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Final Report Summary - ASTRA (Understanding the variability of solar And STellar RAdiative fluxes)

Since the launch of the NIMBUS 7 mission in 1978 we know that the Total Solar Irradiance (TSI, which is the spectrally integrated solar radiative flux at one Astronomical Unit from the Sun), previously known as the solar constant, is not really a constant but instead varies on different time scales.

The interest in solar irradiance variability is by far not limited to the solar community. It has been suggested that the terrestrial climate responds to the decadal variations in solar irradiance and there is also evidence for a longer term influence of solar activity on climate. The variability of solar irradiance is also of high importance for stellar astronomers, who have been comparing it with the variability of other lower main sequence stars. The interest in solar-stellar comparison has been recently rekindled by the unprecedented precision of broadband stellar photometry achieved with the launch of the Kepler and Corot space missions.

A number of models of solar irradiance variability have been created over the last decade. One of the most successful and widely used models is SATIRE (Spectral And Total Irradiance Reconstruction) developed by the Sun-climate group at MPS (host group). In this context, the two main objectives of ASTRA have been:

1. Performing a major upgrade of the SATIRE model of solar irradiance variability;
2. Applying the SATIRE model for understanding variability of stellar brightness.

Following work has been performed during ASTRA:

1. The NLTE (non-local thermodynamic equilibrium) spectral synthesis code NESSY was prepared for calculations of spectra emerging from the quiet Sun and from magnetic features. NESSY is a new version of the COde for SOLAR Irradiance (COSI) whose development was finalised during ASTRA by the researcher and his PhD-student. NESSY is especially suitable for calculating UV irradiance, which justifies its utilisation in ASTRA. The code is descriped in Tagirov et al. 2017. NESSY output has been tested against available measurements (Thuillier et al. 2015,Thuillier et al. 2017).

2. NESSY calculations have been implemented into SATIRE. SATIRE-NESSY irradiance reconstruction has been published in Shapiro et al. 2015. NESSY-SATIRE code has been run in various regimes to pinpoint the spectral features responsible for solar irradiance variability.

3. We employed the high-cadence solar imagery from the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory and SATIRE to recreate the magnetic component of TSI variability on timescale shorter than one day. Recent 3D simulations of solar near-surface convection with MURAM code have been used to calculate the TSI variability caused by convection. This allowed us to compute solar irradiance variability on timescales from minutes to decades. The results are currently being prepared for publication.

4. We have calculated solar brightness variability as it would be seen out of ecliptic. For this we took coverages as seen by an Earth-based observer from full-disc SoHO/MDI and SDO/HMI data and projected them to mimic out-of-ecliptic viewing by an appropriate transformation. The results of this work have been published in Shapiro et al. (2016).

5. We have simulated a magnetically active Sun by filling its surface with an increasing fraction of magnetic features. This allowed us to calculate the variability of stars more active than the Sun. The work is currently in progress.

Main results obtained during the project:

1. New SATIRE-NESSY reconstruction of solar irradiance has been obtained. The reconstruction is free of empirical corrections applied to previous versions of SATIRE;

2. Using the SATIRE-NESSY reconstruction we have shown that the solar irradiance variability in the UV, violet, blue, and green spectral domains is fully controlled by the Fraunhofer lines. The highest peak in absolute SSI variability on timescales from day to decades is associated with the CN violet system between 380 and 390 nm. A quarter of the TSI variability on the 11-year timescale originates in molecular lines;

3. We have demonstrated that solar magnetism and convection can account for TSI variability at all timescales it has ever been measured (sans the 5-minute oscillations from p-modes). We have determined the threshold timescale between TSI variability caused by the magnetic field and by granulation.

4. We have shown that the solar variability measured in Strömgren filters b and y is lower by a factor of three than was thought before. Consequently, most of the Sun-like stars with near-solar levels of activity have photometric variabilities that are significantly larger than solar variability. This, however, does not necessarily imply that the Sun is anomalous with respect to its stellar cohort. We proposed that the low solar variability might be attributed to incidental combination of solar fundamental parameters and location of the Strömgren b and y passbands. In particular, we have shown that a 0.3 dex change of the metallicity can increase solar variability measured in Strömgren filters b and y by almost three times.

5. We have calculated solar brightness variability for the out-of- ecliptic viewing. This allowed us to identify the main drivers of the brightness variations of a star identical to the Sun as observed by ground-based or spaceborne telescopes. In particular, we have shown that rotational solar brightness variability as it would appear in the Kepler and CoRoT passbands from the ecliptic plane is spot-dominated, but that the relative contribution of faculae increases for out-of-ecliptic viewing so that the apparent brightness variations are faculae-dominated for inclinations less than about i = 45⁰.
Over the course of the 11-year activity cycle, the solar brightness variability is faculae-dominated shortwards of 1.2 mm independently of the inclination.

References:

Tagirov et al., 2017, Astron. Astroph., recommended for publication after revision
Thuillier et al., 2015, Sol. Phys., 290., 6., 1581
Thuillier et al., 2017., Astron. Astroph., recommended for publication in after revision
Shapiro et al., 2015, Astron. Astroph., 581, A116
Shapiro et al., 2016, Astron. Astroph., 589, A46

Reported by

MAX-PLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSCHAFTEN EV
Germany
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