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Thomas N Woods - One of the best experts on this subject based on the ideXlab platform.
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an improved solar Spectral Irradiance composite record
2020Co-Authors: Thomas N Woods, Matthew T DelandAbstract:The solar Spectral Irradiance (SSI) data set is a key record for studying and understanding the energetics and radiation balance in Earth’s environment. Understanding the long-term variations of th...
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the maven euvm model of solar Spectral Irradiance variability at mars algorithms and results
2017Co-Authors: E Thiemann, S W Bougher, F G Eparvier, Phillip C Chamberlin, Brian Templeman, Thomas N Woods, Bruce M. JakoskyAbstract:Solar extreme ultraviolet (EUV) radiation is a primary energy input to the Mars atmosphere, causing ionization and driving photochemical processes above approximately 100 km. Because solar EUV radiation varies with wavelength and time, measurements must be Spectrally resolved to accurately quantify its impact on the Mars atmosphere. The Mars Atmosphere and Volatile EvolutioN (MAVEN) EUV Monitor (EUVM) measures solar EUV Irradiance incident on the Mars atmosphere in three bands. These three bands drive a Spectral Irradiance variability model called the Flare Irradiance Spectral Model (FISM)-Mars (FISM-M) which is an iteration of the FISM model by Chamberlin et al. (2007, 2008) for Spectral Irradiance at Earth. In this paper, we report the algorithms used to derive FISM-M and its associated uncertainties, focusing on differences from the original FISM. FISM-M Spectrally resolves the solar EUV Irradiance at Mars from 0.5 to 189.5 nm at 1min cadence, and 0.1 nm resolution in the 6–106 nm range or 1 nm resolution otherwise. FISM-M is suitable for both daily average and flaring Spectral Irradiance estimates and is based on the linear association of the broadband EUVM measurements with Spectral Irradiance measurements, including recent high time cadence 0.1 nm resolution measurements from the EUV Variability Experiment (EVE) on the Space Dynamics Observatory (SDO) between 6 and 106 nm. In addition, we present examples of model outputs for EUV Irradiance variability due to solar flares, solar rotations, Mars orbit eccentricity, and the solar cycle, between October 2015 and November 2016.
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five years of synthesis of solar Spectral Irradiance from sdid sisa and sdo aia images
2016Co-Authors: J M Fontenla, Mihail Codrescu, Mariangel Fedrizzi, T J Fullerrowell, F Hill, E Landi, Thomas N WoodsAbstract:In this paper we describe the synthetic solar Spectral Irradiance (SSI) calculated from 2010 to 2015 using data from the Atmospheric Imaging Assembly (AIA) instrument, on board the Solar Dynamics Observatory spacecraft. We used the algorithms for solar disk image decomposition (SDID) and the Spectral Irradiance synthesis algorithm (SISA) that we had developed over several years. The SDID algorithm decomposes the images of the solar disk into areas occupied by nine types of chromospheric and 5 types of coronal physical structures. With this decomposition and a set of pre-computed angle-dependent spectra for each of the features, the SISA algorithm is used to calculate the SSI. We discuss the application of the basic SDID/SISA algorithm to a subset of the AIA images and the observed variation occurring in the 2010–2015 period of the relative areas of the solar disk covered by the various solar surface features. Our results consist of the SSI and total solar Irradiance variations over the 2010–2015 period. The SSI results include soft X-ray, ultraviolet, visible, infrared, and far-infrared observations and can be used for studies of the solar radiative forcing of the Earth's atmosphere. These SSI estimates were used to drive a thermosphere–ionosphere physical simulation model. Predictions of neutral mass density at low Earth orbit altitudes in the thermosphere and peak plasma densities at mid-latitudes are in reasonable agreement with the observations. The correlation between the simulation results and the observations was consistently better when fluxes computed by SDID/SISA procedures were used.
