Abstract. The shape of the electron energy distribution has long been a central question in the field of highfrequency radio-induced optical emission experiments. This report presents estimates of the electron energy distribution function, f e (E), from 0 to 60 eV, based on optical multiwavelength (6300, 5577, 8446, 4278Å) data and 930-MHz incoherent scatter radar measurements of ion temperature, electron temperature and electron concentration. According to our estimate, the electron energy distribution has a depression at around 2 eV, probably caused by electron excitation of vibrational states in N 2 , and a high energy tail that is clearly supra-thermal. The temporal evolution of the emissions indicates that the electron temperature still plays an important role in providing electrons with energies close to 2 eV. At the higher energies the electron energy distribution has a nonthermal tail.

Abstract. Several explanations have been proposed for Naturally Enhanced ion-acoustic Echoes observed at midand high-latitude Incoherent Scatter observatories. A decisive measure for distinguishing between these explanations is whether or not simultaneously observed up-and downshifted enhancement occur simultaneously, or if they are the result of temporal and/or spatial averaging.The EISCAT Svalbard Radar has two antennas in the same radar system, which can be used as an interferometer when pointed parallel. In observations from 17 January 2002, between 06:46:10 and 06:46:30 UT, we used this possibility, in combination with direct sampling of the received signals, to yield measurements of "naturally enhanced ion-acoustic echoes" with sufficiently high resolution to resolve such averaging, if any. For the first time, radar interferometry has been employed to estimate the sizes of coherent structures. The observations were coordinated with an image intensified video camera with a narrow field of view. Together, this forms the initial study on the causal relationships between enhanced echoes and fine structure in the auroral activity on sub-kilometer, sub-second scales.The results confirm that the enhanced echoes originate from very localised regions (∼300 m perpendicular to the magnetic field at 500 km altitude) with varying range distribution, and with high time variability (≈200 ms). The corresponding increase in scattering cross section, up to 50 dB above incoherent scattering, eliminates theoretical explanations based on marginal stability. The simultaneously observed up-and down-shifted enhanced shoulders, when caused by sufficiently narrow structures to be detected by the interferometer technique, originate predominantly from the same volume. These results have significant impact on theories attempting to explain the enhancements, in particular it is found that the ion-electron two-stream mechanism favoured by many authors is an unlikely candidate to explain the observations. The video data has helped establish a clearCorrespondence to: T. Grydeland (tom.grydeland@phys.uit.no) correlation between the enhanced echoes and auroral activity, on sub-second time scales, showing a threshold connection between the auroral intensity and the triggering of the radar enhancements. It appears that the up-and down-shifted enhanced echoes correlate with fine auroral structures in different ways.

We present evidence of filamented structure in the auroral ionosphere, observed through enhanced radar echoes produced by plasma instabilities in the filaments. Enhancements are observed in up‐ or down‐shifted ion‐acoustic peaks, or both, with power well above thermal levels. Detailed theoretical understanding of these enhancements is still lacking, and several different theories are currently used to explain the observations. Using an interferometric technique, we have measured the horizontal scales of the structures, and their evolution in time, with unprecedented resolution. To explain simultaneous up‐ and down‐shifted enhancements using theories that could only enhance one of these shoulders, spatial and/or temporal averaging has previously been suggested. However, the present observations show that enhancements occur simultaneously and in the same volume. The observed scale size in the plane perpendicular to the magnetic field is comparable to the smaller scale size of optical aurora, which, despite extensive attempts, has not been successfully explained.

Abstract:The Arctic is among the regions with the most rapid changes in climate and has the expected highest increase in temperature. Changes in the timing of phenological phases, such as onset of the growing season observed from remote sensing, are among the most sensitive bio-indicators of climate change. The study area here is the High Arctic archipelago of Svalbard, located between 76°30ʹ and 80°50ʹN. The goal of this study was to use MODIS Terra data (the MOD09Q1 and MOD09A1 surface reflectance products, both with 8-day temporal composites) to map the onset of the growing season on Svalbard for the 2000-2013 period interpreted from field observations. Due to a short and intense period with greening-up and frequent cloud cover, all the cloud free data is needed, which requires reliable cloud masks. We used a combination of three cloud removing methods (State QA values, own algorithms, and manual removal). This worked well, but is time-consuming as it requires manual interpretation of cloud cover. The onset of the growing season was then mapped by a NDVI threshold method, which showed high correlation (r 2 = 0.60, n = 25, p < 0.001) with field observations of flowering of Salix polaris (polar willow). However, large bias was found between NDVI-based mapped onset and field observations in bryophyte-dominated areas, which indicates that the results in these parts must be interpreted with care. On average for the 14-year period, the onset of the growing season occurs after July 1st in 68.

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