The Origin of Extremely Nonradial Solar Wind Outflows

Abstract

The origin of nonradial solar wind flows and their effect on space weather are poorly understood. Here we present a detailed investigation of 12 nonradial solar wind events during solar cycles 23–24, covering the period 1995–2017. In all these events the azimuthal flow angles of the solar wind exceed 6° as measured at the L1 Lagrangian point of the Sun–Earth system, for periods of 24 hr. In addition, all the events were selected during periods when coronal mass ejections (CMEs) and/or corotating interaction regions (CIRs) were absent. For most of the events, the near-Earth solar wind density was <5 cm−3 for periods exceeding 24 hr, similar to the well-known “solar wind disappearance events” wherein near-Earth solar wind densities dropped by two orders of magnitude for periods exceeding 24 hr. The solar source regions determined for all the cases were found to be associated with active region–coronal hole (AR–CH) pairs located around the central meridian. Further, the dynamical evolution of the source regions, studied using both the Extreme-ultraviolet Imaging Telescope and the Michelson Doppler Imager, showed a clear reduction in the CH area accompanied by the emergence of new magnetic flux regions. This dynamic evolution in the AR–CH source regions eventually disturbed the stable CH configurations, thereby giving rise to the extremely nonradial solar wind outflows. We discuss, based on our results, a possible causative mechanism for the origin of these highly nonradial flows that were not associated with either CMEs or CIRs.