The SORCE Mission

The Solar Radiation and Climate Experiment (SORCE) satellite carries four scientific instruments that measure the solar radiation at the top of the Earth's atmosphere. The mission is an important flight component of NASA's Earth Observing System (EOS), which in turn is the major observational and scientific element of the U.S. Global Change Research Program. The scientific objectives of SORCE are to make daily measurements of the total solar irradiance and of spectral solar irradiance from 120 to 2000 nm with additional measurements of the energetic X-rays. Solar radiation provides the dominant energy source for the Earth system and detailed understanding of its variation is essential for atmospheric and climate studies. SORCE was launched on January 25, 2003 and has an expected lifetime through the next solar minimum in about 2007. The spacecraft and all instruments have operated flawlessly during the first 2 years, and this paper provides an overview of the mission and discusses the contributions that SORCE is making to improve understanding of the Sun's influence on the Earth environment.     

Direct and Indirect Thermospheric Heating Sources for Solar Cycles 21–23

Solar variability is often cast in terms of radiative emission and the associated long-term climate response; however, growing societal reliance on technology is creating more interest in day-to-day solar variability. This variability is associated with both solar radiative and solar wind emissions. In this paper we explore the combined effects of radiative and solar wind fluctuations at Earth. The fluctuations in radiative and geomagnetic power create an extended interval of solar maximum for the upper atmosphere. We use a trio of empirical models to estimate, over the last three solar cycles, the relative contributions of solar extreme ultraviolet (UV) power, Joule power, and particle kinetic power to the Earth’s upper atmosphere energy budget. Daily power values are derived from three source models. The SOLAR2000 solar irradiance specification model provides estimates of the daily extreme and far UV solar power input. Geomagnetic power is derived from a combination of satellite-estimated particle precipitation power and an empirical model of Joule power from hemispherically integrated estimates of high-latitude energy deposition. During the interval 1975 to 2003, the average daily contributions were: particles – 36 GW, Joule – 95 GW and solar – 464 GW for a total of 595 GW. Solar wind-driven geomagnetic power provided 22% of the total global upper atmospheric energy. In the top 15 power events, geomagnetic power contributed two-thirds of the total power budget. In each of these events, Joule power alone exceeded solar power. With rising activity, Joule power becomes the most variable element of solar upper atmosphere interactions.     

The Solar Radiation and Climate Experiment (SORCE) Mission for the NASA Earth Observing System (EOS)

The NASA Earth Observing System (EOS) is an advanced study of Earth's long-term global changes of solid Earth, its atmosphere, and oceans and includes a coordinated collection of satellites, data systems, and modeling. The EOS program was conceived in the 1980s as part of NASA's Earth System Enterprise (ESE). The Solar Radiation and Climate Experiment (SORCE) is one of about 20 missions planned for the EOS program, and the SORCE measurement objectives include the total solar irradiance (TSI) and solar spectral irradiance (SSI) that are two of the 24 key measurement parameters defined for the EOS program. The SORCE satellite was launched in January 2003, and its observations are improving the understanding and generating new inquiry regarding how and why solar variability occurs and how it affects Earth's energy balance, atmosphere, and long-term climate changes.     

XUV Photometer System (XPS): Solar Variations during the SORCE Mission

The solar soft X-ray (XUV) radiation is important for upper atmosphere studies as it is one of the primary energy inputs and is highly variable. The XUV Photometer System (XPS) aboard the Solar Radiation and Climate Experiment (SORCE) has been measuring the solar XUV irradiance since March 2003 with a time cadence of 10 s and with about 70% duty cycle. The XPS measurements are between 0.1 and 34 nm and additionally the bright hydrogen emission at 121.6 nm. The XUV radiation varies by a factor of ∼2 with a period of ∼27 days that is due to the modulation of the active regions on the rotating Sun. The SORCE mission has observed over 20 solar rotations during the declining phase of solar cycle 23. The solar XUV irradiance also varies by more than a factor of 10 during the large X-class flares observed during the May–June 2003, October–November 2003, and July 2004 solar storm periods. There were 7 large X-class flares during the May–June 2003 storm period, 11 X-class flares during the October–November 2003 storm period, and 6 X-class flares during the July 2004 storm period. The X28 flare on 4 November 2003 is the largest flare since GOES began its solar X-ray measurements in 1976. The XUV variations during the X-class flares are as large as the expected solar cycle variations.     

