On the azimuthal evolution and geoeffectiveness of the SIR‐associated stream interface

Abstract

AbstractIn this study, the azimuthal evolution of stream interaction regions is investigated, with the goal of predicting the time of arrival of an interface at some later position near 1 AU. A new stream interaction region (SIR) data set is constructed from ACE, STEREO A, and STEREO B in situ measurements, and it is demonstrated that the magnetic pressure and azimuthal flow angle provide the simplest robust estimation of the interface time. This data set was applied in the investigation. In the analysis, the geometric effects of the magnetic spiral angle and the tilt angle of stream interfaces are considered, and it is demonstrated how they may be used to improve forecasts of the arrival time of stream interaction regions from a spacecraft located at 1 AU. The polarity of the interplanetary magnetic field, toward or away from the Sun, observed by consecutive spacecraft measurements is considered for the slow and fast streams straddling a stream interface, in order to investigate whether the geoeffectiveness of the two streams may also be forecast from 1 AU. It is found that the polarity of the magnetic field, associated with a given stream interface, is conserved when observed by two separate spacecraft at azimuthal separations of 20° or less and while in the fast wind, however, the field polarity was not always conserved when observed in the slow wind ahead of the interface. An analysis of the tilt angle evolution during 2008 showed that while the azimuthal tilt angles were generally similar between observations in the same Carrington rotation and in consecutive rotations of the same corotating interaction region, the meridional tilt angles may differ significantly. The forecast analysis showed that the azimuthal evolution of a SIR at 1 AU may be predicted to within a day or two of the actual evolution time, while any discrepancy was most likely caused by changes at the coronal hole on the solar surface, leading azimuthal and radial evolution of the SIR.