First imaging of corotating interaction regions using the STEREO spacecraft

TLDR

Plasma parcels are observed propagating from the Sun to large coronal heights by the Heliospheric Imagers on NASA STEREO during September 2007. The authors propose that a low‑lying plasma enhancement disconnected from a helmet streamer becomes embedded inside a corotating interaction region. They find that these parcels originate near the western boundary of an equatorial coronal hole, corotate with the Sun, evolve into fully developed corotating interaction regions observed by HI, and include a dense parcel near the CIR footpoint that appears before fast wind can overtake slow wind.

Abstract

Plasma parcels are observed propagating from the Sun out to the large coronal heights monitored by the Heliospheric Imagers (HI) instruments onboard the NASA STEREO spacecraft during September 2007. The source region of these out‐flowing parcels is found to corotate with the Sun and to be rooted near the western boundary of an equatorial coronal hole. These plasma enhancements evolve during their propagation through the HI cameras' fields of view and only becoming fully developed in the outer camera field of view. We provide evidence that HI is observing the formation of a Corotating Interaction Region (CIR) where fast solar wind from the equatorial coronal hole is interacting with the slow solar wind of the streamer belt located on the western edge of that coronal hole. A dense plasma parcel is also observed near the footpoint of the observed CIR at a distance less than 0.1AU from the Sun where fast wind would have not had time to catch up slow wind. We suggest that this low‐lying plasma enhancement is a plasma parcel which has been disconnected from a helmet streamer and subsequently becomes embedded inside the corotating interaction region.