Lynch et al. (2010) and the review by Rouillard (2011) (see Fig. 1) paint a rather positive picture for our ability to connect RS observations and IS measurements of certain interplanetary propagating (IP) structures, particularly under solar minimum conditions. The reality, however, is more complicated. Despite the availability of continuous imaging and tracking of IP disturbances from the STEREO heliospheric imagers, CME time-of-arrivals and especially speeds at 1 AU can have errors upwards of 13 h and 200 km s−1, respectively, even for de-projected trajectories (Colaninno et al. 2013). Reconstructions of the CME shape from RS observations rarely agree with in situ ones (e.g., Nieves-Chinchilla et al. 2013; Wood et al. 2017). Some of the discrepancies can be attributed to limitations of the triangulation techniques (whether applied on J-maps or not) arising from the optically thin nature of the white light emission (Howard et al. 2012). Much of the remaining discrepancies, though, should arise from interaction between the structures and the ambient solar wind and/or from internal evolution that can lead to deflections (e.g., Isavnin et al. 2013, 2014), rotations (Nieves-Chinchilla et al. 2012) or distortions (Nieves-Chinchilla et al. 2013). We note, however, that these effects are inferred rather than observed directly in the data. In terms of the 3D global morphology, there is a current debate about distortion or deformation of the internal magnetic field configuration associated to the CMEs. The elongated dark cavity shape associated with the CME flux rope, called “pancaking”, provides an evidence of a highly distorted global structure, probably due to the solar wind interaction (Odstrcil & Pizzo 1999; Riley & Crooker 2004; Owens 2006; Savani et al. 2010; DeForest et al. 2013; Owens et al. 2017). However, another interpretation is that it could be an effect of the projection of a 3D structure in the plane of the sky, creating the appearance of distortion (Vourlidas et al. 2011; Nieves-Chinchilla et al. 2013). In contrast, from the in situ perspective, the magnetic field strength asymmetries observed can be interpreted as a consequence of the local effect of flux rope expansion (Farrugia et al. 1993; Osherovich et al. 1993; Démoulin & Dasso 2009) or erosion (Dasso et al. 2007; Ruffenach et al. 2012), but it could also be a result of the distortion (Hidalgo et al. 2002; Berdichevsky 2013; Nieves-Chinchilla et al. 2018a). Longitudinal deflections during inner heliospheric transit are particularly hard to quantify because heliospheric imaging is taken from and restricted along the ecliptic plane. This is the reason, for example, why the “garden-hose spiral” nature of solar wind streams has never been imaged directly but only inferred by de-projection techniques based on J-maps (Rouillard et al. 2008; Sheeley et al. 2008). While major progress was achieved with STEREO data, establishing the exact longitudinal extent of streamer blowouts (Vourlidas & Webb 2018), whether CMEs deflect longitudinally or merge with each other, where CIRs form or how shocks evolve require observations away from the ecliptic (Gibson et al. 2018). This is an area where Solar Orbiter imaging, particularly from Solar OrbiterHI (Howard et al. 2020) will be trailblazing. However to fully exploit this data we need to adapt our techniques to account for rapidly varying vantage points located outside the ecliptic plane.
Citation: Isavnin et al. 2013
Citation function: Background