Three-dimensional analyses of an aspherical coronal mass ejection and its driven shock

¹ Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, PR China
e-mail: lfeng@pmo.ac.cn
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China
³ Max-Planck-Institut für Sonnensystemforschung, Göttingen, 37077 Lower Saxony, Germany
⁴ Solar-Terrestrial Centre of Excellence-SIDC, Royal Observatory of Belgium, 1180 Brussels, Belgium
⁵ Institute of Geodynamics of the Romanian Academy, 020032 Bucharest-37, Romania

Received: 1 December 2021
Accepted: 10 January 2022

Abstract

Context. Observations reveal that shocks can be driven by fast coronal mass ejections (CMEs) and play essential roles in particle accelerations. A critical ratio, δ, derived from a shock standoff distance normalized by the radius of curvature (ROC) of a CME, allows us to estimate shock and ambient coronal parameters. However, true ROCs of CMEs are difficult to measure due to observed projection effects.

Aims. We investigate the formation mechanism of a shock driven by an aspherical CME without evident lateral expansion. Through three-dimensional (3D) reconstructions without a priori assumptions of the object morphology, we estimate the two principal ROCs of the CME surface and demonstrate how the difference between the two principal ROCs of the CME affects the estimate of the coronal physical parameters.

Methods. The CME was observed by the Sun Earth Connection Coronal and Heliospheric Investigation instruments and the Large Angle and Spectrometric Coronagraph. We used the mask-fitting method to obtain the irregular 3D shape of the CME and reconstructed the shock surface using the bow-shock model. Through smoothings with fifth-order polynomial functions and Monte Carlo simulations, we calculated the ROCs at the CME nose.

Results. We find that (1) the maximal ROC is two to four times the minimal ROC of the CME. A significant difference between the CME ROCs implies that the assumption of one ROC of an aspherical CME could cause overestimations or underestimations of the shock and coronal parameters. (2) The shock nose obeys the bow-shock formation mechanism, considering the constant standoff distance and the similar speed between the shock and CME around the nose. (3) With a more precise δ calculated via 3D ROCs in space, we derive corona parameters at high latitudes of about −50°, including the Alfvén speed and the coronal magnetic field strength.