Active Cavity Radiometer Irradiance Monitor Research Articles

Modeling Quiet Solar Luminosity Variability from TSI Satellite Measurements and Proxy Models during 1980–2018

A continuous record of direct total solar irradiance (TSI) observations began with a series of satellite experiments in 1978. This record requires comparisons of overlapping satellite observations with adequate relative precisions to provide useful long term TSI trend information. Herein we briefly review the active cavity radiometer irradiance monitor physikalisch-meteorologisches observatorium davos (ACRIM-PMOD) TSI composite controversy regarding how the total solar irradiance (TSI) has evolved since 1978 and about whether TSI significantly increased or slightly decreased from 1980 to 2000. The main question is whether TSI increased or decreased during the so-called ACRIM-gap period from 1989 to 1992. There is significant discrepancy between TSI proxy models and observations before and after the gap, which requires a careful revisit of the data analysis and modeling performed during the ACRIM-gap period. In this study, we use three recently proposed TSI proxy models that do not present any TSI increase during the ACRIM-gap, and show that they agree with the TSI data only from 1996 to 2016. However, these same models significantly diverge from the observations from 1981 and 1996. Thus, the scaling errors must be different between the two periods, which suggests errors in these models. By adjusting the TSI proxy models to agree with the data patterns before and after the ACRIM-gap, we found that these models miss a slowly varying TSI component. The adjusted models suggest that the quiet solar luminosity increased from the 1986 to the 1996 TSI minimum by about 0.45 W/m2 reaching a peak near 2000 and decreased by about 0.15 W/m2 from the 1996 to the 2008 TSI cycle minimum. This pattern is found to be compatible with the ACRIM TSI composite and confirms the ACRIM TSI increasing trend from 1980 to 2000, followed by a long-term decreasing trend since.

The 27-day rotational variations in total solar irradiance observations: From SORCE/TIM, ACRIMSAT/ACRIM III, and SOHO/VIRGO

During the last decade, observations from SORCE (Solar Radiation and Climate Experiment)/TIM (Total Irradiance Monitor), ACRIMSAT (Active Cavity Radiometer Irradiance Monitor Satellite)/ACRIM III, and SOHO (Solar and Heliospheric Observatory)/VIRGO (Variability of IRradiance and Gravity Oscillations Sun PhotoMeter) provided Total Solar Irradiance (TSI) measurements with unprecedented accuracy and stability to determine the amount of solar irradiance reaching the top of the atmosphere and how solar irradiance varies on different time scales. These three independent measurements are analyzed using the EEMD (Ensemble Empirical Mode Decomposition) method to characterize the phase and amplitude of the 27-day solar rotational variation in TSI. The mode decomposition clearly identifies a 27-day solar rotational signature on TSI measurements. The rotational variations of TSI from the three independent observations are generally consistent with each other, despite different mean TSI values. During the declining phase of solar cycle 23, the amplitude of TSI 27-day variations is as high as 0.8W/m2 (~0.05%), while during the rising phase of solar cycle 24, the amplitude is up to 0.4W/m2 (~0.04%). During the minimum phase (2008–2009), the amplitude of the rotational mode is only ~0.1W/m2. The correlation of this rotational mode between TIM and ACRIM III is ~0.92 and the slope of the local peak values is ~0.98. The correlation between TIM and VIRGO is ~0.96 and the slope of the local peak values is ~0.98, very similar to the slope with ACRIM III.

Reduction of measurements of total solar flux to a unified scale and uncertainty in estimating its long-term trends

A comparative analysis of measurements of total solar flux in the active cavity radiometer irradiance monitor (ACRIM) and Physikalisch-Meteorologisches Observatorium Davos (PMOD) scales was performed. An analysis of integrated differences of simultaneous measurements of solar flux made it possible to estimate the systematic differences that can lead to the accumulation large errors with time. The measurements of solar flux in ACRIM and PMOD scales were reduced to an intermediate radiometric scale through data correction for systematic errors. The corrected measurements in the unified scale were consistent and there is a compromise between initial data sets. The measurements in the intermediate scale indicate changes in solar flux between the minima of solar activity in 1986, 1996, and 2008. The amplitude of these variations is comparable with uncertainty in their long-term trends, which questions earlier obtained estimates for secular variations of solar luminosity.

