Spinning Spots: Tracking the Rotation of Solar α-Sunspots and What It Means for Other Stars

Emily Joe Lößnitz and collaborators set out to measure how a special type of sunspot, called an α-sunspot, moves across the Sun and how its motion compares to both other sunspot types and the Sun’s quieter surface regions. Sunspots are dark patches on the Sun’s surface where strong magnetic fields poke through, and α-sunspots are generally round, unipolar spots that are often older and more stable. Because the Sun doesn’t spin like a solid ball, its equator rotates faster than its poles, measuring how these spots move can reveal details about how the Sun’s interior rotates. The team also explored how these results could be applied to other stars to understand the way starspots affect their brightness over time.

Observations and Selection Criteria

The researchers manually identified and tracked 105 α-sunspots from over a decade of observations using NASA’s Solar Dynamics Observatory. To be included, a spot had to stay relatively stable for at least 10 days and avoid large nearby magnetic disturbances. They assigned each spot a quality grade, giving more influence to stable, isolated examples. By tracking the exact center of each sunspot with image-processing techniques and correcting for the Sun’s curvature, they measured how fast each one rotated at its latitude.

Modeling the Rotation Law

From these measurements, Lößnitz and colleagues built mathematical “rotation laws” that describe how rotation speed changes from the equator toward the poles. They tested both a simpler two-parameter model (often used for sunspots) and a more complex three-parameter model (better for covering high latitudes). They found that α-sunspots rotate about 1.56% faster than the quiet Sun but 1.35% slower than the average sunspot population, suggesting they are anchored in shallower layers below the surface compared to younger spots. This supports the idea that the depth where magnetic flux tubes are rooted influences how a sunspot moves.

Extrapolating to Other Stars

Because high-latitude spots are rare on the Sun but common on other stars, the team extrapolated their three-parameter law to higher latitudes using statistical simulations. They proposed a way to scale this solar-based rotation curve for stars with different rotation speeds, introducing a “corrected shear factor” to represent how strong the star’s differential rotation is. This method keeps the Sun’s characteristic shape for the rotation curve but stretches or compresses it to match the stellar case, especially useful for modeling starspots seen in brightness measurements.

Light Curve Simulations

To show the astrophysical impact, the team implemented their scaled law into the Stellar Activity Grid for Exoplanets (SAGE) code, which simulates how rotating starspots create changes in a star’s observed brightness, its light curve. They modeled stars with two spots at different latitudes and found that differential rotation can subtly or dramatically change light-curve patterns over time, even producing variations like “scallop-shell” light curves previously linked to clouds or prominences.

Conclusions

The study concludes that α-sunspots are a valuable tracer for solar rotation studies and that their specific rotation profile should be considered when comparing to other stars. Ignoring these effects could lead to systematic errors in estimating stellar rotation rates from light curves. By combining precise solar measurements with stellar modeling, Lößnitz et al. provide a framework for better understanding surface differential rotation across a range of stars and its influence on exoplanet detection and characterization.

Source: Lößnitz