Focus Meeting 9 - Abstracts
Magnetic Features as Drivers of Solar Brightness Variability
Solanki, Sami
Many sources have been proposed for the variable solar Irradiance. Over the years there have been increasing hints that the main cause of irradiance variability at solar rotation and solar cycle timescales is the magnetic features at the solar surface, while at shorter timescales other sources, such as convection are likely to become important as well. However, no final answer could be provided, given the empirical nature of the models of solar irradiance variations. Recently, the use of state-of-the-art 3D MHD simulations incorporating as much physics as possible, to reconstruct solar irradiance variations has introduced a new rigour and has finally provided unambiguous answers for these long-standing questions.
Atomic and Molecular Data Collections for Fine Spectral Analysis and Flux Calculations.
Ryabchikova, Tatiana
Spectral lines provide a substantial contribution to the total absorption of radiation emitted by stars. Therefore improvements to the line data are crucial for realistic reproduction of stellar irradiation. We present a review of the latest experimental and theoretical atomic and molecular data, which are used for fine analysis of stellar and solar spectra as well as for irradiation/flux calculations. In particular, we focus on atomic data for the Fe-peak elements and on transition data for CN and CH molecules that are important for opacity calculations. We compare recent experimental transition probabilities of Fe I lines with the theoretical calculations based on extended set of energy levels and found no significant difference in absolute scales of both sets of values over the wavelength range of 2000 -- 10000 Å. Continuing work on the energy structure of the Iron atom by Peterson et al. (2017) has resulted in measuring and classifying more than 100 high-lying energy levels that helped to identify more than 1000 previously unknown spectral features in the solar and metal-deficient stellar spectra. The average uncertainty of the theoretical data is still as large as ±0.35 dex in logarithmic scale but we see continual and quick improvement in both quality and completeness of the data. We also emphasise the importance of the collisional data in irradiation calculations for the Sun and solar-like stars. Finally, we present a short overview of the most important atomic and molecular databases.
The Influence of Metallicity on Stellar Differential Rotation and Magnetic Activity
Karoff, Christoffer
Over an 11-year cycle the Sun changes its brightness by less than 0.1%, it is however, an open question how strong the Sun's photometric variability was in the distant past. One way to answer this question is to study other Sun-like stars and compare their photometric variability with that of the Sun. Ground-based spectroscopic observations of a 7.4-year cycle in the solar analog HD 173701 complemented with observations from the NASA Kepler space telescope constitute one of the most complete set of observations of a stellar cycle ever obtained for any Sun-like star. These observations thus provide an important inseight in how dynamos work in other Sun-like stars.
Solar Irradiance: from multiple observations to a single composite dataset
Dudok de Wit, Thierry
The solar electromagnetic spectrum and its evolution in time are paramount for understanding solar variability and for quantifying its impact on the Earth’s atmosphere. Even when physics-based irradiance models are continuously growing in maturity, solar irradiance observations remain indispensable.Several instruments have been monitoring the spectrally-resolved (and also the total solar irradiance) since the late 1980’s, mainly in the UV, but also in the visible. Despite being fragmented in time and in wavelength, these observations have deeply impacted our perception of how the Sun varies in time. Today, with the growing demand for understanding how the Sun varies on multi-decadal time scales, and a recent deep solar minimum, there is a clear need for investigating the presence of trends in these observations. Unfortunately, making radiometrically-stable observations is a real challenge; simultaneous observations from different instruments often disagree, which can lead to potentially very different long-term evolutions.Recently, several international teams have tackled this problem by merging individual observations into one single homogeneous composite dataset that would ease their scientific analysis and in particular offer the possibility to explore longer time scales. Here we address these different steps going from individual observations, their inter-calibration, to their fusion into one single composite, with its uncertainties and the critical issues.
