Rotation of flux ropes in the low corona

Abstract

Eruptions of filaments are defined by different parameters, specially, sigmoid handedness and direction of the eruption, which are important parameters for forecasting the geoeffectiveness of consequent interplanetary coronal mass ejection (ICME) or magnetic cloud. Solar filaments often exhibit rotation and deflection during eruptions, which would significantly affect the geoeffectiveness of the coronal mass ejections (CMEs). Therefore, understanding the mechanisms that lead to such lateral displacement of filaments is a great concern to space weather forecasting. Two case studies are discussed. Firstly, the events of September 8 and September 10 2014, were analyzed from the Sun to the Earth. The numerical heliospheric simulation EUHFORIA shows that the handedness of the EUV sigmoid deduced from coronagraph observations was different from the tilt of the ICME at 1~au, suggesting a rotation of the CME in the low corona. A potential undetected low coronal rotation led to erroneous space weather prediction. The second event concerns a filament observed on August 20 2021, which underwent a rotation of 73 degrees during its eruption, implying a significant lateral drifting of the filament material. A data-constrained magnetohydrodynamic (MHD) simulation confirms such a rotation. These two studies reinforce the idea that CMEs are subjected to more significant rotation and deflection in the low corona than during their journey in the heliosphere.

Figures (3)

• Figure 1: Sigmoid observed on September 9 2014, in AR 12158: Top panels: (a) by AIA in 131 Å (b) by HMI, (c) by AIA in 304 Å. The red line indicates the tilt of the active region, and the green dashed line is the polarity inversion line. Bottom panels: Schematic representation of the CME2 orientation inferred from different observational proxies at different locations. (a) Close to 1 $R_\odot$, based on the analysis of the source region (sigmoid); (b) close to 0.1 au, based on the 3D reconstruction of the white-light images; and (c) at 1 au, based on the in situ observations. Adapted from Maharana2023.
• Figure 2: Evolution of $B_z$ for the CME-CME interaction event of September 2014 using EUHFORIA. The co-latitudinal component in the spherical coordinate system is $B_{clt}$, equivalent to $-B_z$ on the ecliptic plane. The red and blue spectra of the colour bar correspond to positive and negative $B_z$, respectively. Each sub-figure shows the view of the equatorial (X-Y) and the meridional (X-Z) planes at a particular time mentioned at its top. (a) CME2 is shown evolving behind CME1 in the early stage of interaction; (b) CME2 compresses the trailing negative $B_z$ part of CME1, creating the geo-effective sheath ahead of itself. Adapted from Maharana2023.
• Figure 3: Observation of the ejection of a filament and change of orientation by CHASE; top panels: intensity in H$\alpha$, middle panels in the H$\alpha$ wing, bottom panels: Doppler shifts. In panels a and b, the spine of the filament is indicated by a dashed line; in panel h, the oval indicates the drainage of the filament towards its drifting footpoint. The CME is on the top right panel, and in the bottom right panel, from the MHD simulation, the initial flux rope is green, and the final flux rope is violet. 73 degrees is the estimated rotation angle of the filament. Adapted from GuoJH2023b.