Abstract
In this study we investigate periods of enhanced ionization in the mesosphere during Northern Hemisphere wintertime. Long‐lasting ionization enhancements (days) are typically produced by solar proton events or by the descent of thermospheric NOX during periods of sustained downward vertical transport associated with a strong underlying polar vortex. Using a new application of ground‐based low‐frequency radio wave remote sensing, we study the mesospheric ionization conditions during the Northern Hemisphere winters spanning 2003–2004, 2004–2005, and 2005–2006. The winter 2003–2004 subionospheric radio wave propagation data from a transmitter in Iceland shows signatures of the descent of NOX through 80 km altitude starting on 13 January 2004, during the occurrence of a strong polar vortex, indicating a thermospheric source for the NOX. Similar analysis of radio wave propagation data in the Northern Hemisphere winter of 2004–2005 does not show a NOX descent event passing through the mesosphere, due to a lack of downward vertical transport as a result of a weak underlying polar vortex, despite the occurrence of significant solar proton ionization during January 2005. In 2005–2006 there were no significant ionization events and also no descent of significant amounts of thermospheric NOX, despite a strong polar vortex and strong vertical transport. We model the signature of the descent of NOX seen in the radio wave propagation data using the Sodankylä Ion Chemistry model, confirming that the levels of NOX in the mesosphere are ∼100 times the usual background levels. The combination of strong NOX sources in the thermosphere and also a strong polar vortex is required for NOX to descend into the stratosphere with significant concentration levels.