A Sunspot Catalog for the Period 1952 – 1986 from Observations Made at the Madrid Astronomical Observatory

The Spectral Irradiance Monitor (SIM) instrument on board the Solar Radiation and Climate Experiment (SORCE) performs daily measurements of the solar spectral irradiance (SSI) from 200 to 2400 nm. Both temporal and spectral corrections for instrument degradation have been built on physical models based on comparison of two independent channels with different solar exposure. The present study derives a novel correction for SIM degradation using the total solar irradiance (TSI) measurements from the Total Irradiance Monitor (TIM) on SORCE. The correction is applied to SIM SSI data from September 2004 to October 2012 over the wavelength range from 205 nm to 2300 nm. The change in corrected, integrated SSI agrees within \(0.1~\mbox{W}\,\mbox{m}^{-2}\) (\(1\sigma\)) with SORCE TIM TSI and independently shows agreement with the SATIRE-S and NRLSSI2 solar models within measurement uncertainties.

     

The Mg II Index from SORCE

The Solar–Stellar Irradiance Comparison Experiment (SOLSTICE) and the Spectral Irradiance Monitor (SIM) on the Solar Radiation and Climate Experiment (SORCE) both measure the solar ultraviolet irradiance surrounding the Mg II doublet at 280 nm on a daily basis. The SIM instrument's resolution (1.1 nm) is similar to the Solar Backscatter Ultraviolet instruments used to compute the standard NOAA Mg II index, while SOLSTICE's resolution is an order of magnitude higher (0.1 nm). This paper describes the technique used to calculate the index for both instruments and compares the resulting time series for the first 18 months of the SORCE mission. The spectral resolution and low noise of the SOLSTICE spectrum produces a Mg II index with a precision of 0.6%, roughly a factor of 2 better than the low-resolution index measurement. The full-resolution SOLSTICE index is able to measure short-timescale changes in the solar radiative output that are lost in the noise of the low-resolution index.     

Comparison of Solar UV Spectral Irradiance from SUSIM and SORCE

Knowledge of solar spectral irradiance (SSI) is important in determining the impact of solar variability on climate. Observations of UV SSI have been made by the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on the Upper Atmosphere Research Satellite (UARS), the Solar-Stellar Irradiance Comparison Experiment (SOLSTICE), and the Solar Irradiance Monitor (SIM), both on the Solar Radiation and Climate Experiment (SORCE) satellite. Measurements by SUSIM and SORCE overlapped from 2003 to 2005. SUSIM and SORCE observations represent ～?20 years of absolute UV SSI. Unfortunately, significant differences exist between these two data sets. In particular, changes in SORCE UV SSI measurements, gathered at moderate and minimum solar activity, are a factor of two greater than the changes in SUSIM observations over the entire solar cycle. In addition, SORCE UV SSI have a substantially different relationship with the Mg ii index than did earlier UV SSI observations. Acceptance of these new SORCE results impose significant changes on our understanding of UV SSI variation. Alternatively, these differences in UV SSI observations indicate that some or all of these instruments have changes in instrument responsivity that are not fully accounted for by the current calibration. In this study, we compare UV SSI changes from SUSIM with those from SIM and SOLSTICE. The primary results are that (1) long-term observations by SUSIM and SORCE generally do not agree during the overlap period (2003?–?2005), (2) SUSIM observations during this overlap period are consistent with an SSI model based on Mg ii and early SUSIM SSI, and (3) when comparing the spectral irradiance for times of similar solar activity on either side of solar minimum, SUSIM observations show slight differences while the SORCE observations show variations that increase with time between spectra. Based on this work, we conclude that the instrument responsivity for SOLSTICE and SIM need to be reevaluated before these results can be used for climate-modeling studies.     