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far and extreme uv solar Spectral Irradiance and radiance from simplified atmospheric physical models
2014Co-Authors: J M Fontenla, E Landi, Martin Snow, Thomas N WoodsAbstract:This article describes an update of the physical models that we use to reconstruct the FUV and EUV Irradiance spectra and the radiance spectra of the features that at any given point in time may cover the solar disk depending on the state of solar activity. The present update introduces important modifications to the chromosphere–corona transition region of all models. Also, the update introduces improved and extended atomic data. By these changes, the agreement of the computed and observed spectra is largely improved in many EUV lines important for the modeling of the Earth’s upper atmosphere. This article describes the improvements and shows detailed comparisons with EUV/FUV radiance and Irradiance measurements. The solar Spectral Irradiance from these models at wavelengths longer than ≈ 200 nm is discussed in a separate article.
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the impact of solar Spectral Irradiance variability on middle atmospheric ozone
2011Co-Authors: A W Merkel, Jerald W Harder, J M Fontenla, Daniel R Marsh, Anne K Smith, Thomas N WoodsAbstract:[1] This study presents the impact of solar Spectral Irradiance (SSI) variability on middle atmospheric ozone over the declining phase of solar cycle 23. Two different types of Spectral forcing are applied to the Whole Atmosphere Community Climate Model (WACCM) to simulate the ozone response between periods of quiet and high solar activity. One scenario uses the solar proxy reconstructions model from the Naval Research Laboratory (NRLSSI), and the other is based on SSI observations from the Solar Radiation and Climate Experiment (SORCE). The SORCE observations show 3–5 times more variability in ultraviolet (UV) radiation than predicted by the proxy model. The NRLSSI forcing had minimal impact on ozone, however, the higher UV variability from SORCE induced a 4% reduction in ozone concentration above 40 km at solar active conditions. The model result is supported by 8 years (2002–2010) of ozone observations from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument. The SABER ozone variations have greater similarity with the SORCE SSI model simulations. The model and satellite data suggests that the ozone response is due to enhanced photochemical activity associated with larger UV variability.
Judith Lean - One of the best experts on this subject based on the ideXlab platform.
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solar Spectral Irradiance variability in cycle 24 observations and models
2016Co-Authors: S V Marchenko, Matthew T Deland, Judith LeanAbstract:Utilizing the excellent stability of the Ozone Monitoring Instrument (OMI), we characterize both short-term (solar rotation) and long-term (solar cycle) changes of the solar Spectral Irradiance (SSI) between 265 and 500 nm during the ongoing cycle 24. We supplement the OMI data with concurrent observations from the Global Ozone Monitoring Experiment-2 (GOME-2) and Solar Radiation and Climate Experiment (SORCE) instruments and find fair-to-excellent, depending on wavelength, agreement among the observations, and predictions of the Naval Research Laboratory Solar Spectral Irradiance (NRLSSI2) and Spectral And Total Irradiance REconstruction for the Satellite era (SATIRE-S) models.
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solar Spectral Irradiance variability in cycle 24 observations and models
2016Co-Authors: S V Marchenko, Matthew T Deland, Judith LeanAbstract:Utilizing the excellent stability of the Ozone Monitoring Instrument (OMI), we characterize both short-term (solar rotation) and long-term (solar cycle) changes of the solar Spectral Irradiance (SSI) between 265-500 nm during the on-going Cycle 24. We supplement the OMI data with concurrent observations from the GOME-2 and SORCE instruments and find fair-to-excellent, depending on wavelength, agreement among the observations and predictions of the NRLSSI2 and SATIRE-S models.
