Cyclic fluctuations in the differential rotation of active stars

AbstractDifferential rotation is described in stellar dynamo models as one of the fundamental phenomena governing the amplification of magnetic fields in active stars.Using indirect imaging methods, the measurement of photospheric differential rotation is now achieved on a growing number of very active stars, a fraction of which exhibit temporal fluctuations of potentially large amplitude in their latitudinal shear, on a time-scale of a few years. I first describe the modeling tools on which such analysis is based, then discuss the implications of this observational work on our understanding of stellar dynamos and of the impact stellar magnetic fields may have on the dynamics of convective envelopes.

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Flux expulsion with dynamics

Journal of Fluid Mechanics ◽

10.1017/jfm.2016.60 ◽

2016 ◽

Vol 791 ◽

pp. 568-588 ◽

Cited By ~ 5

Author(s):

Andrew D. Gilbert ◽

Joanne Mason ◽

Steven M. Tobias

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Lorentz Force ◽

Differential Rotation ◽

Time Scale ◽

Scaling Laws ◽

Dynamical Regime ◽

Kinematic Evolution ◽

Flux Expulsion ◽

The Magnetic Field

In the process of flux expulsion, a magnetic field is expelled from a region of closed streamlines on a $TR_{m}^{1/3}$ time scale, for magnetic Reynolds number $R_{m}\gg 1$ ($T$ being the turnover time of the flow). This classic result applies in the kinematic regime where the flow field is specified independently of the magnetic field. A weak magnetic ‘core’ is left at the centre of a closed region of streamlines, and this decays exponentially on the $TR_{m}^{1/2}$ time scale. The present paper extends these results to the dynamical regime, where there is competition between the process of flux expulsion and the Lorentz force, which suppresses the differential rotation. This competition is studied using a quasi-linear model in which the flow is constrained to be axisymmetric. The magnetic Prandtl number $R_{m}/R_{e}$ is taken to be small, with $R_{m}$ large, and a range of initial field strengths $b_{0}$ is considered. Two scaling laws are proposed and confirmed numerically. For initial magnetic fields below the threshold $b_{core}=O(UR_{m}^{-1/3})$, flux expulsion operates despite the Lorentz force, cutting through field lines to result in the formation of a central core of magnetic field. Here $U$ is a velocity scale of the flow and magnetic fields are measured in Alfvén units. For larger initial fields the Lorentz force is dominant and the flow creates Alfvén waves that propagate away. The second threshold is $b_{dynam}=O(UR_{m}^{-3/4})$, below which the field follows the kinematic evolution and decays rapidly. Between these two thresholds the magnetic field is strong enough to suppress differential rotation, leaving a magnetically controlled core spinning in solid body motion, which then decays slowly on a time scale of order $TR_{m}$.

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Stellar activity and winds shaping the atmospheres of Earth-like planets

Proceedings of the International Astronomical Union ◽

10.1017/s174392131900293x ◽

2018 ◽

Vol 14 (S345) ◽

pp. 181-184

Author(s):

Theresa Lueftinger ◽

Manuel Güdel ◽

Sudeshna Boro Saikia ◽

Colin Johnstone ◽

Beatrice Kulterer ◽

...

Keyword(s):

