Solar Stellar Irradiance Comparison Experiment Research Articles

Solar Irradiance Spectra from the Compact SOLSTICE (CSOL) Experiment: Instrument Design, FUV Calibration, Measurements, and Comparison of the 2018 Rocket Flight

The Compact SOLSTICE, a compact far and mid ultraviolet (FUV and MUV) spectrograph, flew on a sounding rocket on 18 June 2018 to validate and potentially calibrate the SOLar STellar Irradiance Comparison Experiment (SOLSTICE) onboard the Solar Radiation Climate Experiment (SORCE) spacecraft. This article reports the instrument design, the calibration of the FUV channel, and the FUV irradiance measurements. Irradiance measurements are compared to SOLSTICE showing agreement within the combined instrumental uncertainties at most wavelengths, including the H Lyman-alpha emission at 121.6 nm. Some unexplained differences in line ratios between 130.5 nm and 147.5 nm are observed.

SOLar-STellar Irradiance Comparison Experiment II (SOLSTICE II): End-of-Mission Validation of the SOLSTICE Technique

The SOLar-STellar Irradiance Comparison Experiment (SOLSTICE: McClintock, Rottman, and Woods, Solar Phys.230, 225, 2005) onboard the SOlar Radiation and Climate Experiment (SORCE: Rottman, Solar Phys.230, 7, 2005) observed ultraviolet solar spectral irradiance (SSI) from 2003 – 2020. This article gives an overview of the end-of-mission algorithms and calibration of SOLSTICE. Many of the algorithms were updated after the early mission, either due to an improved understanding of the instrument and the space environment, or due to operational constraints as the spacecraft systems aged. We validate the final official data version (V18) with comparisons to other observations and models. The SOLSTICE observations of the solar-cycle variability in the ultraviolet are compared to model estimates.

How long do satellites need to overlap? Evaluation of climate data stability from overlapping satellite records

Abstract. Sensors on satellites provide unprecedented understanding of the Earth's climate system by measuring incoming solar radiation, as well as both passive and active observations of the entire Earth with outstanding spatial and temporal coverage. A common challenge with satellite observations is to quantify their ability to provide well-calibrated, long-term, stable records of the parameters they measure. Ground-based intercomparisons offer some insight, while reference observations and internal calibrations give further assistance for understanding long-term stability. A valuable tool for evaluating and developing long-term records from satellites is the examination of data from overlapping satellite missions. This paper addresses how the length of overlap affects the ability to identify an offset or a drift in the overlap of data between two sensors. Ozone and temperature data sets are used as examples showing that overlap data can differ by latitude and can change over time. New results are presented for the general case of sensor overlap by using Solar Radiation and Climate Experiment (SORCE) Spectral Irradiance Monitor (SIM) and Solar Stellar Irradiance Comparison Experiment (SOLSTICE) solar irradiance data as an example. To achieve a 1 % uncertainty in estimating the offset for these two instruments' measurement of the Mg II core (280 nm) requires approximately 5 months of overlap. For relative drift to be identified within 0.1 % yr−1 uncertainty (0.00008 W m−2 nm−1 yr−1), the overlap for these two satellites would need to be 2.5 years. Additional overlap of satellite measurements is needed if, as is the case for solar monitoring, unexpected jumps occur adding uncertainty to both offsets and drifts; the additional length of time needed to account for a single jump in the overlap data may be as large as 50 % of the original overlap period in order to achieve the same desired confidence in the stability of the merged data set. Results presented here are directly applicable to satellite Earth observations. Approaches for Earth observations offer additional challenges due to the complexity of the observations, but Earth observations may also benefit from ancillary observations taken from ground-based and in situ sources. Difficult choices need to be made when monitoring approaches are considered; we outline some attempts at optimizing networks based on economic principles. The careful evaluation of monitoring overlap is important to the appropriate application of observational resources and to the usefulness of current and future observations.

The dependence of the [FUV-MUV] colour on solar cycle

Solar UV variability is extremely relevant for the stratospheric ozone. It has an impact on Earth's atmospheric structure and dynamics through radiative heating and ozone photochemistry. Our goal is to study the slope of the solar UV spectrum in two UV bands important for the stratospheric ozone production. In order to investigate the solar spectral variability, we use SOLSTICE (the Solar Stellar Irradiance Comparison Experiment) data onboard Solar Radiation and Climate Experiment (SORCE) satellite. Data sets used are far UV (115-180nm) and middle UV (180-310nm), as well as the Mg II index (the Bremen composite). We introduce the SOLSTICE [FUV - MUV] colour to study the solar spectral characteristics, as well as analysis of the colour versus Mg II index. To isolate the 11-year scale variation, we used the Empirical Mode decomposition (EMD) on the data sets. The [FUV - MUV] colour strongly correlates with the Mg II index. More in detail, the [FUV - MUV] colour shows a time dependent behavior when plotted versus Mg II index. To explain this dependence we hypothesize an efficiency reduction of SOLSTICE FUV irradiance using an exponential aging law.

