J. A. Cohen, R. Pradipta, L. M. Burton, A. Labno, M. C. Lee, B. J. Watkins, C. Fallen, S. P. Kuo, W. J. Burke, D. Mabius, and B. Z. See. Generation of ionospheric ducts by the HAARP HF heater. Physica Scripta, volume 142,  2010 .