During the northern hemisphere winter of 2003--2004 significant levels of stratospheric odd nitrogen (NOX) were observed descending from the mesosphere. Here we study subionospheric radio wave propagation data from Ny ¿lesund, Svalbard, Norway to determine the origin of the mesospheric NOX. A clear change in the radio wave diurnal variation is observed, starting on January 13, 2004, lasting for 37 days. The behavior is consistent with the ionization, by Lyman-α, of thermospheric NOX descending into the mesosphere from altitudes above 90 km. Estimates of the concentration of NOX required to produce the observed ionization changes are consistent with the levels of previously published stratospheric mixing ratios after the NOX has descended into the stratosphere. The radio wave data shows that no significant proton or electron precipitation events into the mesosphere occurred at this time, and the mesospheric effects of the large storms in October/November 2003 had abated by late December 2003.