Magnetic Fields ◽

Extreme Environments ◽

Doppler Imaging ◽

Research Network ◽

Theoretical Understanding ◽

Stellar Activity ◽

Active Stars ◽

Earth Like Planets ◽

Stellar Magnetic Fields ◽

Host Stars

AbstractPlanets orbiting young, active stars are embedded in an environment that is far from being as calm as the present solar neighbourhood. They experience the extreme environments of their host stars, which cannot have been without consequences for young stellar systems and the evolution of Earth-like planets to habitable worlds. Stellar magnetism and the related stellar activity are crucial drivers of ionization, photodissociation, and chemistry. Stellar winds can compress planetary magnetospheres and even strip away the outer layers of their atmospheres, thus having an enormous impact on the atmospheres and the magnetospheres of surrounding exoplanets. Modelling of stellar magnetic fields and their winds is extremely challenging, both from the observational and the theoretical points of view, and only ground breaking advances in observational instrumentation and a deeper theoretical understanding of magnetohydrodynamic processes in stars enable us to model stellar magnetic fields and their winds – and the resulting influence on the atmospheres of surrounding exoplanets – in more and more detail. We have initiated a national and international research network (NFN): ‘Pathways to Habitability – From Disks to Active Stars, Planets to Life’, to address questions on the formation and habitability of environments in young, active stellar/planetary systems. We discuss the work we are carrying out within this project and focus on how stellar evolutionary aspects in relation to activity, magnetic fields and winds influence the erosion of planetary atmospheres in the habitable zone. We present recent results of our theoretical and observational studies based on Zeeman Doppler Imaging (ZDI), field extrapolation methods, wind simulations, and the modeling of planetary upper atmospheres.

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V. Sunspots

Transactions of the International Astronomical Union ◽

10.1017/s0251107x00006143 ◽

1985 ◽

Vol 19 (1) ◽

pp. 71-78

Author(s):

V. Buraba

Keyword(s):

Magnetic Fields ◽

Differential Rotation ◽

Solar Physics ◽

Solar Constant ◽

Dwarf Stars ◽

Solar Differential Rotation ◽

Scientific Meetings ◽

Red Dwarf Stars ◽

Stellar Magnetic Fields

Several proceedings of scientific meetings on sunspots appeared during the 1981-1984 period [The Physics of Sunsots, Cram and Thomas (eds.) 1981; see also reports of regional meetings, e.g.. Third European Solar Meeting, Oxford 1981; Nordic Astronomy Meeting, O. Hauge (ed.), Oslo 1983; 11th Regional Consultation on Solar Physics, L. Dezsö and B. Kalman (eds.), Debrecen 1983]. New interest in sunspots was aroused through observations of EUV sunspot spectra from space and was also inspired by the growing number of observations of starspots and other stellar activities [IAU Symposium No. 102, Solar and Stellar Magnetic Fields: Origin and Coronal Effects, J.O. Stenflo (ed.) 1983; Colloquium IAU No. 71 Activity in Red Dwarf Stars, Catania 1982]. Other reasons for the increased interest in sunspots and their energetics were prompted by the correlation between sunspot occurrence and the variations of the solar constant (Hudson et al. 1982) and by the use of sunspot positions for determining solar differential rotation and its change with latitude, depth, and time (Howard et al. 1984, Godoli S Mazzucconi 1982, Balthasar et al. 1984, Tuominen & Kyrolainen 1982, Adam 1983, Koch 1984).

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18. Stellar magnetic fields

Symposium - International Astronomical Union ◽

10.1017/s0074180900237741 ◽

1958 ◽

Vol 6 ◽

pp. 161-165

Author(s):

Horace W. Babcock

Keyword(s):

Magnetic Fields ◽

Large Amplitude ◽

Zeeman Effect ◽

Type A ◽

Magnetic Fluctuations ◽

Sharp Line ◽

Stellar Magnetic Fields ◽

Observational Program

A report is given of a ten-year observational program directed toward the discovery and investigation of the magnetic fields of stars through the Zeeman effect in their spectra. The emphasis has been on the sharp-line stars of type A, of which ten have been found to show irregular magnetic fluctuations without reversal of polarity, ten others to fluctuate irregularly with occasional reversals of polarity, and six to show essentially periodic variations; of the latter group, four are large-amplitude reversers with periods near one week.

