To date, trough studies have been based upon event studies and statistical analyses focusing solely on electron density, and thus, little is known about the predominant processes associated with trough formation on several time scales. Therefore, we have investigated the associated processes on several time scales, especially seasonal variation, solar activity dependence, and rapid temporal variation on a time scale of minutes. To understand these, we first conducted statistical analyses for the seasonal variation and solar activity dependence of the trough, which were based upon EISCAT UHF radar data collected during 1982–2011. Then, we ran high-speed meridional scans with the EISCAT UHF radar and for the first time observed blob deformation within the trough region during a substorm on December 4, 2013. These two investigations led us to the following conclusions.
Regarding the main results obtained for the seasonal variation of the trough:
1. During geomagnetic quiet to moderate conditions, frictional heating caused by plasma flow is active in the auroral region located in the high-latitude side of the EISCAT FOV, and thus, the occurrence rate of the trough is relatively higher in the auroral region. Especially, during summer, the ratio of frictional heating is ~47%
higher on the high-latitude side. Based on comparisons of the occurrence rate with the ratio of frictional heating, we conclude that dissociative recombination accompanied by frictional heating is the main cause of trough formation in sunlit regions.
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2. The increase in �i is suppressed from winter to summer, most likely owing to frictional heating being suppressed by increased ion drag force on the neutral atmosphere in summer.
Regarding the main results concerning the dependence on solar activity:
3. In the winter, the occurrence rate of the trough decreases down to ~20% within the pre-midnight region, while it increases up to ~80% in the post-midnight region on the high-latitude side. In contrast, the occurrence rate increases up to ~80% within the pre-midnight region, while it decreases down to ~20% in the post-midnight region on the low-latitude side. These variations in the occurrence rate are supposedly influenced by the R2 current in the pre-midnight region and the R1 current in the post-midnight region, respectively. Similar behaviors were observed in the equinox under high solar activity. Accordingly, the effects of FACs become dominant with increasing F10.7 during the equinox.
4. Trough depth and frictional heating were found to increase with F10.7 within the post-midnight to morning and dayside to duskside regions during the equinox, and the midnight to morning regions during summer. These data indicate that the trough becomes deeper via dissociative recombination caused by increased �i with increasing F10.7, at least during the equinox and summer seasons.
The new findings on rapid temporal variation within the trough region are summarized as follows:
5. This is the first time that direct observations of blob deformation during a substorm have been reported. The deformation process of the blob applied to the cusp region
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was found within the trough region. Our analysis indicated that the K–H instability and dissociative recombination accompanied by the enhanced plasma flow could have influenced the blob structure, and then smaller scale irregularities could be secondarily created by the localized G-D instability.
Here, we propose two research topics for the future trough studies. The first important topic is the storm and substorm dependences of the trough. As the ionospheric condition is altered drastically by the development of storms and substorms, remarkable trough characteristics can be expected during these extreme conditions. For such studies, a superposed epoch analysis (SEA) using the EISCAT dataset would be effective. The SEA is a statistical method for the analysis of phenomena that develop according to a regular pattern; thus, it would be suitable for phenomena that have specific evolution phases like storms and substorms.
The second potential topic involves trough research using three-dimensional imaging radar. We introduced a new observational technique using EISCAT meridional scans in Chapter 3, but the results still have some drawbacks in regards to the spatial resolution.
To understand the processes of small-scale structures like irregularities, a spatial resolution of tens of kilometers will be necessary. In addition to this, the present EISCAT observations cannot depict the phenomena three-dimensionally. In this sense, three-dimensional imaging observations with good spatial resolution will be necessary to understand the more detailed physical and chemical processes. Such instruments are now available in Poker Flat, Alaska (65°13'N, 147°47'W) and Resolute Bay, Canada (74°7'N, 265°1'E), which are called Advances Modular Incoherent Scatter Radar (AMISR). As the AMISR can grasp the volumetric images of the ionospheric plasma parameters on a more rapid time scale than that of the EISCAT meridional scans, it would be effective for further detailed investigations of the deformation processes of
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blobs. Furthermore, the construction plan of the EISCAT_3D next-generation phased-array IS radar has been proceeding through the activities of member states of the EISCAT association [Wannberg et al., 2010]. The EISCAT_3D radar is a three-dimensional imaging radar, which is supposed to have ten times higher temporal and spatial resolution than the present EISCAT UHF radar. Thus, new findings from such observations will likely be forthcoming.
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