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solar Spectral Irradiance variability in november december 2012 comparison of observations by instruments on the international space station and models
2014Co-Authors: Gerard Thuillier, Judith Lean, A I Shapiro, Natalie A Krivova, Gerhard Schmidtke, Christian Erhardt, Bernd Nikutowski, C Bolduc, Paul CharbonneauAbstract:Onboard the International Space Station (ISS), two instruments are observing the solar Spectral Irradiance (SSI) at wavelengths from 16 to 2900 nm. Although the ISS platform orientation generally precludes pointing at the Sun more than 10 - 14 days per month, in November/December 2012 a continuous period of measurements was obtained by implementing an ISS 'bridging' maneuver. This enabled observations to be made of the solar Spectral Irradiance (SSI) during a complete solar rotation. We present these measurements, which quantify the impact of active regions on SSI, and compare them with data simultaneously gathered from other platforms, and with models of Spectral Irradiance variability. Our analysis demonstrates that the instruments onboard the ISS have the capability to measure SSI variations consistent with other instruments in space. A comparison among all available SSI measurements during November-December 2012 in absolute units with reconstructions using solar proxies and observed solar activity features is presented and discussed in terms of accuracy.
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Analysis of Different Solar Spectral Irradiance Reconstructions and Their Impact on Solar Heating Rates
2014Co-Authors: Germaine Thuillier, S. M. L. Melo, V. I. Fomichev, Patrick Charbonneau, Judith Lean, Chantal Bolduc, A I Shapiro, Natalie A Krivova, Wermer Schmutz, David BolséeAbstract:Proper numerical simulation of the Earth’s climate change requires reliable knowledge of solar Irradiance and its variability on different time scales, as well as the wavelength dependence of this variability. As new measurements of the solar Spectral Irradiance have become available, so too have new reconstructions of historical solar Irradiance variations, based on different approaches. However, these various solar Spectral Irradiance reconstructions have not yet been compared in detail to quantify differences in their absolute values, variability, and implications for climate and atmospheric studies. In this paper we quantitatively compare five different reconstructions of solar Spectral Irradiance changes during the past four centuries, in order to document and analyze their differences. The impact on atmosphere and climate studies is discussed in terms of the calculation of short wave solar heating rates.
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evolution of the sun s Spectral Irradiance since the maunder minimum
2000Co-Authors: Judith LeanAbstract:Because of the dependence of the Sun's irradi- ance on solar activity, reductions from contemporary levels are expected during the seventeenth century Maunder Min- imum. New reconstructions of Spectral Irradiance are de- veloped since 1600 with absolute scales traceable to space- based observations. The long-term variations track the en- velope of group sunspot numbers and have amplitudes con- sistent with the range of Ca II brightness in Sun-like stars. Estimated increases since 1675 are 0.7%, 0.2% and0.07% in broadultraviolet, visible/nearinfraredandinfraredSpectral bands, with a total Irradiance increase of 0.2%.
J M Fontenla - One of the best experts on this subject based on the ideXlab platform.
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five years of synthesis of solar Spectral Irradiance from sdid sisa and sdo aia images
2016Co-Authors: J M Fontenla, Mihail Codrescu, Mariangel Fedrizzi, T J Fullerrowell, F Hill, E Landi, Thomas N WoodsAbstract:In this paper we describe the synthetic solar Spectral Irradiance (SSI) calculated from 2010 to 2015 using data from the Atmospheric Imaging Assembly (AIA) instrument, on board the Solar Dynamics Observatory spacecraft. We used the algorithms for solar disk image decomposition (SDID) and the Spectral Irradiance synthesis algorithm (SISA) that we had developed over several years. The SDID algorithm decomposes the images of the solar disk into areas occupied by nine types of chromospheric and 5 types of coronal physical structures. With this decomposition and a set of pre-computed angle-dependent spectra for each of the features, the SISA algorithm is used to calculate the SSI. We discuss the application of the basic SDID/SISA algorithm to a subset of the AIA images and the observed variation occurring in the 2010–2015 period of the relative areas of the solar disk covered by the various solar surface features. Our results consist of the SSI and total solar Irradiance variations over the 2010–2015 period. The SSI results include soft X-ray, ultraviolet, visible, infrared, and far-infrared observations and can be used for studies of the solar radiative forcing of the Earth's atmosphere. These SSI estimates were used to drive a thermosphere–ionosphere physical simulation model. Predictions of neutral mass density at low Earth orbit altitudes in the thermosphere and peak plasma densities at mid-latitudes are in reasonable agreement with the observations. The correlation between the simulation results and the observations was consistently better when fluxes computed by SDID/SISA procedures were used.
