Virginia M. Silbergleit

Annual variation of geomagnetic activity

Abstract The annual variation of geomagnetic activity is studied through the geomagnetic indices aa, Dst and AE, according to different levels of intensity for each of the indices. For thresholds that correspond from moderate to fairly intense storms (i.e., peak aa⩾ 90), the distribution follows the well-known pattern of a seasonal variation, with maxima around the equinoxes and minima near the solstices. Deviations from this behavior are observed when the distribution refers to levels associated with the occurrence of more intense storms. In particular, the annual distribution of days with a geomagnetic index aa greater than about 90, shows the occurrence of a peak in July. The contribution of very intense storms (aa⩾ 210) to the July peak, seems to be evenly distributed along the 12 solar cycles covered by this index. Furthermore, the indices Dst and AE, although restricted to a much shorter interval of time (they have been recorded only since 1957), seem as well to show the existence of a peak of occurrence for July. The study done for the indices Dst and AE gives some indication for the existence of another peak in November, also for thresholds associated with intense storms. However, due to the lack of longer records for these indices, the real existence of this peak in the geomagnetic activity is questionable. A statistical analysis of the distribution of events according to the levels of intensity of the aa and Dst is also presented. From this analysis it is seen that the number of occurrences of storms above a given level of intensity of those geomagnetic indices, can be approximated by an exponential law. Furthermore, an estimation of the occurrence of storms during a solar cycle as a function of the peak aa (or aa ∗ ) has been also done.

Energy dissipation in substorms: plasmoids ejection

Abstract Intense solar events in March (1989) caused important effects in the Earths magnetosphere. This is demonstrated by the temporal evolution of the auroral electrojet index (AE), the mid-latitude geomagnetic index (SYM-H), the solar wind coupling parameter (ϵ) and the total energy rate dissipated by the magnetosphere ( U T ) for the substorm detected on 5 March (1989). For this event the energy dissipated by plasmoids ejection (PE) in the antisolar direction is estimated using a new approach.

On the occurrence of the largest geomagnetic storms per solar cycle

Abstract The predictive method of Gumbels first asymptotic distribution is used to estimate modal extreme values and return intervals related to the largest geomagnetic storms, characterized by the Dst index, which occurred in the last four solar cycles. A repeat interval of seven solar cycles has been obtained for storms with maximum ¦Dst¦ index values of approximately 600 nT (similar to the extremely large event of 14 March 1989).

Probable Value for the Next Sunspot Minimum

Gumbel’s first distribution is applied to smoothed monthly mean sunspot numbers for solar cycles 10 to 24. According to that, the next minimum for solar cycle 24-25 transition would be the deepest solar minimum of the last 150 years. This study provides an additional insight about changes in the Sun which are currently happening.