions
No full textModelling of N21P (5,3) contamination at the O+ doublet emissions in the region of 732 nm is presented. The method is derived from a known relationship between emission from the N21P (5,3) band and emissions from the N21P (5,2), (4,1) and (3,0) bands. A synthetic molecular spectrum is used to quantify a temperature-dependent emission ratio of these band systems as a function of filter characteristics and emission altitude. Five optical observations of high-energy auroral periods on 9 January 2008 are compared with results from the synthetic spectrum. Two cameras from a high sensitivity, high frame rate (20 Hz) ground based imager in combination with a co-located high resolution spectrograph are used to identify events which are dominated by molecular nitrogen emissions. There is good agreement between the observed and modelled ratios. The temperatures associated with these ratios agree well with temperature profiles extracted from fitting the synthetic spectra to the spectrograph data. A synthetic spectrum is important for future work when the removal of N21P (5,3) contamination from O+ (2P) doublet emissions is required at high temporal resolution
Estimating high-energy electron fluxes by intercalibrating Reimei optical and particle measurements using an ionospheric model
No full textThis paper describes a technique for intercalibrating particle and optical measurements from the Reimei microsatellite using an ionospheric model. Reimei has three auroral cameras (“MAC”), together with electron and ion energy spectrum analysers (“ESA/ISA”). The maximum electron energy measured is 12 keV, which means that during high-energy events, the particle data are often missing an important part of the energy flux. Although the total electron energy flux can be estimated from the optical measurements, the MAC data must be accurately calibrated, which is complicated by an unknown and variable background from sources such as the moon and snow reflection. Using unsaturated ESA measurements of the complete electron spectrum as input for an ionospheric model, the coincident camera observations can be calibrated, allowing estimates to be made of the total electron energy flux at other times during the same event, when the maximum energy is well above that measured by ES
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/
Excitation of tall auroral rays by ohmic heating in field-aligned current filaments at F region heights
No full textThe formation of tall red rays in the ionosphere has been a longstanding unresolved problem of auroral physics. These rays are pencil-like structures which can extend from 150 km at their base to as high as 600 km. At these heights it is very difficult to deposit sufficient power in order to account for the luminosity of tall rays. This work examines ohmic heating by collisional processes in strong field-aligned current sheets to account for visible tall rays. The mechanism is demonstrated by two-dimensional simulation in a fully self-consistent treatment of the ionosphere and coupling to the magnetosphere. We find that a filamentary current density of about 600 µAm-2 over about ten seconds can pump sufficient energy into the ambient oxygen atoms to produce visible auroral red rays. The ohmic heating leads to an electron temperature in excess of 10,000 K in the upper F-region
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)
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Observation 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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