High resolution ground magnetometer data are used in a case study to investigate three effects : (i) the large-scale ionospheric convection pattern appropriate to the two polarities of magnetosheath By; (ii) the temporal evolution of the ionospheric flow from one pattern to another following a sudden switch in the polarity of By; (iii) the imprint on the ionosphere of reconnection processes at the dayside magnctopause, both of the "quasi-steady" (QSR) and the flux transfer event (FTE) variety. The results obtained on these topics are as follows, (i) Asymmetries in the ionospheric convection pattern are observed, and these are different for the two polarities of By. These asymmetries are in qualitative agreement with the results obtained earlier from statistical studies, (ii) We report for the first time observations which show the ionospheric flows to evolve smoothly from one asymmetrical pattern to another, triggered by a single. large, sudden change in the polarity of the magnetosheath By. The flow change is estimated to commence (1.9 ± 0.7) min after the signal reaches ionospheric heights. The time scale associated with the reorientation of the whole ionospheric flow pattern is found to be 15-18 min, which we ascribe to the time for the ionospheric end of a reconnected flux tube to lose its memory of the Bysense under which it was reconnected. (iii) During the time when reconnection was observed by the ISEE spacecraft at the magnetopause (both QSR and FTEs), large ionospheric flows were present at high-latitude stations. Superposed on this large background flow clear and substantial (∼ 20-40% of the total disturbance) fluctuations were recorded at the cusp region stations. These increased and variable flows may constitute the ionospheric imprint of reconnection processes occurring at the higher altitudes, though another plausible explanation is that the transient flow fluctuations may have been caused by solar wind pressure changes. © 1990.
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