The Solar-Stellar Dynamo-Irradiance Connection: What Stellar Lightcurves Tell Us About How the Dynamo Works
Egeland, Ricky
The Sun is the best observed object in all of astronomy. Its proximity allows us to see the patterns of its well-ordered magneticcycle in great detail, at least on the surface. The problem, of course, is that the magnetic dynamo producing the large-scale magnetic field operates beneath the surface, hiding the dynamo processes from view and allowing models a great deal of flexibility. Solar and stellar astronomers have long believed that observing the long-term variability of Sun-like stars might allow us to better constrain the physics of the dynamo by relating observational effects of large-scale magnetism to fundamental properties of the star. Among these effects are small variations of flux in medium and wide-band visible photometry typically obtained in stellar astronomy. Such variability is correlated to more effective proxies of stellar magnetism in interesting ways that can provide discerning tests for physical models of irradiance. In this talk I will review the current observational constraints stellar observations impose on dynamo theory and our understanding of stellar photometric variability from long-term programs such as the Fairborn Observatory Automated Photometric Telescopes and the shorter-term Kepler observations. Finally, we will discuss the potential for new and upcoming programs like TESS, PLATO,and LSST to enable much larger statistical studies on the relationship between irradiance variability and fundamental stellar properties.
Calculations of the solar convection zone with the reduced speed of sound technique
Hotta, Hideyuki
We carry out high-resolution and high-density-contrast calculations with using the Reduced Speed of Sound Technique (RSST). The solar convection zone is filled with highly turbulent thermal convection that is responsible for the energy transport, the angular momentum transport, and the dynamo(s) responsible for the Sun’s small and large scale magnetic field. Modeling the convection zone as a whole is challenging due to the strong density stratification leading to a vast separation of length- and time-scales. In order to cope with this challenge, we adopt the new technique, the RSST, in which the effective speed of sound is reduced with changing the equation of continuity. Besides allowing for significantly larger timesteps, RSST also scales well on massively parallel supercomputers since only local communication is required. High-resolution calculations performed with this method reveal that small-scale magnetic field present in the convection zone has a profound influence on the dynamo action generating the large-scale magnetic field. While previous studies found that coherent large scale field is difficult to maintain at high resolution due to a large degree of small-scale turbulence, we find that small-scale magnetic field suppresses small-scale turbulence and allows for the maintenance of a coherent large-scale magnetic field.In addition, another advantage of the RSST is ability to include the photosphere in deep convection zone calculations. Since the temporal and spatial scales of convection drastically change due to the high density and temperature contrast in the convection zone, there has been no calculation covering whole convection zone. We, for the first time, carry out such a calculation from the base of the convection zone to the solar surface with including the realistic radiation transfer. Detailed influences of the surface region on the deep convection zone are investigated.
Solar Irradiance for Climate Studies
Schmidt, Hauke
There is broad scientific consensus that at least the global warming observed since the 1950s is mainly anthropogenic. However, the contribution of natural forcings (of both solar and volcanic origin) to the evolution of global climate is still poorly quantified, and the uncertainty is considerably larger for periods further back in time. Moreover, regional influences of solar variability may be considerable even on the decadal time scale.In this presentation I will review how the representation of solar irradiance variability in climate models has evolved in the recent decades. Furthermore, I will summarize the current state of knowledge on influences of solar variability on climate, distinguishing in particular the so-called bottom-up and top-down mechanisms. While the further are caused by variability of total solar irradiance (TSI) acting at the Earth’s surface, the latter are caused by changes in solar spectral irradiance (SSI) acting mostly in the Earth’s middle atmosphere but are potentially communicated downward via changes in atmospheric chemistry and dynamics. I will point out how uncertainty in SSI influences the simulated atmospheric response and discuss which role limited understanding of processes in the Earth’s climate system plays for understanding solar-terrestrial effects.
Brightness Variations of Sun-Like Stars
Reiners, Ansgar
Observations of stars other than the Sun provide information that are complementary to long-term solar datasets. They show whether solar variability is typical for a star like the Sun and how the Sun might have looked like at other times. The talk provides a summary about what is known about variability from sun-like stars and how we can use this data to inform our understanding of solar and stellar physics.
