Multi-point observation of a CME on August 16, 2020

On August 16, 2020, at 17:12 UT, a long-duration B1.2-class flare erupted from a filament in the southeastern hemisphere, originating from an active region at $32°$S, $26°$E, near a large equatorial coronal hole (CH). According to LASCO coronagraph images from the L1 point, the associated CME erupted with a principal angle (PA) of approximately $99°$, close to the ecliptic plane, and at a speed of 437 km/s. Consequently, the Lorentz force between the CH’s magnetic field and the CME deflected the CME about $30°$ northward. NOAA’s CME propagation models predicted a possible impact on Earth on August 19 around 22:00 UT. However, no signs of the CME were detected at Earth. Meanwhile, STEREO-A observed a disturbance in the solar wind around 19:00 UT, consistent with a CME impact. Considering that the magnetic field and solar wind plasma move together (i.e., under the frozen-in flux condition), and due to the Sun’s rotation, parcels of solar wind plasma spiral outward from the Sun (forming the Parker spiral). The solar wind spiral motion dragged the CME, deflecting it approximately $30°$ eastward during its propagation to 1 AU, ultimately directing it toward STEREO-A’s location. Furthermore, remote-tracking observations from the two broadband visible-light refracting camera systems (HI-1 and HI-2) aboard STEREO-A captured the CME in J-plot images, tracking it from its eruption to its impact. Additionally, the bi-directional flow of suprathermal electrons observed by STEREO-A during the first 12 h of August 19, 2020, indicates that the CME crossed a high-speed stream (HSS) sector. An isotropic flux of suprathermal electrons observed around 19:00 UT further confirms the CME’s impact in STEREO-A’s vicinity.