Solar irradiance observations with PREMOS filter radiometers on the PICARD mission: In-flight performance and data release

The PREcision Monitoring Sensor (PREMOS) is a solar radiometer on board the French PICARD mission that was launched in June 2010 and decommissioned in April 2014. Aims. The PREMOS radiometer obtains solar irradiance measurements in specific spectral windows in the UV, visible, and near- infrared. In this paper, the PREMOS data and calibration methods are presented. Methods. Using back-up channels, the degradation can theoretically be assessed to correct operational channels. However, a strong degradation within all PREMOS channels requires the application of additional methods, namely using back-up channels and assess- ing the degradation via a proxy-based model. Results. The corrected Level 3 PREMOS data are then used in different contexts in order to be validated. First, the signature of the p-mode are retrieved from the PREMOS data. The Venus transit allows us to empirically determine the intrinsic noise level within the PREMOS high cadence data for the visible and near-infrared channels. We then compare the PREMOS data directly to other data sets, namely from the SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) and the Solar Irradiance Monitor (SIM) instru- ments on board the SOlar Radiation and Climate Experiment (SORCE) spacecraft. Regarding the UV channels, we found an excellent correlation over the lifetime of the PREMOS mission. The ratio between SORCE and PREMOS observations is always less than 1%. Regarding the SSI measurements in the visible and near-infrared, a comparison of short-term variations (i.e. 27-day modulation) shows a rather good correlation by taking into consideration the intrinsic noise within both SIM and PREMOS observations.

A comparison of stratospheric photochemical response to different reconstructions of solar ultraviolet radiative variability

We present calculations of stratospheric chemical abundances variations between different levels of solar activity using a simple photochemistry model in transient chemistry mode. Different models for the reconstruction of the solar spectrum, as well as observations from the SOLar STellar Irradiance Comparison Experiment (SOLSTICE) and Spectral Irradiance Monitor (SIM) on the SOlar Radiation and Climate Experiment (SORCE) satellite, are used as inputs to the calculations. We put the emphasis on the MOnte CArlo Spectral Solar Irradiance Model (MOCASSIM) reconstructions, which cover the spectral interval from 150 to 400nm and extend from 1610 to present. We compare our results with those obtained with the Naval Research Laboratory Solar Spectral Irradiance (NRLSSI) model as well as with the Magnesium-Neutron Monitor (MGNM) model over a period of time spanning the ascending phase of Cycle 22. We also perform the calculations using SORCE composite spectra for the descending phase of Cycle 23 and with the reconstructed MOCASSIM, NRLSSI and MGNM spectra for the same period for comparison. Finally, we compare the chemical abundances obtained for the Maunder Minimum with those obtained for the Cycle 23 minimum (in March 2009) and find that stratospheric ozone concentration was slightly higher during the recent minimum, consequent to the small positive variability between the MOCASSIM spectra for both epochs, especially below 260nm. We find that the response in stratospheric ozone is not only dependent on the variability amplitude in the solar spectrum (especially in the 200–240nm band), but also significantly on the base level of the minimum solar spectrum.

A New SATIRE-S Spectral Solar Irradiance Reconstruction for Solar Cycles 21–23 and Its Implications for Stratospheric Ozone*

