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We present summer Na-densities and atmospheric temperatures measured 80 to110 km above the Arctic Lidar Observatory for Middle Atmosphere Research(ALOMAR). The Weber Na Lidar is part of ALOMAR, located at 69° N inNorway, 150 km north of the Arctic Circle. The sun does not set here duringthe summer months, and measurements require a narrowband Faraday AnomalousDispersion Optical Filter (FADOF).  We discuss an observed sudden enhancement in the Na number density around 22:00 UTon 1 to 2 June 2006. We compare this observation with previous summermeasurements and find a frequent appearance of Na number densityenhancements near local midnight. We describe the time of appearance, thealtitude distribution, the duration and the strength of these enhancementsand compare them to winter observations. We investigate possible formationmechanisms and, as others before, we find a strong link between these Nanumber density enhancements and sporadic E layers

Summer sudden Na number density enhancements measured with the ALOMAR Weber Na Lidar