Bridging EUV and White-Light Observations to Inspect the Initiation Phase of a “Two-Stage” Solar Eruptive Event

Abstract

The initiation phase of coronal mass ejections (CMEs) is a very important aspect of solar physics, as these phenomena ultimately drive space weather in the heliosphere. This phase is known to occur between the photosphere and low corona, where many models introduce an instability and/or magnetic reconnection that triggers a CME, often with associated flaring activity. To this end, it is important to obtain a variety of observations of the low corona to build as clear a picture as possible of the dynamics that occur therein. Here, we combine the EUV imagery of the Sun Watcher using Active Pixel System Detector and Image Processing (SWAP) instrument onboard the Project for Onboard Autonomy (PROBA2) with the white-light imagery of the ground-based Mark-IV K-coronameter (Mk4) at Mauna Loa Solar Observatory (MLSO) to bridge the observational gap that exists between the disk imagery of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) and the coronal imagery of the Large Angle Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO). Methods of multiscale image analysis were applied to the observations to better reveal the coronal signal while suppressing noise and other features. This allowed an investigation into the initiation phase of a CME that was driven by a rising flux-rope structure from a “two-stage” flaring event underlying an extended helmet streamer. It was found that the initial outward motion of the erupting loop system in the EUV observations coincided with the first X-ray flare peak and led to a plasma pile-up of the white-light CME core material. The characterized CME core then underwent a strong jerk in its motion, as the early acceleration increased abruptly, simultaneously with the second X-ray flare peak. The overall system expanded into the helmet streamer to become the larger CME structure observed in the LASCO coronagraph images, which later became concave-outward in shape. Theoretical models for the event are discussed in light of these unique observations, and it is concluded that the formation of either a kink-unstable or torus-unstable flux rope may be the likeliest scenario.