Non-Equilibrium Spectrum Formation affecting Solar Irradiance
RUTTEN, Robert
The theory of solar spectrum formation matured already during the1970s and remains the part of solar physics that is furthest advancedtowards full understanding. Nevertheless, its complexities affectingthe solar irradiance spectrum represent non-trivial or even horrendousobstacles to detailed modeling. For example, modeling of opticalirradiance requires detailed modeling of the ultraviolet, and thatrequires detailed treatment of the millions of spectral linesconstituting the line haze. In the ultraviolet the mechanisms bywhich uibquitous magnetic concentrations constituting network andplage, key irradiance factors, become bright points are not wellstudied yet.I will start with a tutorial overview of spectrum formation conditions(LTE => non-LTE => non-E) affecting solar continua and lines, and thenconcentrate on the complexities and difficulties of particularimportance to irradiance modeling.
Large-scale transport of solar magnetic flux
Isik, Emre
Synoptic observations of the solar surface reveal the key role of bipolar magnetic regions (BMRs) in shaping the solar cycle. As magnetic structures encompassing dark spots and bright faculae, BMRs are subject to the surface flux transport (SFT) process. SFT models provide us with significant insight in our understanding of the solar cycle, in particular when constraining dynamo models. Observational constraints and recent progress in our theoretical understanding favour a Babcock-Leighton type dynamo, which is not far from critical excitation, involving relatively weak nonlinear effects. In addition, observations and models show that the solar cycle is rather sensitive to rare BMRs with extreme properties, which can potentially trigger grand minima and recover normal cycles. The emergence properties of sunspot groups can be reproduced by simulations of flux tubes rising from the convection zone. When the Sun was younger and more rapidly rotating, its surface patterns of activity were likely affected by stronger dynamo action and stronger inertial effects in flux emergence. Forward modelling the emergence and surface transport of magnetic flux is thus crucial in our quest to reverse-model the observed brightness variations of Sun-like stars.
The Build-up of the Sun’s Polar Magnetic Field due to Flux Transport from Decaying Active Regions
Choudhuri, Arnab
The polar magnetic field of the Sun at the end of a solar cycle is an important precursor for the next cycle. The build-up of this polar field by the transport of magnetic flux from decaying active regions at low solar latitudes is an important process to understand the solar cycle. Remarkable progress has been made by the surface flux transport (SFT) model in studying this process. However, some limitations of the SFT model are that the vectorial nature of the magnetic field is not incorporated and the submergence of the meridional circulation underneath the surface at the polar region is not taken into account. These limitations can be overcome in a 3D kinematic dynamo model. Recent results obtained with a 3D kinematic dynamo model (Hazra, Choudhuri & Miesch, 2017, ApJ 835:39) show that the evolution of the magnetic field at the polar region is not handled realistically in the SFT model. Our calculations throw important light on the role of anti-Hale active regions in building up the polar field.
Origin and recovery from grand solar minima in a time delay dynamo model with magnetic noise as an additional poloidal source
Tripathi, Bindesh
We explore a reduced Babcock-Leighton (BL) dynamo model based on delay differential equations using numerical bifurcation analysis. This model reveals hysteresis, seen in the recent mean-field dynamo model and the direct numerical simulations of turbulent dynamos. The BL model with 'magnetic noise' as an additional weak-source of the poloidal field recovers the solar cycle every time from grand minima, which BL source alone cannot do. The noise-incorporated model exhibits a bimodal distribution of toroidal field energy confirming two modes of solar activity. It also shows intermittency and reproduces phase space collapse, an experimental signature of the Maunder Minimum. The occurrence statistics of grand minima in our model agree reasonably well with the observed statistics in the reconstructed sunspot number. Finally, we demonstrate that the level of magnetic noise controls the duration of grand minima and even has a handle over its waiting period, suggesting a triggering effect of grand minima by the noise and thus shutting down the global dynamo. Therefore, we conclude that the 'magnetic noise' due to small-scale turbulent dynamo action (or other sources) plays a vital role even in Babcock-Leighton dynamo models. Thus a serious concern should be given to understand the small-scale turbulent dynamo action, which can help us in predicting the Maunder minimum-like episodes and especially in determining such episodes' duration, along with having an understanding of the associated changes in the solar irradiance.
