Sun-as-a-star spectroscopic observations of the line-of-sight velocity of a solar eruption on October 28, 2021

The propagation direction and true velocity of a solar coronal mass ejection, which are among the most decisive factors for its geo-effectiveness, are difficult to determine through single-perspective imaging observations. Here we show that Sun-as-a-star spectroscopic observations, together with imaging observations, could allow us to solve this problem. Using observations of the Extreme-ultraviolet Variability Experiment onboard the Solar Dynamics Observatory, we found clear blue-shifted secondary emission components in extreme ultraviolet spectral lines during a solar eruption on October 28, 2021. From simultaneous imaging observations, we found that the secondary components are caused by a mass ejection from the flare site. We estimated the line-of-sight (LOS) velocity of the ejecta from both the double Gaussian fitting method and the red-blue asymmetry analysis. The results of both methods agree well with each other, giving an average LOS velocity of the plasma of $\sim 423~\rm{km~s^{-1}}$. From the $304$ \AA~image series taken by the Extreme Ultraviolet Imager onboard the Solar Terrestrial Relation Observatory-A (STEREO-A) spacecraft, we estimated the plane-of-sky (POS) velocity from the STEREO-A viewpoint {to be around $587~\rm{km~s^{-1}}$}. The full velocity of the bulk motion of the ejecta was then computed by combining the imaging and spectroscopic observations, which turns out to be around $596~\rm{km~s^{-1}}$ with an angle of $42.4^\circ$ to the west of the Sun-Earth line and $16.0^\circ$ south to the ecliptic plane.