A. S. Yukimatu

Superposed epoch analysis of the ionospheric convection evolution during substorms: IMF BY dependence

We present superposed epoch analyses of the average ionospheric convection response in the northern and southern hemispheres to magnetospheric substorms occurring under different orientations of the interplanetary magnetic field (IMF). Observations of the ionospheric convection were provided by the Super Dual Auroral Radar Network (SuperDARN) and substorms were identified using the Far Ultraviolet (FUV) instrument on board the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft. We find that during the substorm growth phase the expected IMF BY‐dependent dawn‐dusk asymmetry is observed over the entire convection pattern, but that during the expansion phase this asymmetry is retained only in the polar cap and dayside auroral zone. In the nightside auroral zone the convection is reordered according to the local substorm electrodynamics with any remaining dusk‐dawn asymmetry being more closely related to the magnetic local time of substorm onset, itself only weakly governed by IMF BY. Owing to the preponderance of substorms occurring just prior to magnetic midnight, the substorm‐asymmetry tends to be an azimuthal extension of the dusk convection cell across the midnight sector, a manifestation of the so‐called “Harang discontinuity.” This results in the northern (southern) hemisphere nightside auroral convection during substorms generally resembling the expected pattern for negative (positive) IMF BY. When the preexisting convection pattern in the northern (southern) hemisphere is driven by positive (negative) IMF BY, the nightside auroral convection changes markedly over the course of the substorm to establish this same “Harang” configuration.

Antarctic HF radar observations of irregularities associated with polar patches and auroral blobs: A case study

We report a case study of decameter-scale electron density irregularities associated with polar cap patches and auroral (boundary) blobs in the southern high-latitude F region ionosphere. The observations were carried out on July 14, 1995, with the Antarctic Super Dual Auroral Radar Network HF radars located at Syowa Station and Halley. On that day, 17 irregularity events associated with the patches were identified in the polar cap. The time distribution of these events is consistent with previous model calculations of patch formation and transportation in the northern hemisphere for southward interplanetary magnetic field (IMF) conditions (Bz 0). These patches seem to have been transported into the polar cap from the dayside cusp where the patches had been generated under negative Bz conditions. The striated radar echo patterns due to a series of auroral blobs, clearly observed at Halley in the evening auroral zone, are well explained by previous simulations that calculated the time evolution and transportation of a patch initially located in the polar cap.

Detailed analysis of a substorm event on 6 and 7 June 1989: 2. Stepwise auroral bulge evolution during expansion phase

[1] We have analyzed in detail the auroral bulge evolution during the expansion phase of an isolated substorm, which was observed by the UV imager aboard the Akebono satellite. It was found that there were three distinct stages in the evolution. Stage 1 was characterized by rapid poleward and azimuthal ( predominantly westward) expansions in a short time (about 2 min). Stage 2 was characterized by a very slow poleward and slower and continuous azimuthal expansions. There was a certain period for transition between stage 1 and stage 2, and it was characterized by a very slow poleward and rapid eastward expansions. Stage 3 started about 11 min after the onset and was characterized by a sudden reactivation of the rapid poleward and azimuthal expansions. The reactivation started around the initial onset meridian and then spread both eastward and westward. At the azimuthal front, the expansion first occurred at the lowest latitudes, spread poleward to around the highest latitudes of stage 1, and then spread further poleward after a brief interval. Hence, the local expansion also had three distinct stages similar to the global one. The ground-based observations showed that the highest latitude of the local first stage was very close to the latitude of auroral activity that appeared near the ionospheric plasma sheet boundary layer (PSBL) region a few minutes before the onset. The further poleward expansion during the local third stage started with a significant intensification of the poleward-most auroral activity. During the local third stage, the bright electron auroral region was bifurcated into a poleward expanding part and an equatorward moving part. The proton auroral emission coexisted in the bulge during the local first and second stages and almost disappeared soon after the bifurcation during the local third stage. Based on these observations, we discuss the evolution in the magnetosphere during the expansion phase.