scholarly journals Power spectra of solar brightness variations at various inclinations

2020 ◽  
Vol 636 ◽  
pp. A43 ◽  
Author(s):  
N.-E. Nèmec ◽  
A. I. Shapiro ◽  
N. A. Krivova ◽  
S. K. Solanki ◽  
R. V. Tagirov ◽  
...  
Keyword(s):  
Transport Model ◽  
Solar Rotation ◽  
Power Spectra ◽  
Surface Flux ◽  
The Sun ◽  
Space Telescopes ◽  
Flux Transport ◽  
Total Irradiance ◽  
Solar Brightness ◽  
Magnetic Features

Context. Magnetic features on the surfaces of cool stars lead to variations in their brightness. Such variations on the surface of the Sun have been studied extensively. Recent planet-hunting space telescopes have made it possible to measure brightness variations in hundred thousands of other stars. The new data may undermine the validity of setting the sun as a typical example of a variable star. Putting solar variability into the stellar context suffers, however, from a bias resulting from solar observations being carried out from its near-equatorial plane, whereas stars are generally observed at all possible inclinations. Aims. We model solar brightness variations at timescales from days to years as they would be observed at different inclinations. In particular, we consider the effect of the inclination on the power spectrum of solar brightness variations. The variations are calculated in several passbands that are routinely used for stellar measurements. Methods. We employ the surface flux transport model to simulate the time-dependent spatial distribution of magnetic features on both the near and far sides of the Sun. This distribution is then used to calculate solar brightness variations following the Spectral And Total Irradiance REconstruction approach. Results. We have quantified the effect of the inclination on solar brightness variability at timescales down to a single day. Thus, our results allow for solar brightness records to be made directly comparable to those obtained by planet-hunting space telescopes. Furthermore, we decompose solar brightness variations into components originating from the solar rotation and from the evolution of magnetic features.

scholarly journals Connecting measurements of solar and stellar brightness variations

2020 ◽  
Vol 638 ◽  
pp. A56
Author(s):  
N.-E. Nèmec ◽  
E. Işık ◽  
A. I. Shapiro ◽  
S. K. Solanki ◽  
N. A. Krivova ◽  
...  
Keyword(s):  
Transport Model ◽  
Rotation Axis ◽  
Surface Flux ◽  
Light Curves ◽  
The Sun ◽  
Solar Cycles ◽  
Stellar Rotation ◽  
Flux Transport ◽  
Surface Distribution ◽  
The Difference

Context. A comparison of solar and stellar brightness variations is hampered by the difference in spectral passbands that are used in observations, and also by the possible difference in the inclination of the solar and stellar rotation axes from the line of sight. Aims. We calculate the rotational variability of the Sun as it would be measured in passbands used for stellar observations. In particular, we consider the filter systems used by the CoRoT, Kepler, TESS, and Gaia space missions. We also quantify the effect of the inclination of the rotation axis on the solar rotational variability. Methods. We employed the spectral and total irradiance reconstruction (SATIRE) model to calculate solar brightness variations in different filter systems as observed from the ecliptic plane. We then combined the simulations of the surface distribution of the magnetic features at different inclinations using a surface flux transport model with the SATIRE calculations to compute the dependence of the variability on the inclination. Results. For an ecliptic-bound observer, the amplitude of the solar rotational variability, as observed in the total solar irradiance (TSI), is 0.68 mmag (averaged over solar cycles 21–24). We obtained corresponding amplitudes in the Kepler (0.74 mmag), CoRoT (0.73 mmag), TESS (0.62 mmag), Gaia G (0.74 mmag), Gaia GRP (0.62 mmag), and Gaia GBP (0.86 mmag) passbands. Decreasing the inclination of the rotation axis decreases the rotational variability. For a sample of randomly inclined stars, the variability is on average 15% lower in all filter systems we considered. This almost compensates for the difference in amplitudes of the variability in TSI and Kepler passbands, making the amplitudes derived from the TSI records an ideal representation of the solar rotational variability for comparison to Kepler stars with unknown inclinations. Conclusions. The TSI appears to be a relatively good measure of solar variability for comparisons with stellar measurements in the CoRoT, Kepler, TESS Gaia G, and Gaia GRP filters. Whereas the correction factors can be used to convert the variability amplitude from solar measurements into the values expected for stellar missions, the inclination affects the shapes of the light curves so that a much more sophisticated correction than simple scaling is needed to obtain light curves out of the ecliptic for the Sun.