Solar–Stellar Irradiance Comparison Experiment II (SOLSTICE II): Pre-Launch and On-Orbit Calibrations

The Solar–Stellar Irradiance Comparison Experiment {II (SOLSTICE {II), aboard the Solar Radiation and Climate Experiment (SORCE) spacecraft, consists of a pair of identical scanning grating monochromators, which have the capability to observe both solar spectral irradiance and stellar spectral irradiance using a single optical system. The SOLSTICE science objectives are to measure solar spectral irradiance from 115 to 320 nm with a spectral resolution of 1 nm, a cadence of 6 h, and an accuracy of 5%, to determine its variability with a long-term relative accuracy of 0.5% per year during a 5-year nominal mission, and to determine the ratio of solar irradiance to that of an ensemble of bright B and A stars to an accuracy of 2%. Those objectives are met by calibrating instrument radiometric sensitivity before launch using the Synchrotron Ultraviolet Radiation Facility at the National Institute for Standards and Technology in Gaithersburg, Maryland. During orbital operations irradiance measurements from an ensemble of bright, stable, main-sequence B and A stars are used to track instrument sensitivity. SORCE was launched on 25 January 2003. After spacecraft and instrument check out, SOLSTICE {II first observed a series of three stars to establish an on-orbit performance baseline. Since 6 March 2003, both instruments have been making daily measurements of both the Sun and stars. This paper describes the pre-flight and in-flight calibration and characterization measurements that are required to achieve the SOLSTICE science objectives and compares early SOLSTICE{II measurements of both solar and stellar irradiance with those obtained by SOLSTICE {I on the Upper Atmosphere Research Satellite.     

XUV Photometer System (XPS): Improved Solar Irradiance Algorithm Using CHIANTI Spectral Models

Solar soft X-ray (XUV) radiation is highly variable on all time scales and strongly affects Earth’s ionosphere and upper atmosphere; consequently, the solar XUV irradiance is important for atmospheric studies and for space weather applications. Although there have been several recent measurements of the solar XUV irradiance, detailed understanding of the solar XUV irradiance, especially its variability during flares, has been hampered by the broad bands measured in the XUV range. In particular, the simple conversion of the XUV photometer signal into irradiance, in which a static solar spectrum is assumed, overestimates the flare variations by more than a factor of two as compared to the atmospheric response to the flares. To address this deficiency in the simple conversion, an improved algorithm using CHIANTI spectral models has been developed to process the XUV Photometer System (XPS) measurements with its broadband photometers. Model spectra representative of quiet Sun, active region, and flares are combined to match the signals from the XPS and produce spectra from 0.1 to 40 nm in 0.1-nm intervals for the XPS Level 4 data product. The two XPS instruments are aboard NASA’s Solar Radiation and Climate Experiment (SORCE) and Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) satellites. In addition, the XPS responsivities have been updated for the latest XPS data processing version. The new XPS results are consistent with daily variations from the previous simple conversion technique used for XPS and are also consistent with spectral measurements made at wavelengths longer than 27 nm. Most importantly, the XPS flare variations are reduced by factors of 2 – 4 at wavelengths shorter than 14 nm and are more consistent, for the first time, with atmospheric response to solar flares. Along with the details of the new XPS algorithm, several comparisons to dayglow and photoelectron measurements and model results are also presented to help verify the accuracy of the new XUV irradiance spectra.     

A Computationally Efficient,Time-Dependent Model of the Solar Wind for Use as a Surrogate to Three-Dimensional Numerical Magnetohydrodynamic Simulations

Solar radiation variability spans a wide range in time, ranging from seconds to decadal and longer. The nearly 40 years of measurements of solar irradiance from space established that the total solar irradiance varies by \(\approx 0.1\%\) in phase with the Sun’s magnetic cycle. Specific intervals of the solar spectrum, e.g., ultraviolet (UV), vary by orders of magnitude more. These variations can affect the Earth’s climate in a complex non-linear way. Specifically, some of the processes of interaction between solar UV radiation and the Earth’s atmosphere involve threshold processes and do not require a detailed reconstruction of the solar spectrum. For this reason a spectral UV index based on the (FUV-MUV) color has been recently introduced. This color is calculated using SORCE SOLSTICE integrated fluxes in the FUV and MUV bands. We present in this work the reconstructions of the solar (FUV-MUV) color and Ca ii K and Mg ii indices, from 1749–2015, using a semi-empirical approach based on the reconstruction of the area coverage of different solar magnetic features, i.e., sunspot, faculae and network. We remark that our results are in noteworthy agreement with latest solar UV proxy reconstructions that exploit more sophisticated techniques requiring historical full-disk observations. This makes us confident that our technique can represent an alternative approach which can complement classical solar reconstruction efforts. Moreover, this technique, based on broad-band observations, can be utilized to estimate the activity on Sun-like stars, that cannot be resolved spatially, hosting extra-solar planetary systems.

