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[en] The responses of the Vorticity Area Index (VAI) at 500 mb to large geomagnetic disturbances and to magnetic sector boundary crossings are evaluated for the periods 1947-57 and 1963-74, during which time the geomagnetic response to sector structure were known to be distinctly different. Results indicate that the nature of the VAI response to geomagnetic disturbances is markedly similar between the two subsets. The response does not vanish when only those geomagnetic disturbances not related to sector boundary passage are used in the analysis, which suggests that enhanced geomagnetic activity can independently influence the lower atmosphere. Unlike the geomagnetic disturbance-related effects, the sector-related effects have varied with time in a very complex manner. In view of this it is concluded that geomagnetic disturbances, whose effects have shown pronounced consistency, may prove a better solar signal in future sun-weather studies. (Auth.)

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[en] Variation processes of the H component of the geomagnetic field are discussed. The processes are classified as SI (sudden impulse) and SC (sudden commencement). Currents causing variations of the H component flow on the magnetopause and in the plasma sheet. Ionospheric currents give the most contribution in SI and SC at high latitudes

[en] The latitude dependence of the changes in the ionosphere total electron content (TEC) has been studied during 12 magnetic storms. TEC observations were obtained at Hamilton, Massachusetts, and Arecibo, Puerto Rico. Definite latitude differences are observed in the TEC responses during the magnetic storms: both TEC enhancements and TEC depletions are observed at Hamilton, while only enhancements are measured at Arecibo. A pre-local midnight TEC 'ledge' is frequently observed in the Arecibo storm data but is seldom observed in the higher-latitude Hamilton data. These enhanced ledges in the Arecibo TEC, together with the nighttime TEC depletions measured at Hamilton, produce large south-to-north differences in the ionosphere TEC. The ledges at Arecibo may arise from electrodynamic drift effects associated with westward electric fields; the role of the motion term in the continuity equation for the TEC is assessed under these conditions. The local evening enhancements in the TEC during certain large storms are discussed in the context of the latitude dependence of the storm time magnetic field variations measured from Great Whale River to San Juan. It is shown that the large TEC enhancements appear to be associated with large positive geomagnetic bays at middle to high latitudes

[en] A study has been designed specifically to investigate the dependence of the ionospheric response on the time of occurrence of sudden commencement (SC) and the intensity of the magnetic storms for a low- and a mid-latitude station by considering total electron content and peak electron density data for more than 60 SC-type geomagnetic storms. The nature of the response, whether positive or negative, is found to be determined largely by the local time of SC, although there is a local time shift of about six hours between low- and mid-latitudes. The time delays associated with the positive responses are low for daytime SCs and high for night-time SCs, whereas the opposite applies for negative responses. The time delays are significantly shorter for mid-latitudes than for low-latitudes and, at both latitudes, are inversely related to the intensity of the storm. There is a positive correlation between the intensity of the ionospheric response and that of the magnetic storm, the correlation being greater at mid-latitudes. The results are discussed in the light of the possible processes which might contribute to the storm-associated ionospheric variations. (author)

[en] In this paper, our study is focused on the FACs behavior during the SC event. SC consists of two events which are storm sudden commencement (SSC) and sudden impulse (SI) event. The difference between these two events is that the interplanetary shock due to SSC leads to geomagnetic storm while during SI event, there is no geomagnetic storm afterward. We performed the statistical analysis between the FACs value and the temporal variation of SYMH index (dSYMH/dt) during SC events from 2008 until 2015. The dSYMH/dt describe the interplanetary shock signature during the SC events. The results showed that the dSYMH/dt during SSC event has a better correlation than the dSYMH/dt during SI events. It suggested that during the SSC onset, there is the magnetic reconnection process between magnetosphere and ionosphere. So, the enhancement of ground magnetic field is contributed by both magnetospheric and ionospheric currents. However, during the SI onset, least connection process happen between magnetosphere and ionosphere thus, the ground magnetic disturbance was mainly associated with the magnetospheric current. (paper)

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