scholarly journals Hemispheric injection of magnetic helicity by surface flux transport

2019 ◽  
Vol 631 ◽  
pp. A138 ◽  
Author(s):  
G. Hawkes ◽  
A. R. Yeates
Keyword(s):  
Magnetic Field ◽  
Solar Cycle ◽  
Solar Rotation ◽  
Surface Flux ◽  
Magnetic Helicity ◽  
Solar Cycles ◽  
Flux Transport ◽  
Transport Models ◽  
Radial Magnetic Field ◽  
Helicity Injection

Aims. We estimate the injection of relative magnetic helicity into the solar atmosphere by surface flux transport over 27 solar cycles (1700–2009). Methods. We determine the radial magnetic field evolution using two separate surface flux transport models: one driven by magnetogram inputs and another by statistical active region insertion guided by the sunspot number record. The injection of relative magnetic helicity is then computed from this radial magnetic field together with the known electric field in the flux transport models. Results. Neglecting flux emergence, solar rotation is the dominant contributor to the helicity injection. At high latitudes, the injection is always negative/positive in the northern/southern hemisphere, while at low latitudes the injection tends to have the opposite sign when integrated over the full solar cycle. The overall helicity injection in a given solar cycle depends on the balance between these two contributions. This net injected helicity correlates well with the end-of-cycle axial dipole moment.

Solar and Climate Changes

1997 ◽  
Author(s):  
Douglas V. Hoyt ◽  
Kenneth H. Shatten
Keyword(s):  
Solar Activity ◽  
Time Scales ◽  
Solar Rotation ◽  
Sunspot Cycle ◽  
Climatic Changes ◽  
Decay Rates ◽  
Spectral Irradiance ◽  
The Sun ◽  
Total Irradiance ◽  
Earth’S Climate

Until now we have considered only 11-year variations in solar activity and climate. The sun also varies on longer time scales. Since these variations seem to parallel a number of climatic changes, the sun may contribute to climatic changes on time scales of decades to centuries. We now examine several solar indices that vary in parallel with Earth’s climate change. There exist plausible arguments that these indices are proxy indicators of the sun’s radiative output, but there is no proof. We now present the strongest correlations we have seen for a sun/climate connection. First, as it is the most widely publicized index, we consider the mean level of solar activity. In 1801 Herschel first proposed a relationship between climate and the level of solar activity. Second, we examine solar cycle lengths, which have been studied sporadically since 1905. Third, we look at two closely related indices—sunspot structure and sunspot decay rates. Fourth, we consider variations in the solar rotation rate. Lastly, we examine some major solar and climatic events of the last thousand years to see if any indications of solar influence are evident on climate. Although we present the solar-induced changes as arising from total-irradiance variations, as discussed earlier spectral-irradiance changes may be the primary driver. When Rudolf Wolf reconstructed solar activity based on historical observations of sunspots, he found an 11-year cycle going back to at least 1700. In 1853 Wolf also claimed that there is an 83-year sunspot cycle. This longer term variation becomes evident simply by smoothing the data, as in Socher’s 1939 example. Wolf’s original discovery of an 83-year cycle was forgotten, but the long cycle was rediscovered by H. H. Turner, W. Schmidt, H. H. Clayton, and probably others. After W. Gleissberg also discovered this 80- to 90-year cycle around 1938, he published so much material on the subject that ever since it has been called the Gleissberg cycle. All these rediscoveries of the same phenomenon indicate that the 80- to 90-year cycle may be real but not strictly periodic. Rather, the cycle may be a “persistency” with an 80- to 90-year period. During this period solar activity is quite powerful but fails to exhibit a single sharp spectral peak.

scholarly journals How Good Is the Bipolar Approximation of Active Regions for Surface Flux Transport?

