Periodic Density Structures and the Origin of the Slow Solar Wind

Abstract:

Periodic density structures with length-scales of hundreds to several thousands of Mm and frequencies of tens to hundreds of minutes are observed regularly in the solar wind at 1 AU. These structures coexist with, but are not due to, fluctuations in the plasma resulting from the turbulent cascade. Two lines of evidence – one identifying corresponding changes in compositional data in situ, and another identifying periodic density structures in the inner Heliospheric Imaging data onboard the Solar Terrestrial Relations Observatory (STEREO)/ Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) suite - indicate that periodic density structures are formed in the solar corona as part of the slow solar wind release and/or acceleration processes. The periodic nature of these density structures is an important physical constraint on their origin. In this presentation, we present the results of tracking periodic structures identified in the SECCHI/HI1 images down through the corresponding SECCHI/COR2 images. We demonstrate that the periodic density structures are formed around or below 2.5 solar radii – the inner edge of the COR2 field of view. Further, we compute the occurrence rate of periodic density structures in 10 days of COR2 images as a function of location in the solar corona. We find that this set of periodic density structures occurs preferentially in relation to coronal streamers. Periodic density structures are tracers of solar wind origin and/or acceleration; this study is a pilot for the kinds of investigations that we can carry out with the better temporal and spatial resolution of the heliospheric imagers on Solar Orbiter and Solar Probe Plus.