Semi-empirical modelling of stellar magnetic activity

AbstractSince Galileo, for four hundred years, dark spots have been observed systematically on the surface of the Sun. The monitoring of the sunspot number has shown that their number varies periodically every 11 years. This is the well-known solar activity cycle that is caused by the periodic changes of the magnetic field of the Sun. Not only do spots vary in number on a timescale of a decade, but the total luminosity and other signatures of activity such as flares and coronal mass ejections also increase and decrease with the 11-year cycle. Still unexplained to the present date are periods of decades with almost an absence of activity, where the best known example is the Maunder Minimum. Other stars also exhibit signs of cyclic activity, however the level of activity is usually thousand times higher than the solar one. Obviously, this is due to the difficulty of observing activity at the solar level on most stars. Presently, a method has been developed to detect and study individual solar like spots on the surface of planet-harbouring stars. As the planet eclipses dark patches on the surface of the star, a detectable signature can be observed in the light curve of the star during the transit. The study of a different variety of stars allows for a better understanding of magnetic cycles and the evolution of stars.

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Stellar activity and rotation of the planet host Kepler-17 from long-term space-borne photometry

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833894 ◽

2019 ◽

Vol 626 ◽

pp. A38 ◽

Cited By ~ 4

Author(s):

A. F. Lanza ◽

Y. Netto ◽

A. S. Bonomo ◽

H. Parviainen ◽

A. Valio ◽

...

Keyword(s):

Maximum Entropy ◽

Differential Rotation ◽

Activity Cycle ◽

Magnetic Activity ◽

Model Parameters ◽

The Sun ◽

Level Of Activity ◽

Spot Model ◽

Rotational Evolution ◽

Solar Type

Context. The study of young Sun-like stars is fundamental to understanding the magnetic activity and rotational evolution of the Sun. Space-borne photometry by the Kepler telescope provides unprecedented datasets to investigate these phenomena in Sun-like stars. Aims. We present a new analysis of the entire Kepler photometric time series of the moderately young Sun-like star Kepler-17 accompanied by a transiting hot Jupiter. Methods. We applied a maximum-entropy spot model to the long-cadence out-of-transit photometry of the target to derive maps of the starspot filling factor versus the longitude and the time. These maps are compared to the spots occulted during transits to validate our reconstruction and derive information on the latitudes of the starspots. Results. We find two main active longitudes on the photosphere of Kepler-17, one of which has a lifetime of at least ∼1400 days although with a varying level of activity. The latitudinal differential rotation is of solar type, that is, with the equator rotating faster than the poles. We estimate a minimum relative amplitude ΔΩ/Ω between ∼0.08 ± 0.05 and 0.14 ± 0.05, our determination being affected by the finite lifetime of individual starspots and depending on the adopted spot model parameters. We find marginal evidence of a short-term intermittent activity cycle of ∼48 days and an indication of a longer cycle of 400−600 days characterized by an equatorward migration of the mean latitude of the spots as in the Sun. The rotation of Kepler-17 is likely to be significantly affected by the tides raised by its massive close-by planet. Conclusion. We confirm the reliability of maximum-entropy spot models to map starspots in young active stars and characterize the activity and differential rotation of this young Sun-like planetary host.

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Solar-cycle irradiance variations over the last four billion years

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937128 ◽

2020 ◽

Vol 636 ◽

pp. A83 ◽

Cited By ~ 1

Author(s):

Anna V. Shapiro ◽

Alexander I. Shapiro ◽

Laurent Gizon ◽

Natalie A. Krivova ◽

Sami K. Solanki

Keyword(s):

Solar Activity ◽

Solar Cycle ◽

Solar Irradiance ◽

Activity Cycle ◽

Solar Activity Cycle ◽

Magnetic Activity ◽

The Sun ◽

Earth’S Atmosphere ◽

Solar Magnetic Activity ◽

Earth's Atmosphere

Context. The variability of the spectral solar irradiance (SSI) over the course of the 11-year solar cycle is one of the manifestations of solar magnetic activity. There is strong evidence that the SSI variability has an effect on the Earth’s atmosphere. The faster rotation of the Sun in the past lead to a more vigorous action of solar dynamo and thus potentially to larger amplitude of the SSI variability on the timescale of the solar activity cycle. This could lead to a stronger response of the Earth’s atmosphere as well as other solar system planets’ atmospheres to the solar activity cycle. Aims. We calculate the amplitude of the SSI and total solar irradiance (TSI) variability over the course of the solar activity cycle as a function of solar age. Methods. We employed the relationship between the stellar magnetic activity and the age based on observations of solar twins. Using this relation, we reconstructed solar magnetic activity and the corresponding solar disk area coverages by magnetic features (i.e., spots and faculae) over the last four billion years. These disk coverages were then used to calculate the amplitude of the solar-cycle SSI variability as a function of wavelength and solar age. Results. Our calculations show that the young Sun was significantly more variable than the present Sun. The amplitude of the solar-cycle TSI variability of the 600 Myr old Sun was about ten times larger than that of the present Sun. Furthermore, the variability of the young Sun was spot-dominated (the Sun being brighter at the activity minimum than in the maximum), that is, the Sun was overall brighter at activity minima than at maxima. The amplitude of the TSI variability decreased with solar age until it reached a minimum value at 2.8 Gyr. After this point, the TSI variability is faculae-dominated (the Sun is brighter at the activity maximum) and its amplitude increases with age.

