Solar wind corotating interaction regions: The third dimension

Abstract

A new dimension has recently been added to our understanding of solar wind structure ‐ the dimension out of the solar equatorial plane. Measurements made by the Ulysses space probe at high heliographic latitudes and powerful new 3‐dimensional numerical simulations have revealed that solar wind corotating interaction regions (CIRs) tend to be tilted systematically relative to the solar equatorial plane, at least on the declining phase of the ∼11‐year solar activity cycle. One consequence of this tilt is that with increasing heliocentric distance the forward waves associated with the CIRs tend to propagate toward and across the equator while the reverse waves tend to propagate to higher heliographic latitudes. It has been shown that tilted interaction regions are a natural consequence of the tilt of the solar magnetic dipole relative to the solar rotation axis. This new understanding of the 3‐dimensional nature of CIRs is essential to the interpretation of energetic particle measurements at high solar latitudes, and is useful for explaining certain puzzling aspects of Voyager observations in the very distant heliosphere at lower latitudes. Closer to home, there is an awakening perception of the important role played by CIRs in stimulating recurrent geomagnetic activity. The present paper provides a limited review of recent U.S. work on these topics and on other aspects of corotating interaction regions in the solar wind.