Stellar activity

Stellar activity is variable magnetic behaviour in a star's outer layers, including starspots, atmospheric heating and eruptive phenomena. Rotation carries regions of different brightness and line shape across the visible disc. Their changing contribution alters both the stellar light curve and the apparent Doppler shift, even when the star's centre of mass has no corresponding orbital motion.

This makes stellar activity both a subject of study and a source of stellar noise in exoplanet searches. A changing stellar spectral-line bisector tests whether a line is deforming rather than simply translating in wavelength. Simultaneous brightness measurements supply another diagnostic. Actual observing times matter because an activity-period alias in radial-velocity sampling can place an activity-related signal at a misleading period.

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.^[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.^[ref]

For an illustrative measurement, suppose an apparent front moves from projected radius $3R_{\odot }$ to $7R_{\odot }$ in one hour. Using $R_{\odot }\simeq 6.96\times 10^8\mathrm{m}$ givesThis 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.

Stellar variability that contaminates a planetary radial-velocity or photometric signal. Rotating active regions, changes in convective line shifts, oscillations and granulation can produce apparent velocities. A common spectral shift, line-shape diagnostics, activity indicators and simultaneous photometry provide different tests; none is an automatic numerical probability.

An activity-period alias in radial-velocity sampling is an apparent period created when a stellar activity signal is measured at a restricted set of observing times. For observations exactly separated by $1/f_s$, a sinusoid of frequency $f$ is indistinguishable at those times from appropriate sinusoids at $|f\pm n{f}_s|$, with integer $n$. Thus a genuine stellar rotation signal can produce peaks at periods different from the rotation period.

Real observing times are irregular, so the aliases have different amplitudes and phases rather than being perfectly equivalent. Simulating a candidate signal at those same times helps test which observed peaks it predicts. The method applies to planetary signals as well as stellar noise; a periodogram peak alone does not identify a new exoplanet.^[ref]

The midpoint of the two wings of a stellar absorption line at each depth. A changing line bisector can reveal profile distortions caused by active regions or blends rather than a rigid Doppler shift of the whole spectrum.