Interplanetary conditions: lessons from this minimum

Interplanetary conditions during the Cycle 23-24 minimum have attracted attention because they are noticeably different than those during other minima of the space age, exhibiting more solar wind stream interaction structures in addition to reduced mass fluxes and low magnetic field strengths. In this study we consider the differences in the solar wind source regions by applying Potential Field Source Surface models of the coronal magnetic field. In particular, we consider the large scale coronal field geometry that organizes the open field region locations and sizes, and the appearance of the helmet streamer structure that is another determiner of solar wind properties. The recent cycle minimum had an extraordinarily long entry phase (the decline of Cycle 23) that made it difficult to identify when the actual miminum arrived. In particular, the late 23 ^rd cycle was characterized by diminishing photospheric fields and complex coronal structures that took several extra years to simplify to its traditional dipolar solar minimum state. The nearly dipolar phase, when it arrived, had a duration somewhat shorter than those of the previous cycles. The fact that the corona maintained an appearance more like a solar maximum corona through most of the quiet transitional phase between Cycles 23 and 24 gave the impression of a much more complicated solar minimum solar wind structure in spite of the weaknesses of the mass flux and interplanetary field. The extent to which the Cycle 23-24 transition will affect Cycle 24, and/or represents what happens during weak cycles in general, remains to be seen.