Sources of Differences in On-Orbital Total Solar Irradiance Measurements and Description of a Proposed Laboratory Intercomparison.

There is a 5 W/m2 (about 0.35 %) difference between current on-orbit Total Solar Irradiance (TSI) measurements. On 18–20 July 2005, a workshop was held at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland that focused on understanding possible reasons for this difference, through an examination of the instrument designs, calibration approaches, and appropriate measurement equations. The instruments studied in that workshop included the Active Cavity Radiometer Irradiance Monitor III (ACRIM III) on the Active Cavity Radiometer Irradiance Monitor SATellite (ACRIMSAT), the Total Irradiance Monitor (TIM) on the Solar Radiation and Climate Experiment (SORCE), the Variability of solar IRradiance and Gravity Oscillations (VIRGO) on the Solar and Heliospheric Observatory (SOHO), and the Earth Radiation Budget Experiment (ERBE) on the Earth Radiation Budget Satellite (ERBS). Presentations for each instrument included descriptions of its design, its measurement equation and uncertainty budget, and the methods used to assess on-orbit degradation. The workshop also included a session on satellite- and ground-based instrument comparisons and a session on laboratory-based comparisons and the application of new laboratory comparison techniques. The workshop has led to investigations of the effects of diffraction and of aperture area measurements on the differences between instruments. In addition, a laboratory-based instrument comparison is proposed that uses optical power measurements (with lasers that underfill the apertures of the TSI instruments), irradiance measurements (with lasers that overfill the apertures of the TSI instrument), and a cryogenic electrical substitution radiometer as a standard for comparing the instruments. A summary of the workshop and an overview of the proposed research efforts are presented here.

Wavelet analysis of stellar differential rotation

Future space projects like KEPLER will deliver a vast quantity of high precision light curves of stars. This paper describes a test concerning the observability of rotation and even dierential rotation of slowly rotating stars from such data. Two published light curves of solar total irradiance measures are investigated: the Nimbus-7 Earth Radiation Budget (ERB) observations between 1978 and 1993 and the Active Cavity Radiometer Irradiance Monitor I (ACRIM I) measurements between 1980 and 1989. Light curve analysis show that oscillations on time-scales comparable to solar rotation but of a complex pattern are visible. Neither Fourier analysis nor time-frequency Wavelet analysis yield the true rotation period during the more active phases of the solar cycle. The true rotation period dominates only for a short time during solar minimum. In the light of this study even space-born broad band photometry may turn out an inappropriate instrument to study stellar butterfly diagrams of stars rotating as slow as the Sun. However, it was shown in Papers I and II of this series that chromospheric tracers like Lyman, Mg II h+k and CaII H+K are appropriate instruments to perform this task.

Contribution of the Solar Constant (SOLCON) program to the long‐term total solar irradiance observations

On long timescales the variation of the total solar irradiance (TSI) received by the Earth is believed to be one of the climate change drivers. Therefore accurate and time‐stable measurements of the total solar irradiance are necessary. The Solar Constant (SOLCON) instrument made TSI measurements in April 1992 and during the International Extreme Ultraviolet Hitchhiker (IEH) 3 STS 95 shuttle flight in autumn 1998. We assume that the SOLCON instrument remained stable within 0.01 % in between those measurements, and we verify this assumption as well as possible. From the SOLCON measurements we conclude the following: (1) The 1998 Space Absolute Radiometric Reference (SARR) adjustment coefficient applicable to the Active Cavity Radiometer Irradiance Monitor II (ACRIM II) during the IEH 3 period is 1.000438 with a one sigma uncertainty of 18 ppm, compared to the 1993 SARR adjustment coefficient of 1.000258. (2) The solar monitor on the Earth Radiation Budget Satellite (ERBS), with a 1993 SARR adjustment coefficient of 1.000453, has not aged within a one sigma uncertainty level of 130 ppm; and (3) the 1998 SARR adjustment coefficients for the Variability of Solar Irradiance and Gravity Oscillations (VIRGO) radiometers have been determined with a one sigma uncertainty of 10 ppm: They are 1.000025 for the Differential Absolute Radiometer left channel (DIARAD‐L) and 1.000279 for the version 1.2 data from the PMO6‐VA radiometer.