     

XUV Photometer System (XPS): Overview and Calibrations

The solar soft X-ray (XUV) radiation is highly variable on both short-term time scales of minutes to hours due to flares and long-term time scales of months to years due to solar cycle variations. Because of the smaller X-ray cross sections, the solar XUV radiation penetrates deeper than the extreme ultraviolet (EUV) wavelengths and thus influences the photochemistry and ionization in the mesosphere and lower thermosphere. The XUV Photometer System (XPS) aboard the Solar Radiation and Climate Experiment (SORCE) is a set of photometers to measure the solar XUV irradiance shortward of 34 nm and the bright hydrogen emission at 121.6 nm. Each photometer has a spectral bandpass of about 7 nm, and the XPS measurements have an accuracy of about 20%. The XPS pre-flight calibrations include electronics gain and linearity calibrations in the laboratory over its operating temperature range, field of view relative maps, and responsivity calibrations using the Synchrotron Ultraviolet Radiation Facility (SURF) at the National Institute of Standards and Technology (NIST). The XPS in-flight calibrations include redundant channels used weekly and underflight rocket measurements from the NASA Thermosphere-Ionosphere-Mesosphere-Energetics-Dynamics (TIMED) program. The SORCE XPS measurements have been validated with the TIMED XPS measurements. The comparisons to solar EUV models indicate differences by as much as a factor of 4 for some of the models, thus SORCE XPS measurements could be used to improve these models.     

Titan's atmospheric engine: an overview

Heating occurs in Titan's stratosphere from the absorption of incident solar radiation by methane and aerosols. About 10% of the incident sunlight reaches Titan's surface and causes heating there. Thermal radiation redistributes heat within the atmosphere and cools to space. The resulting vertical temperature profile is stable against convection and a state of radiative equilibrium is established. Equating theoretical and observed temperature profiles enables an empirical determination of the vertical distribution of thermal opacity. A uniformly mixed aerosol is responsible for most of the opacity in the stratosphere, whereas collision-induced absorption of gases is the main contributor in the troposphere. Occasional clouds are observed in the troposphere in spite of the large degrees of methane supersaturation found there. Photochemistry converts CH₄ and N₂ into more complex hydrocarbons and nitriles in the stratosphere and above. Thin ice clouds of trace organics are formed in the winter and early spring polar regions of the lower stratosphere. Precipitating ice particles serve as condensation sites for supersaturated methane vapor in the troposphere below, resulting in lowered methane degrees of supersaturation in the polar regions. Latitudinal variations of stratospheric temperature are seasonal, and lag instantaneous response to solar irradiation by about one season for two reasons: (1) an actual instantaneous thermal response to a latitudinal distribution of absorbing gases, themselves out of phase with the sun by about one season, and (2) a sluggish dynamical response of the stratosphere to the latitudinal transport of angular momentum, induced by radiative heating and cooling. Mean vertical abundances of stratospheric organics and aerosols are determined primarily by atmospheric chemistry and condensation, whereas latitudinal distributions are more influenced by meridional circulations. In addition to preferential scavenging by precipitating ice particles from above, the polar depletion of supersaturated methane results from periodic scavenging by short-lived tropospheric clouds, coupled with the steady poleward march of the continuously drying atmosphere due to meridional transport.     

