New aspects of mid-latitude plasma plumes revealed by
radio and optical observations
Shoichiro Fukao and Mamoru Yamamoto
Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Kyoto, Japan
The mid-latitude ionosphere sometimes shows rapid and turbulent upwelling during sunspot minimum condition, which is called plasma plume after its shape. The plume is characterized by intense coherent radar echoes from 3-m scale field-aligned irregularities (FAIs). Simultaneous coherent and incoherent scatter observations by the MU radar indicates that the FAIs are generated on the bottom side of the F region. The traveling ionospheric disturbances (TIDs) deeply modulate the bottom side of the ionosphere, which may vary the 630 nm band airglow luminosity. The FAIs are detected in the nights when TIDs are intense in amplitude and ionosphere is uplifted. The FAIs form band-like structures that travel to the southwest. Their wavefront and travelling speed are generally consistent with those of the TIDs that are simultaneously detected by the TEC observation. Also, TIDs are associated with perturbation of electric filed in the ionosphere (A. Saito et al., 1995, 1998; Shiokawa et al., 2003). These results suggest that the horizontal gradient of the electric conductivity associated by TIDs and the vertical gradient of the conductivity on the bottom side of the F region ionosphere generates the 3-m scale FAIs through the gradient-drift instability process (A. Saito et al., 2002).
The possible relationship between the F-region and E-region FAIs was suggested early by Kelley and Fukao (1991). S. Saito et al. (2007) shows that band structures of E-region irregularities aligned northwest to southwest drift to the southwest, and their wavefront and propagation direction are the same as those of medium-scale TIDs (MSTIDs) in the F region. Yamamoto et al. (2007) simultaneously observed FAIs by radar imaging technique along the same geomagnetic field lines in the F region with the MU radar and in the E region with a portable coherent radar (F- and E-Region Ionospheric Coupling Study or FERIX campaigns). While the northwest-southeast wavefront with small substructures existed in the F region, quite similar structure was formed in the E region too, and both propagate to the southwest or west. The substructure as small as a few tens of km in the F region propagated to the northwest with various Doppler speeds. This indicates that the F and E regions in the nighttime mid-latitude ionosphere must be considered electrodynamically as a coupled system.
It has been recently found by airglow imager observations at the conjugate locations at mid-latitudes that MSTIDs show an excellent conjugacy with identical structures even to smallest scales between both hemispheres (Otsuka et al., 2004). This may suggest that the E- and F-region coupled system be considered in more global scale.