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The Solar Orbiter Science Activity Plan

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038445 ◽

2020 ◽

Vol 642 ◽

pp. A3 ◽

Cited By ~ 13

Author(s):

I. Zouganelis ◽

A. De Groof ◽

A. P. Walsh ◽

D. R. Williams ◽

D. Müller ◽

...

Keyword(s):

Activity Cycle ◽

Solar Activity Cycle ◽

Coronal Magnetic Field ◽

Space Mission ◽

The Sun ◽

Particle Radiation ◽

Science Activity ◽

Science Operations ◽

Solar Eruptions

Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate?; (2) How do solar transients drive heliospheric variability?; (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere?; (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission’s science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit’s science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans, resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime. This allows for all four mission goals to be addressed. In this paper, we introduce Solar Orbiter’s SAP through a series of examples and the strategy being followed.

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TIME VARIATIONS IN KAMIOKANDE SOLAR NEUTRINO DATA

Modern Physics Letters A ◽

10.1142/s0217732391002165 ◽

1991 ◽

Vol 06 (22) ◽

pp. 2003-2007 ◽

Cited By ~ 8

Author(s):

PROBHAS RAYCHAUDHURI

Keyword(s):

Solar Activity ◽

Solar Neutrino ◽

Statistical Significance ◽

Neutrino Flux ◽

Activity Cycle ◽

Solar Activity Cycle ◽

Neutrino Experiment ◽

The Sun ◽

Flux Data ◽

Solar Neutrino Flux

Solar neutrino flux (Eν ≥ 7.5 MeV ) data from 1st January to April 1990 as measured in Kamiokande solar neutrino experiment have been analyzed statistically and have found that the solar neutrino data varies with the solar activity cycle with very high statistical significance (> 98% confidence level). Average solar neutrino flux data in the sunspot minimum range cannot be equal to twice the average solar neutrino flux data in the sunspot maximum range, which suggests that the neutrino flip through the magnetic field of the convection zone of the sun is not responsible for the solar neutrino flux variation. Thus the variation of solar neutrino flux with the solar activity cycle suggests that the solar activity cycle is due to the pulsating character of the nuclear energy generation inside the core of the sun.

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SOLAR NEUTRINO FLUX AND SUNSPOT DATA

Modern Physics Letters A ◽

10.1142/s0217732388001586 ◽

1988 ◽

Vol 03 (14) ◽

pp. 1319-1322 ◽

Cited By ~ 6

Author(s):

PROBHAS RAYCHAUDHURI

Keyword(s):

Solar Neutrino ◽

Nuclear Energy ◽

Neutrino Flux ◽

Activity Cycle ◽

Solar Activity Cycle ◽

The Sun ◽

Sunspot Maximum ◽

The Core ◽

Solar Neutrino Flux ◽

Sunspot Data

It is shown that the sunspot data and the solar neutrino data anticorrelates except for the period of three years after the sunspot maximum. This suggests that the solar activity cycle is due to the pulsating character of the nuclear energy generation inside the core of the sun.

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The Sun as a Pulsating Rotating Star

Symposium - International Astronomical Union ◽

10.1017/s0074180900237455 ◽

1974 ◽

Vol 59 ◽

pp. 175-175

Author(s):

Kenneth H. Schatten

Keyword(s):

