Co-existing structures from high and low energy precipitation in fine scale aurora

High 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

Observation of O+ 4P-4D0 lines in proton aurora over Svalbard

Spectra of a proton aurora event show lines of O+ 4P-4D0 multiplet (4639–4696 Å) enhanced relative to the N2 +1N(0,2) compared to normal electron aurora. Conjugate satellite particle measurements are used as input to electron and proton transport models, to show that p/H precipitation is the dominant source of both the O+ and N2 +1N emissions. The emission cross-section of the multiplet in p collisions with O and O2 estimated from published work does not explain the observed O+ brightness, suggesting a higher emission cross-section for low energy p impact on O

Multiscale observation of two polar cap arcs occurring on different magnetic field topologies

This paper presents observations of polar cap arc substructure down to scale sizes of meters and temporal resolution of milliseconds. Two case studies containing polar cap arcs occurring over Svalbard are investigated. The first occurred on 4 February 2016 and is consistent with formation on closed field lines; the second occurred on 15 December 2015 and is consistent with formation on open field lines. These events were identified using global‐scale images from the Special Sensor Ultra‐violet Spectrographic Imager (SSUSI) instruments on board Defense Meteorological Satellite Program (DMSP) spacecraft. Intervals when the arcs passed through the small‐scale field of view of the Auroral Structure and Kinetics (ASK) instrument, located on Svalbard, were then found using all sky images from a camera also located on Svalbard. These observations give unprecedented insight into small‐scale polar cap arc structure. The energy and flux of the precipitating particles above these arcs are estimated using the ASK observations in conjunction with the Southampton Ionospheric model. These estimates are then compared to in situ DMSP particle measurements, as well as data from ground‐based instrumentation, to infer further information about their formation mechanisms. This paper finds that polar cap arcs formed on different magnetic field topologies exhibit different behavior at small‐scale sizes, consistent with their respective formation mechanisms

Relation between discrete auroral forms and magnetic field disturbances

Incl. 2 reprints at backSIGLEAvailable from British Library Document Supply Centre- DSC:D66795/86 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Simultaneous imaging of aurora on small scale in OI (777.4 nm) and N21P to estimate energy and flux of precipitation

Simultaneous 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/

Modelling N21P contamination in auroral O+ emissions

Modelling 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

This 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