Long-Term Tracking of Corotating Density Structures using Heliospheric Imaging

TL;DR

This study analyzes long-term heliospheric images to track and characterize corotating density structures in the solar wind, revealing their origins, evolution, and relation to solar wind speed variations during solar minimum.

Contribution

It provides a detailed long-term analysis of corotating density structures using heliospheric imaging, including their evolution, speeds, and origins near the coronal neutral line.

Findings

Most structures are density inhomogeneities advected by slow solar wind.

Mean radial speed of structures is 311 ± 31 km/s.

Speeds track the long-term variation of slow solar wind during solar minimum.

Abstract

The systematic monitoring of the solar wind in high-cadence and high-resolution heliospheric images taken by the Solar-Terrestrial Relation Observatory (STEREO) spacecraft permits the study of the spatial and temporal evolution of variable solar wind flows from the Sun out to 1~AU, and beyond. As part of the EU Framework 7 (FP7) Heliospheric Cataloguing, Analysis and Techniques Service (HELCATS) project, we have generated a catalogue listing the properties of 190 corotating structures well-observed in images taken by the Heliospheric Imager instruments on-board STEREO-A. We present here one of very few long-term analyses of solar wind structures advected by the background solar wind. This analysis confirms that most of the corotating density structures detected by the heliospheric imagers comprises a series of density inhomogeneities advected by the slow solar wind that eventually become entrained by stream interaction regions. We have derived the spatial-temporal evolution of each of these corotating density structures by using a well-established fitting technique. The mean radial propagation speed of the corotating structures is found to be $311 \pm 31$ km~s$^{-1}$. We predicted the arrival time of each corotating density structure at different probes. We show that the speeds of the corotating density structures derived using our fitting technique track well the long-term variation of the radial speed of the slow solar wind during solar minimum years (2007--2008). Furthermore, we demonstrate that these features originate near the coronal neutral line that eventually becomes the heliospheric current sheet.