Physical mechanisms responsible for forming the 4-peak longitudinal structure of the 135.6 nm ionospheric emission: First results from the Canadian IAM

Abstract

The Canadian Ionosphere and Atmosphere Model (Canadian IAM or C-IAM) is a new project aimed at the development of whole atmosphere modeling capabilities in Canada. The first version of the C-IAM is comprised of the extended Canadian Middle Atmosphere (CMAM) and the Murmansk's Upper Atmosphere Model (UAM), currently coupled in a one-way manner. The model extends from the surface to the inner magnetosphere and is able to describe the impact on the upper atmosphere and ionosphere of self-consistently generated lower atmosphere dynamical variability. In the current study, the C-IAM has been used to investigate the physical mechanisms responsible for forming the 4-peak longitudinal structure of the 135.6nm ionospheric emission observed by the IMAGE satellite over the tropics at 20:00 local time from March 20 to April 20, 2002. To perform this study, the C-IAM has been run for this whole observation period taking into account a realistic day-to-day variation in solar and geomagnetic activity. The 4-peak longitudinal structure produced by the model and averaged over the observation period is in a good agreement with the observations. Analysis of the model results suggests that the main mechanism for generating the longitudinal structure in the ionospheric emission after sunset is a modification of the ionospheric electric field in the E region caused in the course of the daytime by differences in the diurnal evolution of the zonal wind in different longitudinal sectors due to waves penetrating from the lower atmosphere. In agreement with the earlier findings, our study showed that this mechanism is driven mainly by the diurnal eastward propagating tide with zonal number 3. A small contribution to the formation of the 4-peak structure is also provided by longitudinal variations in meridional wind and neutral composition at the F2 layer altitudes. The 4 peaks being clearly visible in the monthly mean result are not, however, present every day. Day-to-day variability of both the diurnal wind evolution and geomagnetic activity can significantly modify the longitudinal structure of the ionospheric emission.