Solar Cycle 24: Accurate, Long-Term Solar Spectral Irradiance Measurements from Aura OMI
Martchenko, Serguei
As part of its mission to provide global atmospheric trace-gas measurements since 2004, the Earth-observing Ozone Monitoring Instrument (OMI, on board the Aura spacecraft) acquires daily, moderate-resolution (~0.5 nm) solar irradiance data in the 264-504 nm spectral range. Comprehensive performance evaluations invariably attest to a remarkable OMI stability over the mission lifetime: gradual throughput loss of only ~4%-10%, and small wavelength registration drifts (in general much less than 0.02 nm). The slow, regular instrument changes can be characterized very thoroughly, leading to a solar spectral irradiance (SSI) record of exceptional accuracy, with long-term (solar cycle) uncertainties reduced to ~0.1% in the mid-UV and to ~0.05% in the visible region. We compare OMI observations with various missions, composite data sets, and semi-empirical solar models that are available during solar cycle 24. Wavelength-binned daily OMI measurements demonstrate coherent SSI changes down to the ~0.05% accuracy level. We examine short- and long-term variability in various absorption features (e.g., Mg II, Ca II, Fe I and II blends, Hß) as seen in OMI data. We also demonstrate the gradual transition from the faculae- to sunspot-dominated SSI variability.
Solar Irradiance Reconstruction over the Last 9 Millennia with a Multi-isotope Composite
Wu, Chi-Ju
While direct measurements of the solar irradiance are available for the last four decades, reconstructions of the past solar variability are needed to understand the solar influence on Earth's climate. The longest observational record of solar activity is the sunspot number, going back to 1610 A.D. with uneven quality. To assess solar variability at earlier times, indirect proxies of solar activity, such as concentrations of cosmogenic isotopes 10Be and 14C in terrestrial archives are often used. These isotopes are produced in the terrestrial atmosphere by impinging cosmic rays, whose flux is modulated by both heliospheric magnetic field and geomagnetic field. Therefore, the isotope signals retrieved from various sites around the globe show a very high degree of similarity, reflecting changes in the solar activity. Significant short-and mid-term deviations, however, can be observed due to the different geochemical production and redistribution processes and local climatic conditions. We have taken this into account and developed a state-of-the-art consistent multi-isotope composite reconstruction of solar activity based on six regional 10Be and the global 14C data sets. This composite is then used to reconstruct the total TSI and spectral SSI solar irradiance over the last 9000 years with a semi-empirical model (SATIRE-M) developed in MPS. We also apply statistical analysis to reconstruct the pseudo-solar cycle on millennial time scale.
Observations of UV contrasts and their impacts on the solar irradiance models
Gravet, Romaric
Understanding solar irradiance variations, in particular in the ultraviolet wavelength range, is essential for climate modelling and for space weather. Solar irradiance models are precious for reconstructing the solar spectral irradiance in the absence of observations or when the latter lack stability. However, they come with their assumptions. Here we aim at constraining these by characterising the UV contrast of solar magnetic features at different wavelengths. We consider solar images taken by the Solar Dynamics Observatory between 2010 and 2016. From these we extract the contrast, which we study as a function of position, magnetic field strength and time. As expected, the contrasts in the UV are stronger than those in the visible but we quantify this for the first time and find that the contrast variations are in agreement with the magnetic flux tube picture. The 160nm and 170nm channels show a very similar behaviour; the contrast at 160nm is higher than the one at 170nm, presumably because of the emission of the CIV line from the transition region. The study of solar structures and their segmentation shows that photometric thresholds, both in visible and UV, are necessary to properly segment solar structures because of the coexistence of both dark and bright structures for the same value of the magnetic field. Some pixels that are classified as quiet-Sun by the SATIRE-S model actually belong to faculae, but they are too few to have a significant impact on irradiance reconstructions. Our results highlight the importance of multi-wavelength observations for better constraining the identification of structures. Distinguishing network and faculae is essential for reconstructing the SSI over long time scale, and taking into account for the dependence of the network's contrast with the magnetic field while other structures have a constant contrast improves the SSI reconstruction. Finally, we find no observational evidence for solar cycle variations in the contrast.