Solar Physics ◽  
2020 ◽  
Vol 295 (9) ◽  
Author(s):  
Anthony R. Yeates
Keyword(s):  
Dipole Moment ◽  
Active Region ◽  
Transport Model ◽  
Active Regions ◽  
Surface Flux ◽  
Flux Transport ◽  
Transport Models ◽  
Magnetic Structures ◽  
Axial Dipole ◽  
Flow Parameters

Abstract We investigate how representing active regions with bipolar magnetic regions (BMRs) affects the end-of-cycle polar field predicted by the surface flux transport model. Our study is based on a new database of BMRs derived from the SDO/HMI active region patch data between 2010 and 2020. An automated code is developed for fitting each active region patch with a BMR, matching both the magnetic flux and axial dipole moment of the region and removing repeat observations of the same region. By comparing the predicted evolution of each of the 1090 BMRs with the predicted evolution of their original active region patches, we show that the bipolar approximation leads to a 24% overestimate of the net axial dipole moment, given the same flow parameters. This is caused by neglecting the more complex multipolar and/or asymmetric magnetic structures of many of the real active regions, and may explain why previous flux transport models had to reduce BMR tilt angles to obtain realistic polar fields. Our BMR database and the Python code to extract it are freely available.

scholarly journals Inflection point in the power spectrum of stellar brightness variations

2020 ◽  
Vol 633 ◽  
pp. A32 ◽  
Author(s):  
A. I. Shapiro ◽  
E. M. Amazo-Gómez ◽  
N. A. Krivova ◽  
S. K. Solanki
Keyword(s):  
Power Spectrum ◽  
Inflection Point ◽  
Rotation Period ◽  
Power Spectra ◽  
Light Curves ◽  
Stellar Rotation ◽  
Space Telescopes ◽  
Rotation Periods ◽  
Active Stars ◽  
Total Irradiance

Context. Considerable effort has gone into using light curves observed by such space telescopes as CoRoT, Kepler, and TESS for determining stellar rotation periods. While rotation periods of active stars can be reliably determined, the light curves of many older and less active stars, such as stars that are similar to the Sun, are quite irregular. This hampers the determination of their rotation periods. Aims. We aim to examine the factors causing these irregularities in stellar brightness variations and to develop a method for determining rotation periods for low-activity stars with irregular light curves. Methods. We extended the Spectral And Total Irradiance Reconstruction approach for modeling solar brightness variations to Sun-like stars. We calculated the power spectra of stellar brightness variations for various combinations of parameters that define the surface configuration and evolution of stellar magnetic features. Results. The short lifetime of spots in comparison to the stellar rotation period, as well as the interplay between spot and facular contributions to brightness variations of stars with near solar activity, cause irregularities in their light curves. The power spectra of such stars often lack a peak associated with the rotation period. Nevertheless, the rotation period can still be determined by measuring the period where the concavity of the power spectrum plotted in the log–log scale changes its sign, that is, by identifying the position of the inflection point. Conclusions. The inflection point of the (log–log) power spectrum is found to be a new diagnostic for stellar rotation periods which is shown to work even in cases where the power spectrum shows no peak at the rotation rate.

scholarly journals Dipolar and Quadrupolar Magnetic Field Evolution over Solar Cycles 21, 22, and 23

