Solar Surface Differential Rotation Research Articles

Periodicity in some light curves of the solar analogue V352 Canis Majoris

Our aim was to study the light curve periodicity of the solar analogue V352 CMa and in particular show that the presence or absence of periodicity in low amplitude light curves can be modelled with the Continuous Period Search (CPS) method. We applied the CPS method to 14 years of V-band photometry of V352 CMa and obtained estimates for the mean, amplitude, period and minima of the light curves in the selected datasets. We also applied the Power Spectrum Method (PSM) to these datasets and compared the performance of this frequently applied method to that of CPS. We detected signs of a $11.7 \pm 0.5$ year cycle in in the mean brightness. The long-term average photometric rotation period was $7.24 \pm 0.22$ days. The lower limit for the differential rotation coefficient would be $|k| > 0.12$, if the law of solar surface differential rotation were valid for V352 CMa and the period changes traced this phenomenon. Signs of stable active longitudes rotating with a period of $7.157 \pm 0.002$ days were detected from the epochs of the light minima with the Kuiper method. CPS performed better than the traditional PSM, because the latter always used a sinusoidal model for the data even when this was clearly not the correct model.

SIGNATURE OF DIFFERENTIAL ROTATION IN SUN-AS-A-STAR Ca II K MEASUREMENTS

The characterization of solar surface differential rotation (SDR) from disk-integrated chromospheric measurements has important implications for the study of differential rotation and dynamo processes in other stars. Some chromospheric lines, such as Ca II K, are very sensitive to the presence of activity on the disk and are an ideal choice for investigating SDR in Sun-as-a star observations. Past studies indicate that when the activity is low, the determination of Sun's differential rotation from integrated-sunlight measurements becomes uncertain. However, our study shows that using the proper technique, SDR can be detected from these type of measurements even during periods of extended solar minima. This paper describes results from the analysis of the temporal variations of Ca II K line profiles observed by the Integrated Sunlight Spectrometer (ISS) during the declining phase of Cycle 23 and the rising phase of Cycle 24, and discusses the signature of SDR in the power spectra computed from time series of parameters derived from these profiles. The described methodology is quite general, and could be applied to photometric time series of other Main-Sequence stars for detecting differential rotation.

Investigating stellar surface rotation using observations of starspots

AbstractRapid rotation enhances the dynamo operating in stars, and thus also introduces significantly stronger magnetic activity than is seen in slower rotators. Many young cool stars still have the rapid, primordial rotation rates induced by the interstellar molecular cloud from which they were formed. Also older stars in close binary systems are often rapid rotators. These types of stars can show strong magnetic activity and large starspots. In the case of large starspots which cause observable changes in the brightness of the star, and even in the shapes of the spectral line profiles, one can get information on the rotation of the star. At times even information on the spot rotation at different stellar latitudes can be obtained, similarly to the solar surface differential rotation measurements using magnetic features as tracers. Here, I will review investigations of stellar rotation based on starspots. I will discuss what we can obtain from ground-based photometry and how that improves with the uninterrupted, high precision, observations from space. The emphasis will be on how starspots, and even stellar surface differential rotation, can be studied using high resolution spectra.

Differential rotation on active late-type stars observed with Corot

AbstractOne major topic in studying stellar activity is to explain how phenomena seen on the Sun and stars, and specially magnetic phenomena, depend on stellar properties such as rotation and age. Differential rotation is an important physical process in theories of stellar magnetic field generation. The solar surface differential rotation was initially discovered via the simple method of tracking the rotation rates of individual starspots at different latitudes. Adopting a similar principle, high accuracy light curves of active stars observed with the CoRoT satellite are analyzed using a model based on the rotational modulation of the visibility of active regions.

Prediction of solar active longitudes

Context. After considering the solar surface differential rotation, it was found that two persistent active longitudes of sunspots and of solar X-ray flares, separated by about 180 degrees, exist on the sun. Aims. According to the migration rule of the active longitudes on the Sun, we predict the central positions of the solar active longitudes in Carrington frame. Methods. The migration, M, of an active longitude on the kth day of the ith Carrington rotation in Carrington frame can be described as M = T c Σ Ni i= (Ω i - Ω c ) + k(Ω i - Ω c ), where Ω ι , (sidereal) is the angular velocity of the active longitude in the ith Carrington rotation, Ωc (sidereal), the angular velocity of Carrington frame, and T c , the Carrington rotation period. Solar active longitudes rotate in agreement with solar surface differential rotation law, which can be simply described as Ω ι , = Ω 0 - Bsin 2 (Φ ι ), where Ω 0 is the equatorial angular velocity, B the differential rotation rate, and Φι, the mean latitude of solar activity in the ith rotation. Parameters, Ω 0 and B, can be obtained from observations. If the latitude in the (i + 1)th rotation is approximately that of the ith rotation, the expected angular velocity of an active longitude in the (i + 1)th rotation can be described as Ω(ι+1) p = Ω0- B sin 2 (Φ ι ). The central positions of active longitude, Δ(ι+1)kp, can be expected as well. Results. We predict active longitudes of solar X-ray flares qualitatively between 1997-2006. Active longitudes with half width of 20°-30° cover 80% of C-flares during the minimum of solar activities and X-flares during the maximum. Conclusions. Solar active longitudes can be predicted by differential rotation parameters and latitude of solar activity.

Starspots as tracers of differential surface rotation

Differential rotation is a key ingredient in theories of stellar magnetic field generation. The solar surface differential rotation was initially discovered via the simple method of tracking the rotation rates of individual starspots at different latitudes. Today, the same technique can be applied to rapidly rotating stars, using sequences of Doppler images spanning several stellar rotations. Early results suggest that solar-like differential rotation patterns prevail on rapidly rotating dwarf stars, but much remains to be done in tracing global flow patterns on pre-main sequence stars, giants, and tidally-locked binary components. I outline the relative merits of the three main methods that have so far been used to track the latitude dependence of starspot rotation rates, and discuss the validity of the physical assumptions that underpin them.

The solar surface differential rotation from disk-integrated chromospheric fluxes

Disk-integrated solar chromospheric Caii K-line (3933.68 A) fluxes have been measured almost daily at Sacramento Peak Observatory since 1977. Using observing windows selected to mimic seasonal windows for chromospheric measurements of lower Main-Sequence stars such as those observed by Mount Wilson Observatory's HK Project, we have measured the solar rotation from the modulation of the Caii K-line flux. We track the change of rotation period from the decline of cycle 21 through the maximum of cycle 22. This variation in rotation period is shown to behave as expected from the migration of active regions in latitude according to Maunder's ‘butterfly diagram’, including an abrupt change in rotation period at the transition from cycle 21 to cycle 22. These results indicate the successful detection of solar surface differential rotation from disk-integrated observations. We argue that the success of our study compared to previous investigations of the solar surface differential rotation from disk-integrated fluxes lies primarily with the choice of the length of the time-series window. Our selection of 200 days is shorter than in previous studies whose windows are typically on the order of one year. The 200-day window is long enough to permit an accurate determination of the rotation period, yet short enough to avoid complications arising from active region evolution. Thus, measurements of the variation of rotation period in lower Main-Sequence stars, especially those that appear to be correlated with long-term changes in chromospheric activity (i.e., cycles), are probably evidence for stellar surface differential rotation.