Sunspot decay as a test of the eta-quenching concept

Rüdiger, G.; Kitchatinov, L. L.

doi:10.1002/(sici)1521-3994(200003)321:1<75::aid-asna75>3.0.co;2-b

Cited by 24 publications

(23 citation statements)

References 17 publications

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“…This value is at minimum one order of magnitude higher than that predicted for solar values, which vary from 10 10 cm 2 /s (Dikpati & Charbonneau 1999) to 10 13 cm 2 /s (Rüdiger & Kitchatinov 2000). The diffusion timescale for the magnetic field inside the convection zone (CZ) is given by…”

Section: Stellar Cycle Predictionmentioning

confidence: 74%

“…Even the decay of sunspots, and thus the decay of the surface magnetic flux, is still not fully understood (Rüdiger & Kitchatinov 2000;Martínez Pillet 2002). Analyzing a subset of the Greenwich Photoheliographic Results (GPR), Bumba (1963) proposed a linear decay law of form dA/dt = D (where A is the area of the sunspot usually given in units of millionths of the solar hemisphere, 1 MSH = 3.05 Mm 2 ) for recurrent sunspots (those with two or more disk passages) with Based on data obtained with the STELLA robotic telescopes in Tenerife, an AIP facility jointly operated with IAC.…”

Section: Introductionmentioning

confidence: 99%

“…Krause & Rüdiger (1975) proposed a similar model based on turbulence, where the turbulent diffusion was related to the flux decay by dΦ/dt ∝ η T . Rüdiger & Kitchatinov (2000) used a mean-field formulation of diffusivity quenching and produced quasi-linear decays for both the spot area as well as the magnetic flux.…”

Section: Introductionmentioning

confidence: 99%

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Spot evolution on the red giant star XX Triangulum

Künstler

Carroll

Strassmeier

2015

A&A

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Context. Solar spots appear to decay linearly proportional to their size. The decay rate of solar spots is directly related to magnetic diffusivity, which itself is a key quantity for the length of a magnetic-activity cycle. Is a linear spot decay also seen on other stars, and is this in agreement with the large range of solar and stellar activity cycle lengths? Aims. We investigate the evolution of starspots on the rapidly-rotating (P rot ≈ 24 d) K0 giant XX Tri, using consecutive time-series Doppler images. Our aim is to obtain a well-sampled movie of the stellar surface over many years, and thereby detect and quantify a starspot decay law for further comparison with the Sun. Methods. We obtained continuous high-resolution and phase-resolved spectroscopy with the 1.2-m robotic STELLA telescope on Tenerife over six years, and these observations are ongoing. For each observing season, we obtained between 5 to 7 independent Doppler images, one per stellar rotation, making up a total of 36 maps. All images were reconstructed with our line-profile inversion code iMap. A wavelet analysis was implemented for denoising the line profiles. To quantify starspot area decay and growth, we match the observed images with simplified spot models based on a Monte Carlo approach. Results. It is shown that the surface of XX Tri is covered with large high-latitude and even polar spots and with occasional small equatorial spots. Just over the course of six years, we see a systematically changing spot distribution with various timescales and morphology, such as spot fragmentation and spot merging as well as spot decay and formation. An average linear decay of D = −0.022 ± 0.002 SH/day is inferred. We found evidence of an active longitude in phase toward the (unseen) companion star. Furthermore, we detect a weak solar-like differential rotation with a surface shear of α = 0.016 ± 0.003. From the decay rate, we determine a turbulent diffusivity of η T = (6.3 ± 0.5) × 10 14 cm 2 /s and predict a magnetic activity cycle of ≈26 ± 6 yr. Finally, we present a short movie of the spatially resolved surface of XX Tri.