Gravity Waves ◽

Nuclear Reactions ◽

Activity Cycle ◽

Radial Electric Field ◽

Solar Activity Cycle ◽

Solar Interior ◽

Alfven Wave ◽

Navier Stokes ◽

The Sun ◽

A Charge

Physical arguments are provided which suggest the following:(a) The Sun rotates rapidly internally, with a period near one day. The arguments are based upon a low ‘effective’ plasma dynamic viscosity associated with a negative ‘effective’ magnetic density, ϱM = −B2/4πν2. This low (near zero) viscosity allows several calculations of the angular velocity of the solar core to be made. A fluid dynamical argument based upon the inviscous Navier-Stokes relation shows that for objects seated in a non-expanding magnetohydrodynamic fluid, the usual Kepler law should be replaced by T2 ∝ r4.(b) The rapid rotation suggests that the solar magnetic field is deeply buried and, furthermore, that the Sun violates the Ferraro theorem. This violation results from a radial electric field necessary to support the solar plasma. This radial field requires the Sun to be charged with a charge q = + 2 × 1011 esu. Thus the Sun's differential rotation may be viewed as arising from B']θ=0 in the rest frame of the fluid where Er ≠ 0 or alternatively as a polar spin down by the solar wind flow. Thús in the fluid frame, the solar activity cycle may be viewed as an Alfvén wave, with T= Ω/vA = 20 yr. The velocity of material along the field is such that (G−1 cm−2 s−1), governed by the collapse of mass in the solar core; (4H + 4e → 1He + + 2e) results in a reduction of gas pressure unless ~7 × 1014 gm s−1 of material continually collapse so as to conserve the particle number in the solar interior.(c) This requires the Sun to emit gravity waves with an energy comparable to its luminosity. The virial of the nuclear reactions in the core governs whether the energy goes into gravity waves or heat. A variable positron-electron core allows this.

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Microwave Observations of the Sun During the 22nd Solar Activity Cycle — 17 GHz Radio Heliograph

Environmental and Space Electromagnetics ◽

10.1007/978-4-431-68162-5_34 ◽

1991 ◽

pp. 345-350 ◽

Cited By ~ 1

Author(s):

K. Shibasaki ◽

K. Kai ◽

S. Enome ◽

H. Nakajima ◽

M. Nishio ◽

...

Keyword(s):

Solar Activity ◽

Activity Cycle ◽

Solar Activity Cycle ◽

The Sun

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400 Year record of Δ14C in tree rings: The solar-activity cycle before, during and after maunder minimum and the longer cyclesin tree rings: The solar-activity cycle before, during and after maunder minimum and the longer cycles

Il Nuovo Cimento C ◽

10.1007/bf02507651 ◽

1993 ◽

Vol 16 (4) ◽

pp. 419-436 ◽

Cited By ~ 4

Author(s):

M. R. Attolini ◽

S. Cecchini ◽

M. Galli ◽

G. E. Kocharov ◽

T. Nanni

Keyword(s):

Solar Activity ◽

Tree Rings ◽

Activity Cycle ◽

Solar Activity Cycle ◽

Maunder Minimum

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Starspot rotation rates versus activity cycle phase: Butterfly diagrams of Kepler stars are unlike that of the Sun

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834373 ◽

2019 ◽

Vol 622 ◽

pp. A85 ◽

Cited By ~ 5

Author(s):

M. B. Nielsen ◽

L. Gizon ◽

R. H. Cameron ◽

M. Miesch

Keyword(s):

Effective Temperature ◽

Rotation Rate ◽

Activity Cycle ◽

Magnetic Activity ◽

Activity Level ◽

The Sun ◽

Rotation Rates ◽

Rotation Periods ◽

Activity Cycles ◽

The Mean

Context. During the solar magnetic activity cycle the emergence latitudes of sunspots change, leading to the well-known butterfly diagram. This phenomenon is poorly understood for other stars since starspot latitudes are generally unknown. The related changes in starspot rotation rates caused by latitudinal differential rotation can, however, be measured. Aims. Using the set of 3093 Kepler stars with measured activity cycles, we aim to study the temporal change in starspot rotation rates over magnetic activity cycles, and how this relates to the activity level, the mean rotation rate of the star, and its effective temperature. Methods. We measured the photometric variability as a proxy for the magnetic activity and the spot rotation rate in each quarter over the duration of the Kepler mission. We phase-folded these measurements with the cycle period. To reduce random errors, we performed averages over stars with comparable mean rotation rates and effective temperature at fixed activity-cycle phases. Results. We detect a clear correlation between the variation of activity level and the variation of the starspot rotation rate. The sign and amplitude of this correlation depends on the mean stellar rotation and – to a lesser extent – on the effective temperature. For slowly rotating stars (rotation periods between 15 − 28 days), the starspot rotation rates are clearly anti-correlated with the level of activity during the activity cycles. A transition is observed around rotation periods of 10 − 15 days, where stars with an effective temperature above 4200 K instead show positive correlation. Conclusions. Our measurements can be interpreted in terms of a stellar “butterfly diagram”, but these appear different from that of the Sun since the starspot rotation rates are either in phase or anti-phase with the activity level. Alternatively, the activity cycle periods observed by Kepler are short (around 2.5 years) and may therefore be secondary cycles, perhaps analogous to the solar quasi-biennial oscillations.

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