Energy and flux variations across thin auroral arcs

Two discrete auroral arc filaments, with widths of less than 1 km, have been analysed using multi-station, multi-monochromatic optical observations from small and medium field-of-view imagers and the EISCAT radar. The energy and flux of the precipitating electrons, volume emission rates and local electric fields in the ionosphere have been determined at high temporal (up to 30 Hz) and spatial (down to tens of metres) resolution. A new time-dependent inversion model is used to derive energy spectra from EISCAT electron density profiles. The energy and flux are also derived independently from optical emissions combined with ion-chemistry modelling, and a good agreement is found. A robust method to obtain detailed 2-D maps of the average energy and number flux of small scale aurora is presented. The arcs are stretched in the north-south direction, and the lowest energies are found on the western, leading edges of the arcs. The large ionospheric electric fields (250 mV m?1) found from tristatic radar measurements are evidence of strong currents associated with the region close to the optical arcs. The different data sets indicate that the arcs appear on the boundaries between regions with different average energy of diffuse precipitation, caused by pitch-angle scattering. The two thin arcs on these boundaries are found to be related to an increase in number flux (and thus increased energy flux) without an increase in energ

Fine-scale electric fields and Joule heating from observations of the Aurora

Optical measurements from three selected wavelengths have been combined with modelling of emissions from an auroral event to estimate the magnitude and direction of small-scale electric fields on either side of an auroral arc. The temporal resolution of the estimates is 0.1 seconds, which is much higher resolution than measurements from SuperDARN in the same region, with which we compare our estimates. Additionally, we have used the SCANDI instrument to measure the neutral wind during the event in order to calculate the height integrated Joule heating. Joule heating obtained from the small scale electric fields gives larger values (17 ± 11 and 6 ± 9 mWm−2 on average on each side of the arc) than the Joule heating obtained from more conventionally used SuperDARN data (5 mWm−2). This result is significant, because Joule heating will cause changes in the thermosphere from thermal expansion and thermal conductivity, and may also affect the acceleration of the neutral wind. Our result indicates that high spatial and temporal resolution electric fields may play an important role in the dynamics of the magnetosphere-ionosphere-thermosphere system

Results from the intercalibration of optical low light calibration sources 2011

Following the 38th Annual European Meeting on Atmospheric Studies by Optical Methods in Siuntio in Finland, an intercalibration workshop for optical low light calibration sources was held in Sodankylä, Finland. The main purpose of this workshop was to provide a comparable scale for absolute measurements of aurora and airglow. All sources brought to the intercalibration workshop were compared to the Fritz Peak reference source using the Lindau Calibration Photometer built by Wilhelm Barke and Hans Lauche in 1984. The results were compared to several earlier intercalibration workshops. It was found that most sources were fairly stable over time, with errors in the range of 5–25%. To further validate the results, two sources were also intercalibrated at UNIS, Longyearbyen, Svalbard. Preliminary analysis indicates agreement with the intercalibration in Sodankylä within about 15–25%

Timescales of Birkeland Currents driven by the IMF

We obtain current densities from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), alongside By and Bz from the Interplanetary Magnetic Field (IMF) for March 2010. For each AMPERE spatial coordinate, we cross‐correlate current density with By and Bz, finding the maximum correlation for lags up to 360 min. The patterns of maximum correlation contain large‐scale structures consistent with the literature. For the correlation with By, the lags on the dayside are 10 min at high latitudes but up to 240 min at lower latitudes. Lags on the nightside are 90–150 min. For Bz, the shortest lags on the dayside are 10–20 min; on the equatorward edge of the current oval, 60–90 min; and on the nightside, predominantly 90–150 min. This novel approach enables us to see statistically the timescales on which information is electrodynamically communicated to the ionosphere after magnetic field lines reconnect on the dayside and nightside

Effect of water vapour absorption on hydroxyl temperatures measured from Svalbard

We model absorption by atmospheric water vapour of hydroxyl airglow emission using the HIgh-resolution TRANsmission molecular absorption database (HITRAN2012). Transmission coefficients are provided as a function of water vapour column density for the strongest OH Meinel emission lines in the (8–3), (5–1), (9–4), (8–4), and (6–2) vibrational bands. These coefficients are used to determine precise OH(8–3) rotational temperatures from spectra measured by the High Throughput Imaging Echelle Spectrograph (HiTIES), installed at the Kjell Henriksen Observatory (KHO), Svalbard. The method described in this paper also allows us to estimate atmospheric water vapour content using the HiTIES instrument

A new automatic method for estimating the peak auroral emission height from all-sky camera images

This paper presents a new fully automatic method for quickly finding the average peak emission height of a single auroral structure from a pair of all-sky camera images with overlapping fields of view. The peak emission height of the aurora must be estimated in order to calculate several other important parameters, such as horizontal spatial scales, optical flow velocities, and ionospheric electric fields. In most cases the height is not measured, but a value is assumed, often about 110 km. It is unclear how accurate this assumption is. A future statistical study of the auroral height in which the method presented here will be applied to many years of observations will lead to more accurate assumptions of the height with quantitative error estimates, and therefore more accurate estimates of parameters derived using these assumed auroral heights. In the present work the performance of the new method is compared to another recent automatic method. It is found that the new method measures the peak emission height regardless of the shape of the volume emission rate profile, unlike the other recent method. However, the new method is less suitable than the other method for analysis of very wide auroral arcs (>30 km) or for aurora in the magnetic zenith of one of the images