Asymmetric expansion of coronal mass ejections in the low corona

Abstract

Aims. Understanding how magnetic fields are structured within coronal mass ejections (CMEs), and how they evolve from the low corona into the heliosphere, is a major challenge for space weather forecasting and for solar physics. The study of CME morphology is a particularly auspicious approach to this problem, given that it holds a close relationship with the CME magnetic field configuration. Although earlier studies have suggested an asymmetry in the width of CMEs in orthogonal directions, this has not been inspected using multi-viewpoint observations. Methods. The improved spatial, temporal, and spectral resolution, added to the multiple vantage points offered by missions of the Heliophysics System Observatory, constitute a unique opportunity to gain insight into this regard. We inspect the early evolution (below ten solar radii) of the morphology of a dozen CMEs occurring under specific conditions of observing spacecraft location and CME trajectory, favorable to reduce uncertainties typically involved in the 3D reconstruction used here. These events are carefully reconstructed by means of a forward modeling tool using simultaneous observations of the Solar-Terrestrial Relations Observatory (STEREO) Extreme Ultraviolet Imager (EUVI) and the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) as input when originating low in the corona, and followed up in the outer fields of view of the STEREO and the Solar and Heliospheric Observatory (SOHO) coronagraphs. We then examine the height evolution of the morphological parameters arising from the reconstructions.Results. The multi-viewpoint analysis of this set of CMEs revealed that their initial expansion -below three solar radii-is considerably asymmetric and non-self-similar. Both angular widths, namely along the main axes of CMEs (AW L ) and in the orthogonal direction (AW D , representative of the flux rope diameter), exhibit much steeper change rates below this height, with the growth rate of AW L found to be larger than that of AW D , also below that height. Angular widths along the main axes of CMEs are on average ≈ 1.8 times larger than widths in the orthogonal direction AW D . The ratios of the two expansion speeds, namely in the directions of CMEs main axes and in their orthogonal, are nearly constant in time after ∼ 4 solar radii, with an average ratio ≈ 1.6. Heights at which the width change rate is defined to stabilize are greater for AW L than for AW D .

Connecting 3D Evolution of Coronal Mass Ejections to Their Source Regions

“…We find that for most of the CMEs, the width became constant in the height range of 2.5 − 3 R . This is in agreement with the results of Cremades et al (2020) who reported that the width of the CMEs changed considerably below 3 R . We also note from panel (b) that the mode of both the distributions fall in the overlapping height range of 2.5 − 3 R .…”

“…A sample of 460 CMEs observed by twin STEREO spacecrafts was used to compare the single-viewpoint and 3D kinematics (via triangulation) of CMEs in COR-2 FOV (Balmaceda et al 2018a). It is worth noting that although these studies provide 3D kinematic information, most of the analysis (Cremades et al (2020) and Cabello et al (2016) being exceptions in this context) are carried out starting from the COR-2 FOV, and hence the essential information in the inner corona is left out. Thus the major gap in our understanding of CME kinematics is to understand their 3D evolution in the inner corona.…”

The Deflection of Coronal Mass Ejections by the Ambient Coronal Magnetic Field Configuration

“…(2015) also found that the flare‐related CME on 7 January 2014 changes its propagation direction by around 28° in latitude and 43° in longitude near the Sun attributing this to the “channeling” by the AR CMF rather than to deflection by nearby structures. In addition, besides the CME deflections, the ambient magnetic field configuration can effect the rotation and expansion of CMEs and lead to anisotropic expansion or deformation (i.e., Cremades et al., 2020; Kay et al., 2017; Zuccarello et al., 2012), which should be taken into consider and will be discussed in the future.…”

Investigating Width Distribution of Slow and Fast CMEs in Solar Cycles 23 and 24

“…Recently, it has been reported that the CMEs evolve non selfsimilarly in the inner corona (see Cremades et al, 2020;Majumdar et al, 2020). In the CDAW catalog, the width of a CME is defined as the maximum angle subtended by a CME on the center of the Sun when the CME enters the LASCO C3 field of view (FOV) where the width appears to approach a constant value (Gopalswamy, 2004).…”

“…Apart from their radial propagation, CMEs are also known to exhibit lateral expansion that leads to an increase in their angular width as they propagate outwards (Kay et al, 2015;Cremades et al, 2020;Majumdar et al, 2020) and that it is the Lorentz force at their source region that is closely responsible for translating and expanding them (Subramanian et al, 2014). In this regard, Zhao et al (2017) reported on the importance of the angular width of a CME, in determining whether the corresponding interplanetary CME and the preceding shock will reach Earth.…”