Assessing an uncertainty of the Total Solar Irradiance composite
Schmutz, Werner
The TSI instrument PREMOS on the French satellite PICARD was operational from July 27, 2010 until March 4, 2014. Recently, we have re-evaluated the PREMOS data reduction and released version 2 of its TSI time series. The presentation will present the specific problems of PREMOS in establishing its long-term stability. Different solutions for assessing the sensitivity change of the two PREMOS TSI channels give an estimate of the uncertainty of the PREMOS long-term stability. This allows to compare PREMOS to the three other space radiometers PMO6/VIRGO and DIARAD/VIRGO on SOHO, ACRIM III on ACRIM-SAT, and TIM on SORCE, Combining the four different TSI time series allows to construct empirically a TSI composite over the almost four years when four instruments have been in operation simultaneously. Comparing the individual instruments time series to the composite then allows to get an estimate for each instrument's long-term stability and to extrapolate an uncertainty for the long-term stability over the instruments operational time. This extrapolation then gives an empirical estimate for the TSI composite from 1996 to today.
Using MHD simulations of the solar atmosphere to reproduce irradiance variations: challenges and recent progresses
Criscuoli, Serena
Several irradiance reconstruction techniques rely on the use of static, one-dimensional, semi-empirical atmosphere models. These models do not capture the high dynamical and fine spatial structuring of the solar atmosphere. Moreover, they are derived from measurements of the solar spectrum that they aim to reproduce. Magneto Hydrodynamic (MHD) simulations of the solar atmosphere are known to better reproduce observed properties of the Sun, so that their use in modeling irradiance variations seems a natural evolution of the current techniques. Nevertheless, the development of such new techniques have been for long hampered by several factors. In this contribution I will review the main problems related to the use of MHD simulations in reproducing irradiance variations, and the recent efforts undertaken by the scientific community to overcome them. I will in particular describe recent development in simulating the higher layers of the solar atmosphere (where most of the observed irradiance variability originates) and the challenges imposed by numerical radiative transfer, in both LTE and NLTE treatment.
Statistical properties of starspots on solar-type stars and their correlation with flare activity
Maehara, Hiroyuki
Recent high-precision photometry by Kepler mission found thousands of “superflares” on solar-type stars. Most of superflare stars show quasi-periodic brightness modulations caused by the rotation of the star with starspots. We analyzed the statistical properties of starspots on solar-type stars and their correlation with the flare activity by using the data from the Kepler mission. The analysis shows that the fraction of stars showing large-amplitude rotational variations, which are thought to be the signature of the large starspots, decreases as the rotation period increases. Assuming simple relations between temperatures of spot and photosphere and between spot area and lifetime, we compared the size distribution of large starspot groups on slowly-rotating solar-type stars with that of sunspot groups observed during recent 140 years. The size distribution of starspots shows the power-law distribution and the size distribution of relatively large sunspots lies on this power-law line. This result implies that the large starspots with the area of >2-3x104 MHS (micro solar hemispheres; 1 MSH=3x1016 cm2) could appear once in a few hundred years on the slowly-rotating solar-type stars like our Sun. We also found that the frequency-energy distributions for flares originating from spots with different sizes are the same for solar-type stars with superflares and the Sun and the frequency of flare with a given energy is roughly proportional to the spot area. These results suggest that the magnetic activity on solar-type stars with superflares and that on the Sun are caused by the same physical processes.