Abstract The authors present a revised and extended total and spectral solar irradiance (SSI) reconstruction, which includes a wavelength-dependent uncertainty estimate, spanning the last three solar cycles using the Spectral and Total Irradiance Reconstruction—Satellite era (SATIRE-S) model. The SSI reconstruction covers wavelengths between 115 and 160 000 nm and all dates between August 1974 and October 2009. This represents the first full-wavelength SATIRE-S reconstruction to cover the last three solar cycles without data gaps and with an uncertainty estimate. SATIRE-S is compared with the Naval Research Laboratory Spectral Solar Irradiance (NRLSSI) model and ultraviolet (UV) observations from the Solar Radiation and Climate Experiment (SORCE) Solar Stellar Irradiance Comparison Experiment (SOLSTICE). SATIRE-S displays similar cycle behavior to NRLSSI for wavelengths below 242 nm and almost twice the variability between 242 and 310 nm. During the decline of the last solar cycle, between 2003 and 2008, the SSI from SORCE SOLSTICE versions 12 and 10 typically displays more than 3 times the variability of SATIRE-S between 200 and 300 nm. All three datasets are used to model changes in stratospheric ozone within a 2D atmospheric model for a decline from high solar activity to solar minimum. The different flux changes result in different modeled ozone trends. Using NRLSSI leads to a decline in mesospheric ozone, while SATIRE-S and SORCE SOLSTICE result in an increase. Recent publications have highlighted increases in mesospheric ozone when considering version 10 SORCE SOLSTICE irradiances. The recalibrated SORCE SOLSTICE version 12 irradiances result in a much smaller mesospheric ozone response than that of version 10, and this smaller mesospheric ozone response is similar in magnitude to that of SATIRE-S. This shows that current knowledge of variations in spectral irradiance is not sufficient to warrant robust conclusions concerning the impact of solar variability on the atmosphere and climate.

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.

Comparison of Magnesium II core-to-wing ratio observations during solar minimum 23/24

The MgII index is an important proxy for solar activity; in particular it correlates well with the EUV which is important for understanding the upper atmosphere. We compare the measurements from all of the instruments making daily measurements during the most recent solar minimum. After adjustments to the data to account for instrumental effects, we find that there are still some discrepancies between the various time series. The data from the primary channel of the SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) on SORCE requires a correction factor starting in early 2006 in order to bring it into agreement with the redundant SOLSTICE channel and with the other datasets. Once this correction has been applied, all the MgII measurements are in good agreement throughout the solar minimum interval.

Patterns of carbon monoxide in the middle atmosphere and effects of solar variability

We determine the spatial-time patterns of zonally averaged carbon monoxide (CO) in the middle atmosphere by applying Principle Component Analysis to the CO data obtained from the Microwave Limb Sounder (MLS) measurements on the Aura satellite in 2004–2012. The first two principal components characterize more than 90% of the CO variability. Both principal components are localized in the low thermosphere near the mesopause. The first principal component is asymmetric relative to the poles. It has opposite signs in the Northern and Southern Hemisphere at mid to high latitudes and strongly oscillates with an annual periodicity. The second principal component has the same sign in both hemispheres and oscillates mainly with a semi-annual frequency. Both principal components are modulated by the 11-year solar cycle and display short-term variations. To test possible correlations of these variations with the short term solar ultraviolet (UV) variability we use the simultaneous measurements of the UV solar radiance from the Solar-Stellar Irradiance Comparison Experiment (SOLSTICE) on the Solar Radiation and Climate Experiment (SORCE) satellite to investigate the correlation between CO in the middle atmosphere and solar UV in 2004–2012. Using a wavelet coherence technique a weak, intermittent 27-day signal is detected in high-frequency parts of the CO principal components.

Interannual variations of MLS carbon monoxide induced by solar cycle

More than eight years (2004–2012) of carbon monoxide (CO) measurements from the Aura Microwave Limb Sounder (MLS) are analyzed. The mesospheric CO, largely produced by the carbon dioxide (CO2) photolysis in the lower thermosphere, is sensitive to the solar irradiance variability. The long-term variation of observed mesospheric MLS CO concentrations at high latitudes is likely driven by the solar-cycle modulated UV forcing. Despite of different CO abundances in the southern and northern hemispheric winter, the solar-cycle dependence appears to be similar. This solar signal is further carried down to the lower altitudes by the dynamical descent in the winter polar vortex. Aura MLS CO is compared with the Solar Radiation and Climate Experiment (SORCE) total solar irradiance (TSI) and also with the spectral irradiance in the far ultraviolet (FUV) region from the SORCE Solar-Stellar Irradiance Comparison Experiment (SOLSTICE). Significant positive correlation (up to 0.6) is found between CO and FUV/TSI in a large part of the upper atmosphere. The distribution of this positive correlation in the mesosphere is consistent with the expectation of CO changes induced by the solar irradiance variations.

