= Coronal mass ejection
A <coronal mass ejection> is an expulsion of <plasma> and embedded <magnetic field> from the <solar corona> into interplanetary space. It carries matter as well as magnetic structure. A <solar flare> instead denotes a rapid atmospheric energy-release event observed through enhanced radiation; the two phenomena can accompany the same eruption without being interchangeable.<Coronal Mass Ejections by NOAA Space Weather Prediction Center>[Definition and distinction between expelled plasma/field and associated solar activity]{ref}
A <coronagraph> suppresses the bright solar disc so that much fainter coronal structures can be followed outward. A sequence of images gives projected size, motion and morphology. SOHO's LASCO observations provide an important example of the instrument-based diagnosis used in space-weather analysis.<Coronal Mass Ejections by NOAA Space Weather Prediction Center>[Coronagraph imagery, LASCO observations and inferred size, speed and direction]{ref}
For an illustrative measurement, suppose an apparent front moves from projected radius $3R_\odot$ to $7R_\odot$ in one hour. Using $R_\odot\simeq6.96\times10^8\,\mathrm m$ gives
$$
v_{\rm projected}=\frac{4R_\odot}{3600\,\mathrm s}\simeq7.73\times10^5\,\mathrm{m\,s^{-1}}.
$$
This is a plane-of-sky speed, not automatically the true radial speed. Projection, the choice of visible front and evolution between frames affect the inference. Motion mainly towards the observer can produce a surrounding halo in projection rather than a simple sideward trajectory.
A passing interplanetary disturbance can also be studied through local <solar wind> density, velocity and magnetic-field measurements. A leading <shock wave> and the magnetized ejecta are different parts of that disturbance. Combining remote imaging with local measurements is more informative than treating brightness or apparent speed alone as a complete description of the eruption.
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