Abstract. We present summer Na-densities and atmospheric temperatures measured 80 to 110 km above the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR). The Weber Na Lidar is part of ALOMAR, located at 69 • N in Norway, 150 km north of the Arctic Circle. The sun does not set here during the summer months, and measurements require a narrowband Faraday Anomalous Dispersion Optical Filter (FADOF).We discuss an observed sudden enhancement in the Na number density around 22:00 UT on 1 to 2 June 2006. We compare this observation with previous summer measurements and find a frequent appearance of Na number density enhancements near local midnight. We describe the time of appearance, the altitude distribution, the duration and the strength of these enhancements and compare them to winter observations. We investigate possible formation mechanisms and, as others before, we find a strong link between these Na number density enhancements and sporadic E layers.

Abstract. Several possible mechanisms for the production of sporadic sodium layers have been discussed in the literature, but none of them seem to explain all the accumulated observations. The hypotheses range from direct meteoric input, to energetic electron bombardment on meteoric smoke particles, to ion neutralization, to temperature dependent chemistry. The varied instrumentation located on Andøya and near Tromsø in Norway gives us an opportunity to test the different theories applied to high latitude sporadic sodium layers. We use the ALOMAR Weber sodium lidar to monitor the appearance and characteristics of a sporadic sodium layer that was observed on 5 November 2005. We also monitor the temperature to test the hypotheses regarding a temperature dependent mechanism. The EISCAT Tromsø Dynasonde, the ALOMAR/UiO All-sky camera and the SKiYMET meteor radar on Andøya are used to test the suggested relationships of sporadic sodium layers and sporadic E-layers, electron precipitation, and meteor deposition during this event. We find that more than one candidate is eligible to explain our observation of the sporadic sodium layer.

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Temperature measurements from the ALOMAR Weber Na lidar together with cosmic radio noise absorption measurements from IRIS and particle measurements from NOAA 15, 16 and 17 are used to study effects of geomagnetic activity on the polar winter upper-mesospheric temperature. On 21-22 January 2005 we have 14 h of continuous temperature measurement with the Na lidar coinciding with strong geomagnetic activity in the declining phase of one of the hardest and most energetic Solar Proton Event (SPE) of solar cycle 23. According to measurements by the imaging riometer IRIS in northern Finland, the temperature measurements coincide with two periods of increased cosmic radio noise absorption. Particle measurements from the three satellites, NOAA 15, 16 and 17 that pass through and near our region of interest confirm that the absorption events are probably due to particle precipitation and not due to changes in e.g. the electron recombination coefficient.The measured temperature variation at 85 and 90 km is dominated by a 7.6-h wave with downward phase propagation and a vertical wavelength of approximately 10 km. Assuming that the wave is due to a lower altitude source independent of the particle precipitation, we do not find any temperature modification that seems to be related to the absorption events. The average temperature is larger than expected above 90 km based on MSIS and the monthly mean from falling spheres, which could be due to particle precipitation and Joule heating prior to our measurement period. There is also a possibility that the identified wave phenomenon is an effect of the geomagnetic activity itself. Earlier studies have reported of similar wavelike structures in wind observations made by the EISCAT VHF radar during SPEs, and found it Correspondence to: H. Nesse Tyssøy (hilde.nesse@ift.uib.no) conceivable that the wave could be excited by the effect of energetic particles precipitating into the mesosphere.

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