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solar Spectral Irradiance solar activity and the near ultra violet
2015Co-Authors: J M Fontenla, P C Stancil, E LandiAbstract:The previous calculations of the Solar Spectral Irradiance (SSI) by the Solar Radiation Physical Modeling, version 2 system, are updated in this work by including new molecular photodissociation cross-sections of important species, and many more levels and lines in its treatment of non-LTE radiative transfer. The current calculations including the new molecular photodissociation opacities produce a reduced over-ionizaton of heavy elements in the lower chromosphere and solve the problems with prior studies of the UV SSI in the wavelength range 160–400 nm and now reproduce the available observations with much greater accuracy. Calculations and observations of the near-UV at 0.1 nm resolution and higher are compared. The current set of physical models includes four quiet-Sun and five active-region components, from which radiance is computed for ten observing angles. These radiances are combined with images of the solar disk to obtain the SSI and Total Solar Irradiance and their variations. The computed SSI is compared with measurements from space at several nm resolution and agreement is found within the accuracy level of these measurements. An important result is that the near-UV SSI increase with solar activity is significant for the photodissociation of ozone in the terrestrial atmosphere because a number of highly variable upper chromospheric lines overlap the ozone Hartley band.
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far and extreme uv solar Spectral Irradiance and radiance from simplified atmospheric physical models
2014Co-Authors: J M Fontenla, E Landi, Martin Snow, Thomas N WoodsAbstract:This article describes an update of the physical models that we use to reconstruct the FUV and EUV Irradiance spectra and the radiance spectra of the features that at any given point in time may cover the solar disk depending on the state of solar activity. The present update introduces important modifications to the chromosphere–corona transition region of all models. Also, the update introduces improved and extended atomic data. By these changes, the agreement of the computed and observed spectra is largely improved in many EUV lines important for the modeling of the Earth’s upper atmosphere. This article describes the improvements and shows detailed comparisons with EUV/FUV radiance and Irradiance measurements. The solar Spectral Irradiance from these models at wavelengths longer than ≈ 200 nm is discussed in a separate article.
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high resolution solar Spectral Irradiance from extreme ultraviolet to far infrared
2011Co-Authors: J M Fontenla, J. Harder, Martin Snow, W Livingston, T N WoodsAbstract:[1] This paper presents new extremely high-resolution solar Spectral Irradiance (SSI) calculations covering wavelengths from 0.12 nm to 100 micron obtained by the Solar Irradiance Physical Modeling (SRPM) system. Daily solar Irradiance spectra were constructed for most of Solar Cycle 23 based on a set of physical models of the solar features and non-LTE calculations of their emitted spectra as function of viewing angle, and solar images specifying the distribution of features on the solar disk. Various observational tests are used to assess the quality of the spectra provided here. The present work emphasizes the effects on the SSI of the upper chromosphere and full-non-LTE radiative transfer calculation of level populations and ionizations that are essential for physically consistent results at UV wavelengths and for deep lines in the visible and IR. This paper also considers the photodissociation continuum opacity of molecular species, e.g., CH and OH, and proposes the consideration of NH photodissociation which can solve the puzzle of the missing near-UV opacity in the Spectral range of the near-UV. Finally, this paper is based on physical models of the solar atmosphere and extends the previous lower-layer models into the upper-transition-region and coronal layers that are the dominant source of photons at wavelengths shorter than ∼50 nm (except for the He II 30.4 nm line, mainly formed in the lower-transition-region).