The role of the solar irradiance variability in the evolution of the middle atmosphere during 2004-2009

[1] Recent measurements by the Spectral Irradiance Monitor (SIM) and the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) onboard the Solar Radiation and Climate Experiment satellite have revealed the spectral solar irradiance (SSI) changes in the ultraviolet between 2004 and 2009 to be several times higher than it was shown by all previous SSI measurements and reconstructions. In this paper, we simulate the O3, OH, and temperature responses to solar irradiance variability using four different SSI data sets trying to define which one gives the best agreement between the simulated and observed responses. First, we apply the 1-D radiative-convective model with interactive photochemistry to determine the regions of the atmosphere where the O3, OH, and temperature are most sensitive to the spectrum discrepancies between the different SSI data sets. As the comparison with observations can be only made taking into account dynamics and all known forcings of the atmosphere, we then apply the 3-D chemistry-climate model SOCOL to simulate the atmosphere evolution from May 2004 to February 2009. We compare the modeled OH, O3, and temperature changes with atmospheric data measured by several space instruments. Overall, the comparison shows that the atmospheric changes simulated with the 3-D SOCOL model driven by the SIM and SOLSTICE SSI are closest to the atmospheric measurements.

Middle atmosphere response to different descriptions of the 11-yr solar cycle in spectral irradiance in a chemistry-climate model

Abstract. The 11-yr solar cycle in solar spectral irradiance (SSI) inferred from measurements by the SOlar Radiation & Climate Experiment (SORCE) suggests a much larger variation in the ultraviolet than previously accepted. We present middle atmosphere ozone and temperature responses to the solar cycles in SORCE SSI and the ubiquitous Naval Research Laboratory (NRL) SSI reconstruction using the Goddard Earth Observing System chemistry-climate model (GEOSCCM). The results are largely consistent with other recent modeling studies. The modeled ozone response is positive throughout the stratosphere and lower mesosphere using the NRL SSI, while the SORCE SSI produces a response that is larger in the lower stratosphere but out of phase with respect to total solar irradiance above 45 km. The modeled responses in total ozone are similar to those derived from satellite and ground-based measurements, 3–6 Dobson Units per 100 units of 10.7-cm radio flux (F10.7) in the tropics. The peak zonal mean tropical temperature response using the SORCE SSI is nearly 2 K per 100 units F10.7 – 3 times larger than the simulation using the NRL SSI. The GEOSCCM and the Goddard Space Flight Center (GSFC) 2-D coupled model are used to examine how the SSI solar cycle affects the atmosphere through direct solar heating and photolysis processes individually. Middle atmosphere ozone is affected almost entirely through photolysis, whereas the solar cycle in temperature is caused both through direct heating and photolysis feedbacks, processes that are mostly linearly separable. This is important in that it means that chemistry-transport models should simulate the solar cycle in ozone well, while general circulation models without coupled chemistry will underestimate the temperature response to the solar cycle significantly in the middle atmosphere. Further, the net ozone response results from the balance of ozone production at wavelengths less than 242 nm and destruction at longer wavelengths, coincidentally corresponding to the wavelength regimes of the SOLar STellar Irradiance Comparison Experiment (SOLSTICE) and Spectral Irradiance Monitor (SIM) on SORCE, respectively. A higher wavelength-resolution analysis of the spectral response could allow for a better prediction of the atmospheric response to arbitrary SSI variations.

Solar UV variations during the decline of Cycle 23

Previous satellite measurements of solar UV variability show consistent solar cycle irradiance changes within instrumental uncertainties, and also show consistent spectral dependence for both rotational and solar cycle variations. Empirical solar irradiance models produce solar UV variations that agree well with observational data. Recent UV irradiance data from the Solar Radiation and Climate Experiment (SORCE) Spectral Irradiance Monitor (SIM) and Solar Stellar Irradiance Comparison Experiment (SOLSTICE) instruments longward of 170nm covering the declining phase of Cycle 23 show solar variations that greatly exceed both previous measurements and predicted irradiance changes over this period. The spectral dependence of the SIM and SOLSTICE variations differs from previous results. However, short-term solar variability derived from SIM and SOLSTICE UV irradiance data agrees with other concurrent solar UV measurements and previous results, suggesting no change in solar physics. The SORCE long-term UV results can be explained by undercorrection of instrument response changes during early on-orbit measurements.

UV solar irradiance low during recent solar minimum

Solar irradiance, which varies with the 11‐year solar cycle and on longer time scales, can affect temperatures and winds in the atmosphere, influencing Earth's climate. As the Sun currently wakes up from a period of low sunspot activity, researchers want to know how irradiance during the recent solar minimum compares to historical levels. In addition to understanding the total received power, it is important to know how various spectral bands behave, in particular, the ultraviolet, which causes heating and winds in the stratosphere. Lockwood analyzed solar ultraviolet spectral irradiance data from May 2003 to August 2005 from both the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instrument on board the Upper Atmosphere Research Satellite (UARS) and the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) instrument on the Solar Radiation and Climate Experiment (SORCE) satellite. Using several different methods to intercalibrate the data, he developed a data composite that can be used to determine differences between the recent solar minimum and previous minima. The author found that solar irradiance during the recent sunspot minimum has been especially low. (Journal of Geophysical Research‐Atmospheres, doi:10.1029/2010JD014746, 2011)

Was UV spectral solar irradiance lower during the recent low sunspot minimum?

