Evolution of an ''Ice-void'' in a NLC-display Observed from the Ground

Large, “virtually ice-free” regions have been detected in the Polar Mesospheric Cloud fields by the CIPS instrument on the AIM satellite. These regions have been referred to as “ice voids” (Rusch et al. JASTP, 2009). In the CIPS images ice voids appear as oval shape dark spots with diameters varying from tens to more than hundreds of kilometres but typically about 300 km. It has been hypothesized that they could be caused by heating due to energy deposition through breaking of gravity waves from localized, perhaps tropospheric sources. Due to the way in which the image is sampled by the CIPS instrument only very limited information on the dynamical behaviour on the relevant time scales of these features is, so far, available. Now, for the first time, an ice void (or at least a structure strongly resembling ice voids as identified in the CIPS-images) has been registered by a ground-based camera in a noctilucent cloud (NLC) display north of Stockholm, Sweden on July 4, 2010. The camera takes an image every 30 s and the evolution of the "void", its growth and its motion could be followed during, at least 1 hour. The observed feature is an oval shaped structure (about 200 km x 300 km) centred around 61°N 15°E. The void remained stable for about an hour but finally was filled with weak NLCs. We will provide more detailed analysis of the temporal development of this event. Geo-located, Rayleigh background removed "ice-void" image from a NLC display over Scandinavia on July 4, 2010