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the impact of solar Spectral Irradiance variability on middle atmospheric ozone
2011Co-Authors: A W Merkel, Jerald W Harder, J M Fontenla, Daniel R Marsh, Anne K Smith, Thomas N WoodsAbstract:[1] This study presents the impact of solar Spectral Irradiance (SSI) variability on middle atmospheric ozone over the declining phase of solar cycle 23. Two different types of Spectral forcing are applied to the Whole Atmosphere Community Climate Model (WACCM) to simulate the ozone response between periods of quiet and high solar activity. One scenario uses the solar proxy reconstructions model from the Naval Research Laboratory (NRLSSI), and the other is based on SSI observations from the Solar Radiation and Climate Experiment (SORCE). The SORCE observations show 3–5 times more variability in ultraviolet (UV) radiation than predicted by the proxy model. The NRLSSI forcing had minimal impact on ozone, however, the higher UV variability from SORCE induced a 4% reduction in ozone concentration above 40 km at solar active conditions. The model result is supported by 8 years (2002–2010) of ozone observations from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument. The SABER ozone variations have greater similarity with the SORCE SSI model simulations. The model and satellite data suggests that the ozone response is due to enhanced photochemical activity associated with larger UV variability.
S K Solanki - One of the best experts on this subject based on the ideXlab platform.
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solar total and Spectral Irradiance reconstruction over the last 9000 years
2018Co-Authors: N A Krivova, S K Solanki, I G UsoskinAbstract:Changes in solar Irradiance and in its Spectral distribution are among the main natural drivers of the climate on Earth. However, Irradiance measurements are only available for less than four decades, while assessment of solar influence on Earth requires much longer records. The aim of this work is to provide the most up-to-date physics-based reconstruction of the solar total and Spectral Irradiance (TSI/SSI) over the last nine millennia. The concentrations of the cosmogenic isotopes 14C and 10Be in natural archives have been converted to decadally averaged sunspot numbers through a chain of physics-based models. TSI and SSI are reconstructed with an updated SATIRE model. Reconstructions are carried out for each isotope record separately, as well as for their composite. We present the first ever SSI reconstruction over the last 9000 years from the individual 14C and 10Be records as well as from their newest composite. The reconstruction employs physics-based models to describe the involved processes at each step of the procedure. Irradiance reconstructions based on two different cosmogenic isotope records, those of 14C and 10Be, agree well with each other in their long-term trends despite their different geochemical paths in the atmosphere of Earth. Over the last 9000 years, the reconstructed secular variability in TSI is of the order of 0.11%, or 1.5 W/m2. After the Maunder minimum, the reconstruction from the cosmogenic isotopes is consistent with that from the direct sunspot number observation. Furthermore, over the nineteenth century, the agreement of Irradiance reconstructions using isotope records with the reconstruction from the sunspot number by Chatzistergos et al. (2017) is better than that with the reconstruction from the WDC-SILSO series (Clette et al. 2014), with a lower chi-square-value.
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models of solar total and Spectral Irradiance variability of relevance for climate studies
2013Co-Authors: N A Krivova, S K SolankiAbstract:The variable radiative output of the Sun is a prime external driver of the Earth’s climate system. Just how effective this driver is has remained relatively uncertain, however, partly due to missing knowledge on the exact variation of the Sun’s Irradiance over time in different parts of the solar spectrum. Due to the limited length of the time series of measured Irradiance and inconsistencies between different measurements, models of solar Irradiance variation are particularly important. Here we provide an overview of progress over the last half decade in the development and application of the SATIRE family of models. For the period after 1974, the model makes use of the full-disc magnetograms of the Sun and reproduces up to 97 % of the measured Irradiance variation. Over this time frame, there is no evidence for any non-magnetic change in the solar Irradiance on time scales longer than about a day. We have also been able to compute total solar Irradiance since the Maunder minimum and further into the past throughout the whole Holocene. The Sun’s Spectral Irradiance from the Lyman α line in the UV to the far infrared has also been reconstructed throughout the telescopic era.