Characteristics of the radiation emerging from the top of a Rayleigh atmosphere—II Total upward flux and albedo

The solution of the equation of radiative transfer in a medium exhibiting Rayleigh scattering, as developed by S. Chandrasekhar, has been used for an extensive series of computations(³) of the characteristics of the scattered and diffusely reflected radiation emerging from the top of an atmospheric model which corresponds in many respects to the sunlit portion of the earth's atmosphere. The first part of this two-part discussion dealt with the intensity, degree of polarization, plane of polarization and the neutral points of the emergent light as functions of sun elevation, direction in the downward hemisphere, optical thickness of the model atmosphere and reflectivity of the underlying surface. This second part is concerned with the upward flux obtained by an integration of the intensity over the entire hemisphere, for the incident radiation (a) being independent of wavelength or (b) having the spectral distribution of the extra-terrestrial solar radiation. Integration with respect to wavelength in the latter case, together with an approximation for the sphericity of the atmosphere, yields a value of 7.6 per cent for the earth's planetary albedo due to scattering by the clear atmosphere. An approximation for ozone absorption decreases the computed albedo to 6.9 per cent.     

Effects of the Absorption of Solar XUV Radiation by the Earth's Upper Atmosphere at Altitudes of 100–500 km in the X-ray Solar Images Obtained Onboard the Coronas-I (TEREK Telescope) and Coronas-F (SPIRIT X-ray Complex) Satellites

We consider the effects of the absorption of solar XUV radiation by the Earth's atmosphere that were observed in the solar images obtained with the TEREK-K telescope onboard the Coronas-I satellite in May–June 1994 at low solar activity and with the SPIRIT instrumentation onboard the Coronas-F satellite in October–November 2001 at maximum solar activity. The solar images were recorded during the satellite occultation: in the 175- and 304-A spectral ranges onboard Coronas-I with the TEREK-K telescope and in the 175-, 304-, and 8.42-A ranges onboard Coronas-F with the SPIRIT instrumentation. Based on the XUV solar images obtained during atmospheric sounding, apart from the total absorption, we can determine the direction of the atmospheric density gradient and study the local absorption variations with altitude on spatial scales of less than 1 km. The described method can significantly supplement the data obtained in studies of the upper atmosphere by the methods of mass spectrometry, incoherent radar scattering, and the drag of orbital spacecraft.     

Atmospheric effect on the upwelling radiation at the top of the atmosphere over a stream

A new version is adopted for the evaluation of the upwelling radiation from atmosphere bounded by the surface, where the surface is composed of two half semi-infinite Lambert surfaces and a stream is inserted between them. The contrast of the stream is discussed with respect to the atmospheric effect. The width of the stream is considered to be 0.5, 1, and 3km; The solar and observational direction is located in the normal plane to the stream. The observational site is located at altitude 30km. The horizontal distance of observational site to the stream is fixed to 6.28 . The atmosphere is assumed to be homogeneous, which is composed of aerosol and molecules, where the model aerosol is of the oceanic type.In the computational procedure, a probability of radiation interacting with respective half surfaces and the stream are calculated based on the assumption of single scattering in the atmosphere, where isotropic scattering is undertaken. By use of this probability, the emergent radiation at the top of the atmosphere is calculated approximately by considering the radiative interactions between atmosphere and surfaces up to twice. The numerical simulation exhibits the extraordinary effect near the stream. The contrast of the stream depends upon the albedo of the surrounding surfaces. It increases with the increase of the stream width and decreases with the optical thickness.     

Diffusion of radiation in planetary atmospheres

The problem of interaction of the solar radiation with the turbid Earth atmosphere, containing complicated polydispersive aerosol systems, is discussed in this paper. Equations for computing the angular functions ofn-th order scattering are derived. On the basis of these functions the spectral radiance, radiation flows and radiation balance of the atmosphere in the short-wave spectral range are calculated. The relations obtained can be used to calculate the complex index of refraction, distribution function and other characteristics of the submicron aerosol fraction, by solving the inverse problems.     

