Decameter Structure in Heater-Induced Airglow at the High Frequency Active Auroral Research Program Facility

Abstract

On 28 October 2008, small-scale rayed artificial airglow was observed at the High frequency Active Auroral Research Program (HAARP) heating facility by the HAARP telescopic imager. This airglow occurred during an experiment at twilight from 0255–1600 UT (1855–2000 LT) and with estimated scale sizes of 100 m (at assumed 225 km altitude) constitutes the smallest structure observed in artificial airglow to date. The rays appeared to be oriented along the geomagnetic field lines. During this period, other instruments, SuperDARN, GPS receivers, stimulated electromagnetic emissions receivers, also recorded unusual data sets with the general characteristic of time scales longer than anticipated for features to form. The experiment took place at the commencement of a small geomagnetic disturbance (Kp of 4.3). This unique observation is as yet unexplained. The airglow features start as large scale structures and then become smaller as heating continues in apparent contradiction to current theories on irregularity development. A thermal gradient instability at boundary of the ionospheric footprint of the plasmapause may be responsible for causing the small-scale structuring. Observations of 427.8 nm N₂⁺ (first negative group) emissions indicate the presence of ionization.