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towards a long term record of solar total and Spectral Irradiance
2011Co-Authors: Natalie A Krivova, S K Solanki, Y C UnruhAbstract:Abstract The variation of total solar Irradiance (TSI) has been measured since 1978 and that of the Spectral Irradiance for an even shorter amount of time. Semi-empirical models are now available that reproduce over 80% of the measured Irradiance variations. An extension of these models into the more distant past is needed in order to serve as input to climate simulations. Here we review our most recent efforts to model solar total and Spectral Irradiance on time scales from days to centuries and even longer. Solar Spectral Irradiance has been reconstructed since 1947. Reconstruction of solar total Irradiance goes back to 1610 and suggests a value of about 1 – 1.5 W / m 2 for the increase in the cycle-averaged TSI since the end of the Maunder minimum, which is significantly lower than previously assumed but agrees with other modern models. First steps have also been made towards reconstructions of solar total and Spectral Irradiance on time scales of millennia.
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Spectral Irradiance variations comparison between observations and the satire model on solar rotation time scales
2008Co-Authors: Y C Unruh, Natalie A Krivova, J. Harder, S K Solanki, Greg KoppAbstract:Aims. We test the reliability of the observed and calculated Spectral Irradiance variations between 200 and 1600 nm over a time span of three solar rotations in 2004. Methods. We compare our model calculations to Spectral Irradiance observations taken with SORCE/SIM, SoHO/VIRGO, and UARS/SUSIM. The calculations assume LTE and are based on the SATIRE (Spectral And Total Irradiance REconstruction) model. We analyse the variability as a function of wavelength and present time series in a number of selected wavelength regions covering the UV to the NIR. We also show the facular and spot contributions to the total calculated variability. Results. In most wavelength regions, the variability agrees well between all sets of observations and the model calculations. The model does particularly well between 400 and 1300 nm, but fails below 220 nm, as well as for some of the strong NUV lines. Our calculations clearly show the shift from faculae-dominated variability in the NUV to spot-dominated variability above approximately 400 nm. We also discuss some of the remaining problems, such as the low sensitivity of SUSIM and SORCE for wavelengths between approximately 310 and 350 nm, where currently the model calculations still provide the best estimates of solar variability.
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the solar Spectral Irradiance since 1700
2000Co-Authors: M Fligge, S K SolankiAbstract:The change in the Irradiance spectrum of the Sun from 1700 to the last solar minimum is determined and compared to the change in the spectrum between activity minimum and maximum. For this purpose we have used detailed model flux spectra of solar magnetic features. Also, time-series of the solar Spectral Irradiance since 1700 in different wavelength bands are reconstructed. We expect that these reconstructions are more accurate than previously published ones, although they suffer (like all reconstructions of solar Irradiance on such time-scales) from uncertainties in our knowledge of the evolution of the solar network with time.
David Bolsée - One of the best experts on this subject based on the ideXlab platform.
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Solar Spectral Irradiance at 782 nm as Measured by the SES Sensor Onboard Picard
2016Co-Authors: Mustapha Meftah, David Bolsée, Alain Hauchecorne, Abdanour Irbah, Gaël Cessateur, Slimane Bekki, Luc Damé, Nuno PereiraAbstract:Picard is a satellite dedicated to the simultaneous measurement of the total and solar Spectral Irradiance, the solar diameter, the solar shape, and to the Sun’s interior through the methods of helioseismology. The satellite was launched on June 15, 2010, and pursued its data acquisitions until March 2014. A Sun Ecartometry Sensor (SES) was developed to provide the stringent pointing requirements of the satellite. The SES sensor produced an image of the Sun at 782±2.5 nm. From the SES data, we obtained a new time series of the solar Spectral Irradiance at 782 nm from 2010 to 2014. During this period of Solar Cycle 24, the amplitude of the changes has been of the order of ± 0.08 %, corresponding to a range of about 2×10−3 Wm−2nm−1. SES observations provided a qualitatively consistent evolution of the solar Spectral Irradiance variability at 782 nm. SES data show similar amplitude variations with the semi-empirical model Spectral And Total Irradiance REconstruction for the Satellite era (SATIRE-S), whereas the Spectral Irradiance Monitor instrument (SIM) onboard the SOlar Radiation and Climate Experiment satellite (SORCE) highlights higher amplitudes.
