We investigate the hypothesis that large-scale (≈ 100 km) plasma density enhancements (or “blobs”) found in the auroral F layer become structured via a magnetic-flux-tube interchange (MFTI) process. In such a process, plasma structure is produced when spatially irregular electric fields transport higher number density plasma (from within the blob) into a region containing lower number density plasma (the background ionosphere), and vice versa. Direct experimental evidence of this process can be obtained by measuring concurrently the spatial distributions of F region plasma density and electric field. Using the tristatic EISCAT radar facility, we measured these quantities in a two-dimensional plane transverse to the geomagnetic field, at 300-km altitude. We show, in a case study, that plasma density structure found along the poleward wall of a blob was indeed accompanied by similar-scale variations in the ionospheric electric field, and that the sense of relative motion between high and low number density plasma is consistent with ongoing structuring of the plasma via an MFTI process. From the estimated growth rate of 3×10−3 s−1, the observed plasma structure could have been produced in several minutes by the irregular electric field pattern. The source of the MFTI process, however, is not clear. The MFTI process did not appear to be driven by F region polarization electric fields, a conclusion based on (1) the apparent lack of inverse correlation between plasma density and “slip” velocity (i.e., difference velocity between plasma and neutrals) patterns, and (2) the positive growth rate found along the poleward wall of the blob in the presence of a westward Pedersen current. This conclusion excludes (at least for this data set) the gradient drift and current-convective instabilities as primary sources of the ongoing structuring process.
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