Local-time variations of geomagnetic disturbances during intense geomagnetic storms and possible association with their interplanetary causes

In the present paper the local-time variations in the disturbance of the geomagnetic-field horizontal component (H) for eight intense geomagnetic storms that occurred during the descending phase of solar cycle 23 have been analyzed. The study was based on the plot of contour lines of the H-depletion intensity in the plane local time versus universal time (LT–UT maps) with the objective of observing how the morphology and evolution of the ring current is mapped into the surface of the Earth in presence of intense geomagnetic storms.The criterion for the selection of the events was a peak Dst below about – 200 nT. The values of the horizontal geomagnetic field were obtained from six mid-latitude stations, with 1 min resolution, and the disturbances were computed by a process similar to that for obtaining the Sym-H geomagnetic index. By means of a cubic spline, the results were interpolated to all longitudes with 6 min in LT spacement. The observed contour maps show, as expected, that the region of largest depletion in H are situated around dusk, as a consequence of the formation of a partial ring current, mainly located in the noon-dusk-midnight hemisphere. However, H disturbances are also observed around midnight and, to a less extend, at the noon-dawn sector.In order to detect a prevalent pattern for the behavior of the geomagnetic-disturbance distribution, a statistical analysis was done by means of occurrence histograms, for different levels of the relative intensity of the storm as a function of local time. The relative intensity for each event was defined as a parameter varying between 0 and 1, with 0 (1) corresponding to the maximum (minimum) horizontal field disturbance during that particular event. Although this analysis does not show the temporal evolution of the disturbance, it confirms the above conclusions about its LT distribution. When only the main phases of the storms are considered in the statistics, the basic differences are the dawn peak is lightly shifted towards noon and that the noon-dawn contribution becomes larger.The observed distributions were tentatively associated with the corresponding interplanetary causes of the events. Since among the eight considered storms four are associated to magnetic cloud with a shock interplanetary structure (sMC) and three to sheath regions followed by a magnetic cloud (SH/MC), the statistical study was also performed for the two subsets of storms separately. For the first group (sMC) the LT distribution looks very similar to that of the whole set of storms. On the other hand, it was observed that for the last group (SH/MC) the peak around midnight was not present. This result might be a consequence that these type of storms are probably not associated to the presence of substorms.