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Analysis of Different Solar Spectral Irradiance Reconstructions and Their Impact on Solar Heating Rates
2014Co-Authors: Germaine Thuillier, S. M. L. Melo, V. I. Fomichev, Patrick Charbonneau, Judith Lean, Chantal Bolduc, A I Shapiro, Natalie A Krivova, Wermer Schmutz, David BolséeAbstract:Proper numerical simulation of the Earth’s climate change requires reliable knowledge of solar Irradiance and its variability on different time scales, as well as the wavelength dependence of this variability. As new measurements of the solar Spectral Irradiance have become available, so too have new reconstructions of historical solar Irradiance variations, based on different approaches. However, these various solar Spectral Irradiance reconstructions have not yet been compared in detail to quantify differences in their absolute values, variability, and implications for climate and atmospheric studies. In this paper we quantitatively compare five different reconstructions of solar Spectral Irradiance changes during the past four centuries, in order to document and analyze their differences. The impact on atmosphere and climate studies is discussed in terms of the calculation of short wave solar heating rates.
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the solar Spectral Irradiance as a function of the mg ii index for atmosphere and climate modelling
2012Co-Authors: Gerard Thuillier, David Bolsée, A I Shapiro, Matthew T Deland, W Schmutz, S. M. L. MeloAbstract:We present a new method to reconstruct the solar spectrum Irradiance in the Ly α – 400 nm region, and its variability, based on the Mg ii index and neutron-monitor measurements. Measurements of the solar Spectral Irradiance available in the literature have been made with different instruments at different times and different Spectral ranges. However, climate studies require harmonised data sets. This new approach has the advantage of being independent of the absolute calibration and aging of the instruments. First, the Mg ii index is derived using solar spectra from Ly α (121 nm) to 410 nm measured from 1978 to 2010 by several space missions. The variability of the spectra with respect to a chosen reference spectrum as a function of time and wavelength is scaled to the derived Mg ii index. The set of coefficients expressing the Spectral variability can be applied to the chosen reference spectrum to reconstruct the solar spectra within a given time frame or Mg ii index values. The accuracy of this method is estimated using two approaches: direct comparison with particular cases where solar spectra are available from independent measurements, and calculating the standard deviation between the measured spectra and their reconstruction. From direct comparisons with measurements we obtain an accuracy of about 1 to 2%, which degrades towards Ly α. In a further step, we extend our solar Spectral-Irradiance reconstruction back to the Maunder Minimum introducing the relationship between the Mg ii index and the neutron-monitor data. Consistent measurements of the Mg ii index are not available prior to 1978. However, we remark that over the last three solar cycles, the Mg ii index shows strong correlation with the modulation potential determined from the neutron-monitor data. Assuming that this correlation can be applied to the past, we reconstruct the Mg ii index from the modulation potential back to the Maunder Minimum, and obtain the corresponding solar Spectral-Irradiance reconstruction back to that period. As there is no direct measurement of the Spectral Irradiance for this period we discuss this methodology in light of the other proposed approaches available in the literature. The use of the cosmogenic-isotope data provides a major advantage: it provides information about solar activity over several thousands years. Using technology of today, we can calibrate the solar Irradiance against activity and thus reconstruct it for the times when cosmogenic-isotope data are available. This calibration can be re-assessed at any time, if necessary.
