B. Gustavsson, R. Newsome, T. B. Leyser, M. J. Kosch, L. Norin, M. McCarrick, T. Pedersen, and B. J. Watkins. First observations of X-mode suppression of O-mode HF enhancements at 6300 A . Geophys. Res. Lett., volume 36, 2009 .