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Section: Stellar Cycle Predictionmentioning

confidence: 74%

Section: Introductionmentioning

confidence: 99%

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Spot evolution on the red giant star XX Triangulum

Künstler

Carroll

Strassmeier

2015

A&A

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“…The systematic deviations from a strict parabolic law were explained by Petrovay et al (1999) with a generalization of the turbulent erosion model of sunspot decay when flux tubes are embedded in a pre-existing homogeneous plage field. It is worth noting that the theoretical decay rate from 2-D modelling of a sunspot turned out to be close to linear for both the spot area and the magnetic flux (Rüdiger and Kitchatinov 2000). This confusing situation may partly be resolvable with new high-resolution solar observing techniques and instruments and, in addition, by comparing to starspots.…”

Section: Starspot Lifetimesmentioning

confidence: 98%

Starspots

Strassmeier

2009

Astron Astrophys Rev

434

378

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Starspots are created by local magnetic fields on the surfaces of stars, just as sunspots. Their fields are strong enough to suppress the overturning convective motion and thus block or redirect the flow of energy from the stellar interior outwards to the surface and consequently appear as locally cool and therefore dark regions against an otherwise bright photosphere (Biermann in Astronomische Nachrichten 264: 361, 1938; Z Astrophysik 25:135, 1948). As such, starspots are observable tracers of the yet unknown internal dynamo activity and allow a glimpse into the complex internal stellar magnetic field structure. Starspots also enable the precise measurement of stellar rotation which is among the key ingredients for the expected internal magnetic topology. But whether starspots are just blown-up sunspot analogs, we do not know yet. This article is an attempt to review our current knowledge of starspots. A comparison of a white-light image of the Sun (G2V, 5 Gyr) with a Doppler image of a young solar-like star (EK Draconis; G1.5V, age 100 Myr, rotation 10 × Ω Sun ) and with a mean-field dynamo simulation suggests that starspots can be of significantly different appearance and cannot be explained with a scaling of the solar model, even for a star of same mass and effective temperature. Starspots, their surface location and migration pattern, and their link with the stellar dynamo and its internal energy transport, may have far reaching impact also for our understanding of low-mass stellar evolution and formation. Emphasis is given in this review to their importance as activity tracers in particular in the light of more and more precise exoplanet detections around solar-like, and therefore likely spotted, host stars.

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“…To date the decay of sunspots has been primarily addressed by simplified turbulent decay models, such as Petrovay and van Driel-Gesztelyi (1997), Rüdiger and Kitchatinov (2000) and Kitchatinov and Olemskoi (2006). With proper assumptions about the quenching of the turbulent magnetic diffusivity these models can describe the overall flux loss rates and shape of the decay curve, a more detailed modeling requires taking fully into account the interaction of the sunspot with the surrounding convection zone as well large scale flows (moat).…”

Section: Sunspot Evolution Past Emergence Processmentioning

confidence: 99%

Sunspot Modeling: From Simplified Models to Radiative MHD Simulations

Rempel

Schlichenmaier

2011

Living Rev. Solar Phys.

126

100

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We review our current understanding of sunspots from the scales of their fine structure to their large scale (global) structure including the processes of their formation and decay. Recently, sunspot models have undergone a dramatic change. In the past, several aspects of sunspot structure have been addressed by static MHD models with parametrized energy transport. Models of sunspot fine structure have been relying heavily on strong assumptions about flow and field geometry (e.g., flux-tubes, "gaps", convective rolls), which were motivated in part by the observed filamentary structure of penumbrae or the necessity of explaining the substantial energy transport required to maintain the penumbral brightness. However, none of these models could self-consistently explain all aspects of penumbral structure (energy transport, filamentation, Evershed flow). In recent years, 3D radiative MHD simulations have been advanced dramatically to the point at which models of complete sunspots with sufficient resolution to capture sunspot fine structure are feasible. Here, overturning convection is the central element responsible for energy transport, filamentation leading to fine structure, and the driving of strong outflows. On the larger scale these models are also in the progress of addressing the subsurface structure of sunspots as well as sunspot formation. With this shift in modeling capabilities and the recent advances in high resolution observations, the future research will be guided by comparing observation and theory.

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Sunspot decay as a test of the eta-quenching concept

Cited by 24 publications

References 17 publications

Spot evolution on the red giant star XX Triangulum

Spot evolution on the red giant star XX Triangulum

Starspots

Sunspot Modeling: From Simplified Models to Radiative MHD Simulations

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