[1] A detailed analysis is presented of solar UV spectral irradiance for the period between May 2003 and August 2005, when data are available from both the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instrument (on board the Upper Atmosphere Research Satellite (UARS) spacecraft) and the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) instrument (on board the Solar Radiation and Climate Experiment (SORCE) satellite). The ultimate aim is to develop a data composite that can be used to accurately determine any differences between the “exceptional” solar minimum at the end of solar cycle 23 and the previous minimum at the end of solar cycle 22 without having to rely on proxy data to set the long-term change. SUSIM data are studied because they are the only data available in the “SOLSTICE gap” between the end of available UARS SOLSTICE data and the start of the SORCE data. At any one wavelength the two data sets are considered too dissimilar to be combined into a meaningful composite if any one of three correlations does not exceed a threshold of 0.8. This criterion removes all wavelengths except those in a small range between 156 nm and 208 nm, the longer wavelengths of which influence ozone production and heating in the lower stratosphere. Eight different methods are employed to intercalibrate the two data sequences. All methods give smaller changes between the minima than are seen when the data are not adjusted; however, correcting the SUSIM data to allow for an exponentially decaying offset drift gives a composite that is largely consistent with the unadjusted data from the SOLSTICE instruments on both UARS and SORCE and in which the recent minimum is consistently lower in the wave band studied.

The SORCE SIM Solar Spectrum: Comparison with Recent Observations

The Spectral Irradiance Monitor (SIM) on board the NASA SORCE satellite (Solar Radiation and Climate Experiment) was launched on 25 January 2003 and has been making twice-daily measurements of solar variability in the 220 to 1630 nm range and daily measurements in the 1600 to 2400 nm range. This study presents preflight and postlaunch calibration activities of the SIM instrument and its flight spare components as well as in-flight comparisons with the ATLAS 3 composite spectrum (Atmospheric Laboratory for Applications and Science) in the ultraviolet (UV), visible, and near infrared (NIR) as well as comparisons with the SOLSTICE (Solar Stellar Irradiance Comparison Experiment) in the UV. In the 258 to 1350 nm range, the SIM agrees with ATLAS 3 with a fractional difference of −0.021±0.021 (k=1, estimated standard deviation) and with the additional corrections discussed herein the agreement improves to −0.008±0.021 (k=1). In the ultraviolet (220–307 nm) the agreement between all the instruments in this study is better than 5%, but fractional differences reveal other instrument- and calibration-related differences. In the 1350 to 2400 nm range the agreement between SIM and ATLAS 3 is about 8%, so these SIM data are corrected to agree with ATLAS 3 in this range.

Solar spectral irradiance variability in the ultraviolet from SORCE and UARS SOLSTICE

The SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) on the SOlar Radiation and Climate Experiment (SORCE) has been measuring the solar spectral irradiance on a daily basis since early 2003. This time period includes near-solar maximum conditions, the Halloween storms of 2003, and solar minimum conditions. These results can be compared to observations from the SOLSTICE I experiment that flew on the Upper Atmosphere Research Satellite (UARS) during the decline of the previous solar cycle as well as with currently operating missions. We will discuss similarities and differences between the two solar cycles in the long-term ultraviolet irradiance record.

Solar–Stellar Irradiance Comparison Experiment II (Solstice II): Examination of the Solar–Stellar Comparison Technique

The Solar-Stellar Irradiance Comparison Experiment (SOLSTICE) measures the solar spectral irradiance from 115 to 320 nm with a resolution of 0.1 nm. The Sun and stars are both observed with the same optics and detector, changing only the apertures and integration times. Pre-launch calibration at SURF allows us to measure both with an absolute accuracy of 5%. The in-flight sensitivity degradation is measured relative to a set of stable, early-type stars. The ensemble of stars form a calibration reference standard that is stable to better than 1% over timescales of centuries. The stellar irradiances are repeatedly observed on a grid of wavelengths and our goal is to measure changes in the absolute sensitivity of the instrument at the 0.5% per year level. This paper describes the details of the observing technique and discusses the level of success in achieving design goals.

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.