How to model the variations of the Ly-a spectral irradiance ?
Kretzschmar, Matthieu
We analyse the 43 spectrally resolved SOHO/SUMER observations of the H Ly-a (121.567nm) irradiance over Solar Cycle 23 and discuss how to extrapolate them in order to provide the community with Ly-a spectral irradiance profile at other times. We end up with a simple but performant model that computes the irradiance spectral profile from 1947 to present. Such a model is relevant for the study of many astronomical environments, from planetary atmospheres to interplanetary medium. This empirical model, which uses the Ly-a disk-integrated irradiance composite, reproduces the temporal variability of the observed profile and matches the independent SORCE/SOLSTICE spectral observations from 2003 to 2007 with an accuracy better than 10%.
The latest TSI measurement results from the Compact Lightweight Absolute Radiometer (CLARA) onboard the NorSat-1 micro satellite
Walter, Benjamin
Continuous and precise Total Solar Irradiance (TSI) measurements are indispensable to evaluate the influence of short- and long-term solar radiative emission variations on the Earth’s energy budget. The existence of potentially long-term trends in the Sun’s activity and their effects on Earth climate is a societally-important field of research. The Compact Lightweight Absolute Radiometer (CLARA) is one of PMOD/WRC’s future contributions to the almost seamless series of space-borne TSI measurements since 1978. CLARA was end-to-end calibrated against the SI-traceable cryogenic radiometer of the TSI Radiometer Facility (TRF) in Boulder (Colorado). The absolute measurement uncertainties for the three SI-traceable TSI detectors within CLARA are 567, 576 and 912 ppm (k = 1). CLARA is one of three payloads on the Norwegian micro-satellite NorSat-1, which was launched July 14th, 2017.We present the latest TSI observations of CLARA together with the lessons learned about the instrument behavior on NorSat-1 including: i) pointing instabilities of the satellite platform and its influence on TSI results, ii) degradation of the detector sensitivity, iii) instrument sensitivity to temperature variations, iv) CLARA’s ability for measuring TSI during solar eclipses for determining the Sun’s radius, and v) comparison of the TSI results to other space radiometers. The comparison between CLARA’s most stable Channel B preliminary first light observations of 1359.84 W m-2 and VIRGO’s new scale TSI observation (1360.14 W m-2) show that they are in good agreement within the instrument uncertainties.
Atmosphere models from inversion of high-resolution spectropolarimetric observations for solar irradiance reconstructions
Ermolli, Ilaria
In semi-empirical models of solar irradiance variability, the radiative output of the Sun is reconstructed by combining the information about the spatial distribution of solar surface magnetism in resolved full-disc observations with the intensity contrast of solar surface features calculated by the solution to the radiative transfer equation in atmosphere models representative of the various features observed on the solar disc. Up till recently these atmospheres were 1D semi-empirical models presented in the literature, while now they can also be 3D atmospheres from magneto-convection simulations. We aimed to further extend the current semi-empirical models of solar irradiance variability to incorporate atmosphere models from state-of-the-art high-resolution solar observations. We analyzed spectropolarimetric data of the Fe I 630 nm line pair and of the Ca II 854.2 nm line obtained in regions that are representative of the granular quiet-Sun pattern and of small- and large-scale magnetic features, both bright and dark with respect to the quiet-Sun. We derived atmosphere models of the observed regions from data inversion and discussed the obtained atmospheres with respect to several atmosphere models presented in the literature. We herewith present the accuracy of our spectral synthesis computations on our observation-based atmospheres for application in solar irradiance estimates.