2010 ◽  
Vol 6 (S271) ◽  
pp. 94-101 ◽  
Author(s):  
M. L. DeRosa ◽  
A. S. Brun ◽  
J. T. Hoeksema
Keyword(s):  
Magnetic Field ◽  
Spherical Harmonic ◽  
Transport Model ◽  
Activity Cycle ◽  
Surface Flux ◽  
Solar Activity Cycle ◽  
Solar Photosphere ◽  
Solar Cycles ◽  
Flux Transport ◽  
The Past

AbstractTime series of photospheric magnetic field maps from two observatories, along with data from an evolving surface-flux transport model, are decomposed into their constituent spherical harmonic modes. The evolution of these spherical harmonic spectra reflect the modulation of bipole emergence rates through the solar activity cycle, and the subsequent dispersal, shear, and advection of magnetic flux patterns across the solar photosphere. In this article, we discuss the evolution of the dipolar and quadrupolar modes throughout the past three solar cycles (Cycles 21–23), as well as their relation to the reversal of the polar dipole during each solar maximum, and by extension to aspects of the operation of the global solar dynamo.

scholarly journals Simulations of solar wind variations during an 11-year cycle and the influence of north–south asymmetry

2018 ◽  
Vol 84 (5) ◽  
Author(s):  
B. Perri ◽  
A. S. Brun ◽  
V. Réville ◽  
A. Strugarek
Keyword(s):  
Solar Wind ◽  
Mass Loss ◽  
Loss Rate ◽  
Transport Model ◽  
Magnetic Energy ◽  
Mass Loss Rate ◽  
Phase Lag ◽  
The Sun ◽  
Flux Transport ◽  
South Asymmetry

We want to study the connections between the magnetic field generated inside the Sun and the solar wind impacting Earth, especially the influence of north–south asymmetry on the magnetic and velocity fields. We study a solar-like 11-year cycle in a quasi-static way: an asymmetric dynamo field is generated through a 2.5-dimensional (2.5-D) flux-transport model with the Babcock–Leighton mechanism, and then is used as bottom boundary condition for compressible 2.5-D simulations of the solar wind. We recover solar values for the mass loss rate, the spin-down time scale and the Alfvén radius, and are able to reproduce the observed delay in latitudinal variations of the wind and the general wind structure observed for the Sun. We show that the phase lag between the energy of the dipole component and the total surface magnetic energy has a strong influence on the amplitude of the variations of global quantities. We show in particular that the magnetic torque variations can be linked to topological variations during a magnetic cycle, while variations in the mass loss rate appear to be driven by variations of the magnetic energy.

scholarly journals The Large-scale Coronal Structure of the 2017 August 21 Great American Eclipse: An Assessment of Solar Surface Flux Transport Model Enabled Predictions and Observations

2018 ◽  
Vol 853 (1) ◽  
pp. 72 ◽  
Author(s):  
Dibyendu Nandy ◽  
Prantika Bhowmik ◽  
Anthony R. Yeates ◽  
Suman Panda ◽  
Rajashik Tarafder ◽  
...  
Keyword(s):  
Large Scale ◽  
Transport Model ◽  
Solar Surface ◽  
Surface Flux ◽  
Coronal Structure ◽  
Flux Transport

Large-scale transport of solar and stellar magnetic flux

2018 ◽  
Vol 14 (A30) ◽  
pp. 347-350
Author(s):  
Emre Işık
Keyword(s):  
Magnetic Fields ◽  
Magnetic Flux ◽  
Large Scale ◽  
Surface Flux ◽  
Current Understanding ◽  
Solar Photosphere ◽  
The Sun ◽  
Forward Modelling ◽  
Flux Transport ◽  
Brightness Variability

AbstractSurface flux transport (SFT) models have been successful in reproducing how magnetic flux at the solar photosphere evolves on large scales. SFT modelling proved to be useful in reconstructing secular irradiance variations of the Sun, and it can be potentially used in forward modelling of brightness variations of Sun-like stars. We outline our current understanding of solar and stellar SFT processes, and suggest that nesting of activity can play an important role in shaping large-scale patterns of magnetic fields and brightness variability.

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