A New Analysis of the Spectra Obtained by the <Emphasis Type="Italic">Venera</Emphasis> Missions in the Venusian Atmosphere. I. The Analysis of the Data Received from the <Emphasis Type="Italic">Venera-11</Emphasis> Probe at Altitudes Below 37 km in the 0.44–0.66 µm Wavelength Range

The processes of the solar radiation extinction in deep layers of the Venus atmosphere in a wavelength range from 0.44 to 0.66 µm have been considered. The spectra of the solar radiation scattered in the atmosphere of Venus at various altitudes above the planetary surface measured by the Venera-11 entry probe in December 1978 are used as observational data. The problem of the data analysis is solved by selecting an atmospheric model; the discrete-ordinate method is applied in calculations. For the altitude interval from 2–10 km to 36 km, the altitude and spectral dependencies of the volume coefficient of true absorption have been obtained. At altitudes of 3–19 km, the spectral dependence is close to the wavelength dependence of the absorption cross section of S₃ molecules, whence it follows that the mixing ratio of this sulfur allotrope increases with altitude from 0.03 to 0.1 ppbv.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 4, 2005, pp. 304–320.Original Russian Text Copyright © 2005 by Maiorov, Ignat’ev, Moroz, Zasova, Moshkin, Khatuntsev, Ekonomov.     

Emergent radiation from the atmosphere bounded by the two half Lambert surfaces composed of different albedo,II

For the evaluation of the effect of the non-uniform surface albedo on the emergent radiation from the atmosphere, the emergent radiation from the atmosphere bounded by the two half Lambert surfaces composed of different albedo is computed. This paper is the improved version of the previous paper (Takashima and Masuda, 1991). The atmosphere is assumed to be homogeneous, which is composed of aerosol, molecules, and absorbent gases. Their optical thicknesses are (1) 0.25, 0.23, and 0.02, and (2) 0.75, 0.23, and 0.02, respectively. The model aerosol is of the oceanic and water soluble types.In the computational procedure, the emergent radiation is calculated approximately by the contributions due to the multiple scattering in the atmosphere, and due to the diffusely or directly transmitted radiation through the atmosphere which is reflected by the surfaces once (4 interactive radiative modes between atmosphere and surface). Furthermore, to perform the hemispherical integration processing the radiative interaction, the transmission function based on the single scattering in the atmosphere is introduced and then the transmission function is averaged over the hemisphere with weighting function. The numerical simulation exhibits the extraordinary effect near the two half surface boundary of different albedoes. The effect decreases exponentially with the distance from the boundary. The effect depends on the atmospheric aerosol type, optical thickness, and surface albedo. The present version enables us to quantitatively discuss the radiative transfer trend near the boundary of two half surfaces. The upward radiance would simply be evaluated using the present scattering approximation method if the surface albedo is less than 0.3. The present method is thought of as a first step extending the one-dimensional radiative transfer model to two-dimensional using the doubling-adding method.     

Modeling Total Solar Irradiance with San Fernando Observatory Ground-Based Photometry: Comparison with ACRIM, PMOD, and RMIB Composites

We model total solar irradiance (TSI) using photometric irradiance indices from the San Fernando Observatory (SFO), and compare our model with measurements compiled from different space-based radiometers. Space-based measurements of TSI have been obtained recently from ACRIM-3 on board the ACRIMSAT. These data have been combined with other data sets to create an ACRIM-based composite. From VIRGO on board the Solar and Heliospheric Observatory (SOHO) spacecraft two different TSI composites have been developed. The VIRGO irradiance data have been combined by the Davos group to create a composite often referred to as PMOD (Physikalisch-Meteorologisches Observatorium Davos). Also using data from VIRGO, the Royal Meteorological Institute of Belgium (RMIB) has created a separate composite TSI referred to here as the RMIB composite. We also report on comparisons with TSI data from the Total Irradiance Monitor (TIM) experiment on board the Solar Radiation and Climate Experiment (SORCE) spacecraft. The SFO model correlates well with all four experiments during the seven-year SORCE interval. For this interval, the squared correlation coefficient R ² was 0.949 for SORCE, 0.887 for ACRIM, 0.922 for PMOD, and 0.924 for RMIB. Long-term differences between the PMOD, ACRIM, and RMIB composites become apparent when we examine a 21.5-year interval. We demonstrate that ground-based photometry, by accurately removing TSI variations caused by solar activity, is useful for understanding the differences that exist between TSI measurements from different spacecraft experiments.