Central axial field direction in magnetic clouds and its relation to southward interplanetary magnetic field events and dependence on disappearing solar filaments

Abstract

Extended periods of strong southward interplanetary magnetic field Bs are the primary cause of intense geomagnetic storms. Most such intervals are associated with magnetic clouds in the solar wind. We define a magnetic cloud Bs event as the interval of southward interplanetary magnetic field observed as a magnetic cloud passes by the Earth. For the 26 well‐characterized events studied here, we find that (1) magnetic cloud central axial field directions are almost evenly distributed between −90° and 90° ecliptic latitude, the longitudinal distribution is slightly peaked around the east and west, (2) the duration and intensity of magnetic cloud Bs events correlate linearly with the direction of the cloud's central axial field, and (3) cloud central axial field directions are correlated with the central axial field directions of the associated disappearing filament on the Sun. These findings are useful in predicting from solar observations the duration and intensity of those magnetic cloud Bs events that hit Earth.