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Measurements of stellar magnetic fields: Empirical constraints on stellar dynamo and rotational evolution theories

Cool Stars, Stellar Systems, and the Sun - Lecture Notes in Physics ◽

10.1007/3-540-18653-0_104 ◽

1987 ◽

pp. 44-46 ◽

Cited By ~ 9

Author(s):

Jeffrey L. Linsky ◽

Steven H. Saar

Keyword(s):

Magnetic Fields ◽

Rotational Evolution ◽

Stellar Magnetic Fields ◽

Stellar Dynamo

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Temporal fluctuations in the differential rotation of cool active stars

Monthly Notices of the Royal Astronomical Society ◽

10.1046/j.1365-2966.2003.07101.x ◽

2003 ◽

Vol 345 (4) ◽

pp. 1187-1199 ◽

Cited By ~ 93

Author(s):

J.- F. Donati ◽

A. Collier Cameron ◽

P. Petit

Keyword(s):

Differential Rotation ◽

Temporal Fluctuations ◽

Active Stars

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Observation of solar photospheric magnetic fields and differential rotation

EAS Publications Series ◽

10.1051/eas:2003097 ◽

2003 ◽

Vol 9 ◽

pp. 159-159

Author(s):

N. Meunier

Keyword(s):

Magnetic Fields ◽

Differential Rotation

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An Enhanced Intrinsic Time-Scale Decomposition Method Based on Adaptive Lévy Noise and Its Application in Bearing Fault Diagnosis

Symmetry ◽

10.3390/sym13040617 ◽

2021 ◽

Vol 13 (4) ◽

pp. 617

Author(s):

Jianpeng Ma ◽

Shi Zhuo ◽

Chengwei Li ◽

Liwei Zhan ◽

Guangzhu Zhang

Keyword(s):

Fault Diagnosis ◽

Time Scale ◽

Lévy Noise ◽

Model Parameters ◽

Rolling Bearings ◽

Intrinsic Time ◽

Time Scale Decomposition ◽

Weak Fault ◽

Levy Noise ◽

The Impact

When early failures in rolling bearings occur, we need to be able to extract weak fault characteristic frequencies under the influence of strong noise and then perform fault diagnosis. Therefore, a new method is proposed: complete ensemble intrinsic time-scale decomposition with adaptive Lévy noise (CEITDALN). This method solves the problem of the traditional complete ensemble intrinsic time-scale decomposition with adaptive noise (CEITDAN) method not being able to filter nonwhite noise in measured vibration signal noise. Therefore, in the method proposed in this paper, a noise model in the form of parameter-adjusted noise is used to replace traditional white noise. We used an optimization algorithm to adaptively adjust the model parameters, reducing the impact of nonwhite noise on the feature frequency extraction. The experimental results for the simulation and vibration signals of rolling bearings showed that the CEITDALN method could extract weak fault features more effectively than traditional methods.

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GNSS-to-GNSS time offsets: study on the broadcast of a common reference time

GPS Solutions ◽

10.1007/s10291-020-01082-y ◽

2021 ◽

Vol 25 (2) ◽

Author(s):

Ilaria Sesia ◽

Giovanna Signorile ◽

Tung Thanh Thai ◽

Pascale Defraigne ◽

Patrizia Tavella

Keyword(s):

Time Scales ◽

Time Scale ◽

Reference Time ◽

Single Time ◽

Common Reference ◽

Prediction Quality ◽

Common Time ◽

Time Offset ◽

The Impact ◽

Low Uncertainty

AbstractWe present two different approaches to broadcasting information to retrieve the GNSS-to-GNSS time offsets needed by users of multi-GNSS signals. Both approaches rely on the broadcast of a single time offset of each GNSS time versus one common time scale instead of broadcasting the time offsets between each of the constellation pairs. The first common time scale is the average of the GNSS time scales, and the second time scale is the prediction of UTC already broadcast by the different systems. We show that the average GNSS time scale allows the estimation of the GNSS-to-GNSS time offset at the user level with the very low uncertainty of a few nanoseconds when the receivers at both the provider and user levels are fully calibrated. The use of broadcast UTC prediction as a common time scale has a slightly larger uncertainty, which depends on the broadcast UTC prediction quality, which could be improved in the future. This study focuses on the evaluation of two different common time scales, not considering the impact of receiver calibration, at the user and provider levels, which can nevertheless have an important impact on GNSS-to-GNSS time offset estimation.

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