Observations of traveling ionospheric disturbances and 3-m scale irregularities in the nighttime F-region ionosphere with the MU radar and a GPS network

Abstract

The nighttime traveling ionospheric disturbances (TIDs) and the F-region 3-m scale field-aligned irregularities were simultaneously observed with the MU radar and GEONET, a GPS network, during the FRONT (F-region Radio and Optical measurement of the Nighttime TID) campaign periods in May 1998 and August 1999. The vertical profile of electron density detected by the incoherent scatter observation of the MU radar clarified that ionized atmosphere on the bottomside of the ionospheric F-region was deeply modulated by TIDs, which would cause the variations of the 630 nm band airglow luminosity. The coherent echoes from the 3-m scale fieldaligned irregularities were detected also on the bottomside of the F-region in the nights when TIDs were intense in amplitude and the ionosphere was uplifted. Two-dimensional structures of the field-aligned irregularities detected by the multi-beam observation of the MU radar revealed that the 3-m scale irregularities formed band-like structures and traveled to the southwest in several nights. Their wave vector and traveling velocity were coincident with those of the nighttime TIDs that were simultaneously detected by the TEC observation of GEONET. The intense Doppler velocities of the coherent echoes indicate that the polarization electric field is generated inside the TIDs. We consider that the horizontal gradient of the electric conductivity associated by TIDs and the vertical gradient of the conductivity on the bottomside of the F-region ionosphere generates the 3-m scale irregularities through the gradient-drift instability process. The anti-correlation of the occurrence rate of the F-region field-aligned irregularities to the solar activity would be caused by the anti-correlation of the amplitude of TIDs and of the vertical gradient of the Pedersen conductivity.

Imaging observations of nighttime mid-latitude F-region field-aligned irregularities by an MU radar ultra-multi-channel system

“…10. According to Saito et al (2002), in the TEC depletion region (dN e <0), the polarization electric field, E p , would be northeastward and lift up the ionosphere. On the other hand, in the TEC enhancement region (dN e >0), the polarization electric field, E p , would be southwestward.…”

“…Using an MU radar multi-beam experiment without interferometry, Saito et al (2002) reported that echoes tend to appear at the northern slope of MSTIDs. However, our experiment with better spatial resolution showed that these echoes with positive range rate were distributed over the TEC depletion region of the MSTID irrespective of TEC gradients.…”

“…Compared with those in the E-region, such as quasi-periodic (QP) echoes associCorrespondence to: S. Saito (susaito@enri.go.jp) ated with the sporadic-E (E s ) layer , they have not been studied well. Only a few studies in Japan (Fukao et al, 1988Saito et al, 1998Saito et al, , 2002 and in Puerto Rico (Swartz et al, 2000) using radars have reported the mid-latitude F-region FAIs. This is partly because the echoes associated with the mid-latitude F-region FAIs are relatively weak and a powerful radar is required to observe them.…”

“…Nighttime mid-latitude F-region FAIs associated with medium-scale traveling ionospheric disturbances (MSTIDs) with typical wavelengths of 100-500 km have been observed (Saito et al, 1998(Saito et al, , 2002. However, F-region FAIs are less frequently observed than MSTIDs.…”

“…Using the middle-and-upper (MU) atmosphere radar (Fukao et Published by Copernicus Publications on behalf of the European Geosciences Union. al., 1985a, b) multi-beam experiment, Saito et al (2002) reported that echoes tend to appear at the northern slope of MSTIDs. However, it was difficult to see the precise relationship between the MSTIDs and FAIs, nor fine scale structures of FAIs, as their observations were conducted without interferometry.…”