Northeastward motion of nighttime medium‐scale traveling ionospheric disturbances at middle latitudes observed by an airglow imager

Abstract

Nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) observed in 630‐nm airglow images at middle latitudes are known to have a predominantly northwest‐southeast phase surface and to move southwestward in the Northern Hemisphere of Earth. However, the mechanisms of MSTID generation and their systematic southwestward motion have not been clarified. In this paper, we report the “northeastward” motion of the MSTIDs observed at Paratunka, Far East Russia (52.97°N, 158.25°E), using an all‐sky 630‐nm airglow imager at 2000–2300 LT on 19 August 2007. The MSTIDs moved first southwestward but then back northeastward in the northern part of the images. The northeastward motion of the MSTIDs took place coincident with a F layer height decrease observed by an ionosonde at Paratunka. The F layer height decrease was also confirmed by an enhancement of the 630‐nm airglow intensity, which seemed to propagate from northeast to southwest. This fact suggests that the F layer height decrease was caused by poleward wind enhancement rather than westward electric field. These observations imply that the F layer height decrease or the poleward thermospheric wind has some role in the northeastward turning of the MSTID propagation direction.

Medium‐scale traveling ionospheric disturbances observed with the SuperDARN Hokkaido radar, all‐sky imager, and GPS network and their relation to concurrent sporadic E irregularities

“…Meanwhile, we note that the propagation direction of nighttime MSTIDs detected with the Hokkaido radar is not always southwestward. In some cases the propagation direction changes from southwestward to northeastward before midnight: see an example reported by Shiokawa et al [2008], who have pointed out some role of F layer altitude decrease or poleward thermospheric wind in changing the direction.…”

Coordinated observations of nighttime medium‐scale traveling ionospheric disturbances in 630‐nm airglow and HF radar echoes at midlatitudes

“…It should be noted that the lower‐latitude portions of the images are masked by obstacles. Figure 2b shows the same images as those in Figure 2a, the only difference being that the airglow intensity represented in percentage deviations from 1‐h running averages [e.g., Shiokawa et al , 2008]. The MSTID structures in Figure 2b can be recognized with better clarity than those in Figure 2a.…”

“…The all‐sky imager at Paratunka began the automatic airglow imaging observations in August 2007 as part of the Optical Mesosphere Thermosphere Imagers (OMTIs) [ Shiokawa et al , 2008, 2009]. The all‐sky imager has a fish‐eye lens with a 180° FOV and is equipped with a cooled CCD camera with 512 × 512 pixels (operated with 2 × 2 binning, i.e., 256 × 256 pixels).…”

Insights into the nature of northwest‐to‐southeast aligned ionospheric wavefronts from contemporaneous Very Large Array and ionosonde observations

“…The results found by Shiokawa et al [2008] and Helmboldt et al [2012] discussed in section 1 provide motivation to explore how the direction of propagation of these MSTIDs depends on the change in F ‐region height. This was done by first estimating the h′F temporal gradient, ∂h′F/∂t, for each VLSS summer nighttime observation.…”

“…The Cosgrove [2007] simulations also showed that the strength of the coupling was reduced when a significant amount of structure was present within the E s layer. Recent studies based on observations of disturbances over Japan [ Shiokawa et al , 2008] and the southwestern United States [ Helmboldt et al , 2012] have also shown that summer nighttime MSTIDs can propagate toward the northeast, sometimes reversing direction from previously being directed southwestward. These northeastward‐directed waves have been tentatively linked to significant drops in F ‐region height, which may provide a clue to the mechanisms that determine the direction of motion of nighttime, midlatitude MSTIDs.…”