On the preferential occurrence of interplanetary shocks in July and November: Causes (solar wind annual dependence) and consequences (intense magnetic storms)

摘要

An annual dependence of interplanetary shock rate near 1 AU was found with preferential shock occurrence in July and in November. Although a solar origin for this dependence can not be discarded, it is shown that at least part of this shock distribution can be explained or caused by the annual dependence of solar wind parameters. Such annual dependence in plasma and magnetic field parameters makes the shock formation easier, in the second half year, due to the lower average solar wind bulk and characteristic speeds (Alfvenic and magnetosonic). These asymmetric interplanetary space propagation conditions are probably due to the Earth being exposed to solar wind from different solar magnetic hemispheres in both half years. Furthermore, the peaks in shock distribution are coincident with the peaks in the very intense geomagnetic storms distribution found by Clúa de Gonzalez et al. [2001]. It is suggested that the irregularity in geomagnetic activity is associated with the shock rate annual dependence. These results might imply that solar wind emitted from the opposite solar magnetic hemispheres has slightly different properties.