Shear Of Differential Rotation Research Articles

Grand Activity Minima and Maxima via Dual Dynamos

Reconstructions of past solar activity based on cosmogenic radioisotopes have reavealed that the Sun spends a significant fraction ( ${\approx}\, 20$ %) of its time in aperiodically recurring states of so-called Grand Minima or Grand Maxima, namely epochs of strongly supressed and markedly above-average levels of magnetic activity, respectively. The physical origin of these episodes is not yet understood. In this article we present a dual-dynamo model of the solar cycle, combining a dominant dynamo based on differential-rotation shear and surface decay of bipolar active regions, and a weak, deep-seated turbulent dynamo. The resulting dynamo simulations are found to exhibit the equivalent of observed Grand Minima and Maxima. By adjusting the magnitude and saturation level of the secondary turbulent dynamo, we can reproduce well the duration and waiting-time distributions of Grand Minima and Maxima inferred from the cosmogenic-isotope record. The exit from Grand Minima episodes is typically characterized by strong hemispheric asymmetries, in agreement with sunspot observations during the 1645 – 1715 Maunder Minimum. In these simulations, Grand Maxima can be unambiguously identified as a distinct dual-dynamo state resulting from constructive interference between the two dynamos mechanisms operating within the simulation. This interaction leads to the autonomous production of long quasi-periodicities in the millennial range, commensurate with the Halstatt cycle. Such a quasi-periodic modulation, readily produced through dynamical backreaction on large-scale flows in non-kinematic dynamo models, is quite uncommon in a purely kinematic solar-cycle model such as the one developed herein. We argue that these long periodicities are set by the long diffusion time of magnetic field accumulating in the stable layers underlying the turbulent convection zone.

Pulsation, Rotation and Flares in A Stars

AbstractNew observations of Kepler δ Scuti stars show that our understanding of pulsation in these stars is incomplete. A large fraction of A and B stars exhibit rotational modulation in light, suggesting that spots exist in stars with radiative envelopes. Flares are seen in some A stars, as may be expected if starspots are present. Differential rotation shear increases from M to F but decreases for A stars; it reaches a maximum among the γ Doradus variables. Current views of stars with radiative envelopes may need to be reviewed in the light of these observations.

Transport and mixing in the radiation zones of rotating stars

The purpose of this paper is to improve the modelization of the rotational mixing which occurs in stellar radiation zones, through the combined action of the thermally driven meridional circulation and of the turbulence generated by the shear of differential rotation. The turbulence is assumed to be anisotropic, due to the stratification, with stronger transport in the horizontal directions than in the vertical. The main difference with the former treatments by Zahn (1992) and Maeder & Zahn (1998) is that we expand here the departures from spherical symmetry to higher order, and include explicitly the differential rotation in latitude, to first order. This allows us to treat simultaneously the bulk of a radiation zone and its tachocline(s). Moreover, we take fully into account the non-stationarity of the problem, which will enable us to tackle the rapid phases of evolution. The system of partial differential equations, which govern the transport of angular momentum, heat and chemical elements, is written in a form which makes it ready to implement in a stellar evolution code. Here the effect of a magnetic field is deliberately ignored; it will be included in forthcoming papers.

The 11 year history of starspots on V1149 ORI = HD 37824

view Abstract Citations (12) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The 11 Year History of Starspots on V1149 Ori=HD 37824 Hall, Douglas S. ; Fekel, Francis C. ; Henry, Gregory W. ; Barksdale, William S. Abstract All available radial-velocity measures, published and unpublished, yield an improved period and a new (assumed circular) solution. The period is 53.58 d +/- 0.02 d, and conjunction (K1 giant in front) was at 2 444 325.93 +/- 0.12 d. Eleven years of V-band photometry, published and unpublished, between 1978-1979 and 1989-1990 are analyzed. Eighteen data groups are fit with a two-spot light-curve-modeling technique. Six spots existing sometime during the 11 years are identified, and the 4-percent range of their rotation periods is used to estimate a differential rotation coefficient of k = 0.08 +/- 0.02. Observed lifetimes of those six spots are consistent with times calculated on the assumption that large spots are disrupted by the shear of differential rotation. The two best observed spots each lasted about five years. Publication: The Astronomical Journal Pub Date: November 1991 DOI: 10.1086/116004 Bibcode: 1991AJ....102.1808H Keywords: Starspots; Stellar Spectrophotometry; K Stars; Orbital Elements; Radial Velocity; Stellar Rotation; Astronomy; STARS: CHROMOSPHERES full text sources ADS | data products SIMBAD (2)