The SORCE Solar Spectral Irradiance Record: Fifteen Years of SSI
Snow, Martin
The Solar Radiation and Climate Experiment (SORCE) has measured solar spectral irradiance (SSI) from the ultraviolet through the infra-red on a daily basis for more than a solar cycle. This fifteen year record includes the decline of solar cycle 23 and the entirety of solar cycle 24. We will present the measurements from the SOLar STellar Irradiance Comparison Experiment (SOLSTICE) and the Solar Irradiance Monitor (SIM) along with uncertainties in both absolute calibration and uncertainties in on-orbit degradation tracking. These observations will also be compared with irradiance models as well as observations from overlapping missions such as UARS/SUSIM, AURA/OMI and ISS/SOLSPEC. Initial results from the next generation of SSI instruments, TSIS-1/SIM and the compact SOLSTICE, will also be shown. Finally, an overview of the future of SSI measurements from space will be described.
Solar brightness variations as they would be observed by Kepler telescope
Nemec, Nina-Elisabeth
Recent planet-hunting missions, such as CoRoT and Kepler opened new perspectives for studying stellar photometric variabilities on timescales of stellar rotation and below. Understanding stellar variability on such timescales, and how it compares to that of the Sun is of particular interest. While stars are observed from arbitrary directions, solar brightness variations are measured from the equatorial plane due to the special position of the Earth-bound observer. Thus, comparison studies of stars and the Sun are not straight forward and the effect of inclination has to be taking into account. Here, we model solar brightness variations on timescales of the solar rotational period and below as they would be observed out of ecliptic. For that the distribution of the magnetic features on the solar surface is calculated with the Surface Flux Transport Model developed at MPS. Using this tool allows us to disentangle two superposed effects: Contributions from the rotation of the magnetic features and contributions from their evolution. Moreover, we study how variability is affected by metallicity, where different inclinations are taking into account. We find, that for an observer from the equatorial plane the solar variability on the rotational timescale would appear to be spot-dominated, whereas for an observer out of the ecliptic it would appear to be faculae-dominate.
Fast spectral synthesis for a new generation of solar and stellar brightness variability models
Cernetic, Miha
Recently, realistic three-dimensional (3D) magnetohydrodynamic (MHD) simulations of the solar near-surface magneto-convection achieve excellent agreement with observations. Such calculations pave the way for a new generation of models of solar and stellar brightness variability, which calculate the radiative transfer for a huge set of rays through such 3D MHD cubes. If using the conventional method for the radiative transfer for such a huge amount of rays, it becomes computationally very expensive. This is due to the treatment of the spectral lines: The spectra of the Sun and Sun-like stars contain several millions of molecular and atomic lines that play an important role in the irradiance variability in the UV and visible spectral domains. To achieve faster calculations while taking spectral lines into account, we propose a solution in the form of modified opacity distribution functions (ODFs). Standard ODFs allow taking into account the effect of spectral lines by approximating their complex structure in opacity. We investigate the procedure used for ODF computations and develop a novel approach for fast calculations of a) integrated flux and its variability in passbands of interest, e.g. in Strömgren-, Kepler-, and Plato- filters; b) the total solar irradiance (TSI) and its variability. In particular, we show that it is sufficient to calculate the radiative transfer of just about a hundred frequency points to accurately reproduce the TSI and its variability. Our method and the related speed-up in radiative transfer calculations allows the current models to be taken to a new level.
Long-Term Model-Based Reconstructions of the Solar Irradiance
Ball, Will
To understand how our climate has changed in the past, and will evolve in the future, it is important to estimate changes in Earth's primary energy source: the Sun. Solar irradiance is a fundamental component of any climate modelling study. Variations in both the total, and spectral, solar irradiance are essential to determine the magnitude of the impact that the Sun has made, and will make, to the climate. Here, I will provide an up-to-date review of the main long-term total and spectral irradiance reconstructions relevant to forcing climate models, which can show a large range of both total and spectral variations on decadal and centennial timescales. To put in context the century-scale reconstructions, for which direct irradiance observations do not exist, it is important to also discuss the irradiance models with respect to the observations that have accumulated over the last 40 years, since the beginning of the satellite era. Therefore, I will also give an overview of the uncertainties that remain in the observations used to constrain and verify models, and the forcing range that the long-term reconstructions will provide to reconstruct past climate behaviour, and make future projections.