Long-Term Tracking of Corotating Density Structures Using Heliospheric Imaging

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 catalog listing the properties of 190 corotating structures well-observed in images taken by the Heliospheric Imager (HI) instruments onboard STEREO-A (ST-A). Based on this catalog, we present here one of very few long-term analyses of solar wind structures advected by the background solar wind. We concentrate on the subset of plasma density structures clearly identified inside corotating structures. 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 ± 31 km s −1 . Such a low mean value corresponds to the terminal speed of the slow solar wind rather than the speed of stream interfaces, which is typically intermediate between the slow and fast solar wind speeds (∼ 400 km s −1 ). Using our fitting technique, we predicted the arrival time of each corotating density structure at different probes in the inner heliosphere. We find that our derived speeds are systematically lower by ∼ 100 km s −1 than those measured in situ at the predicted impact times. Moreover, for cases when a stream interaction region is clearly detected in situ at the estimated impact time, we find that our derived speeds are lower than the speed of the stream interface measured in situ by an average of 55 km s −1 at ST-A and 84 km s −1 at STEREO-B (ST-B). 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.

The Temporal and Spatial Scales of Density Structures Released in the Slow Solar Wind During Solar Activity Maximum

“…We make the assumption that all of the white light observed in a pixel has been scattered by a blob. We further assume that blobs have their origin at the neutral line (Plotnikov et al 2016) and move at a constant velocity radially outwards. In the case of a highly tilted neutral line, this places the blobs on a 3D spiral centered on the Sun corotating with the neutral line.…”

“…In each HI J-map, we can see that both the real tracks (left-hand column) and the fitted tracks (central column) tend to converge, a characteristic signature of blob emissions from a single corotating source region on the Sun as viewed from the vantage point of STEREO-A prior to superior conjunction (Rouillard et al 2008). The properties of those blobs propagating in the ecliptic plane that constitute the ecliptic substructure of the CIR were derived by Plotnikov et al (2016) and are listed in the CIR catalogue (the so-called CIRCAT) produced by these authors for the FP7 HELCATS project.…”

“…Similar transient structures had previously been detected in in situ measurements alone (e.g., Crooker et al 1996Crooker et al , 2004Kilpua et al 2009;Yu et al 2016). More recently, Plotnikov et al (2016) mapped blob emission along the ecliptic plane over nearly a complete solar cycle. The authors found that, statistically, blobs emerge from the coronal neutral line.…”

Coronal mass ejection-related particle acceleration regions during a simple eruptive event

“…The catalog uses the fitting technique developed by Rouillard et al (2010a,b), which is based on J-maps. Through a systematic analysis, Plotnikov et al (2016) show that all CIRs measured in situ were also detected in white-light imagery be- • to 74…”

Reconstructing Solar Wind Inhomogeneous Structures From Stereoscopic Observations in White Light: Small Transients Along the Sun‐Earth Line

“…We find the peak region at the period of 27 days at the distance bellow 50 Rs where large‐scale transients dominate but less than 50 Rs where small‐scale transients can be seen. This is reasonable because most notable small‐scale transients near the ecliptic plane come from coronal streamers above solar active regions (Plotnikov et al, ), which may live for as long as several Carrington rotations. A more detailed analysis of the periodic transients is given in the next section.…”