Variation of mesospheric ozone during the highly relativistic electron event in May 1992 as measured by the High Resolution Doppler Imager instrumenton UARS

Highly relativistic electron precipitation events (HREs) include long-lived enhancements of the Aux of electrons with E > 1 MeV into the Earth's atmos phere. HREs also contain increased fluxes of electrons with energies above 100 keV that have been predicted to cause targe depletions of mesospheric o zone. For some of the measured instantaneous values of the electron fluxes during the HRE of May 1992, relative depletions greater than 22% were predi cted to occur between altitudes of 55 and 80 km, where HOx reactions cause local minima in both the ozone number density and mixing ratio altitude pro files. These ozone depictions should follow the horizontal distribution of the electron precipitation, having a distinct boundary equatorward of the L = 3 magnetic shell. To search for these effects, we have analyzed ozone da ta from the High Resolution Doppler Imager (HRDI) instrument on UARS. Owing to the multiple, off-track viewing angles of HRDI, observations in the reg ion affected by the electrons are taken at similar local solar times before , during, and after the electron flux increase. Our analysis limits the rel ative ozone depletion to values < 10% during the very intense May 1992 HRE. We do observe decreases in the ozone mixing ratio at several points in the diurnal cycle that may be associated with the transport of water vapor int o the mesosphere during May 1992. This masking of the precipitating electro n effects by the seasonal variations In water vapor can complicate the dete ction 1 of those effects.