Total Solar Irradiance Trend During Solar Cycles 21 and 22

Results from Active Cavity Radiometer Irradiance Monitor (ACRIM) experiments show an upward trend in total solar irradiance of 0.036 percent per decade between the minima of solar cycles 21 and 22. The trend follows the increasing solar activity of recent decades and, if sustained, could raise global temperatures. Trends of total solar irradiance near this rate have been implicated as causal factors in climate change on century to millennial time scales.

Solar total irradiance observations from spacecraft: 1992–1993

A brief review of the simultaneous observations of solar total irradiance performed by the UARS ACRIM II (upper atmosphere research satellite active cavity radiometer irradiance monitor), EURECA SOVA 1 and SOVA 2 (European retrievable carrier platform solar variability experiments), ERBS ERBE (Earth Radiation Budget Satellite Earth Radiation Budget Experiment), Nimbus 7 ERB, and Atlas 1 and 2 in 1992–1993 (a period characterized as the declining phase of solar cycle 22), is presented in this paper. Statistical comparison of the irradiance values and the linear relationships among the time series are examined for the overlap period August 1992 to May 1993. The mean irradiance at 1 astronomical unit (AU) observed by the UARS ACRIM II, EURECA SOVA 1, EURECA SOVA 2, and ERBS ERBE is within the range of 1365.5–1366.9 W m−2, and high correlation (r2 = 0.94) exists among the time series. Linear regression models based on the measurements are used in estimating the observed irradiance and predicting the missing values. The challenge in establishing a homogeneous long‐term database from the independent observations of shorter duration requires a clear understanding of the compatibility of the data sets and any instrument‐related drift or degradation.

Variations in total solar irradiance during solar cycle 22

In this study, we have attempted to model the variations in total solar irradiance from two spacecraft. Specifically, we have modeled the Earth Radiation Budget on the Nimbus 7 spacecraft and the active cavity radiometer irradiance monitor (ACRIM‐I) on the Solar Maximum Mission (SMM) spacecraft using ground‐based photometry of sunspots and faculae from the San Fernando Observatory (SFO). Additionally, for some cases, solar backscatter ultraviolet/2 data on the Mg II core‐to‐wing ratio from the NOAA 9 spacecraft was used. We have found that most of the solar cycle variation in the total solar irradiance can be accounted for by sunspots and faculae/network. The unexplained variation is not greater than approximately 0.0022% (22 ppm) per year for most of solar cycle 22. Using Nimbus 7 data from March 2, 1985, to December 13, 1993 (1281 data points), as the dependent variable, with the SFO photometric sunspot index (PSI) and the NOAA 9 Mg II core‐to‐wing ratio for the faculae/network as independent variables (the best model for this interval), we obtained a multiple correlation coefficient squared (R2) of 0.848. The rms noise in the residuals is approximately 0.221 W m−2 (162 ppm). This rms noise appears to be dominated by noise in the spacecraft data. For the same model, but for the time interval from March 2, 1985, to July 14, 1989, we obtained an R2 of 0.838 for 718 data points. The same type of model for this same interval, substituting SMM/ACRIM‐I total irradiance for Nimbus 7, gave an R2 of 0.857 for 685 data points. Our best correlation, however, came from a three‐parameter model, fitting Nimbus 7 data to the SFO digital PSI, the SFO facular index PFIFA, and the NOAA 9 data for the interval May 30, 1988, to December 13, 1993, giving an R2 of 0.887 (745 data points). These strong correlations suggest that most of the variation in solar irradiance is associated with known solar magnetic features. Whether or not these magnetic features can explain all of the solar irradiance variability will require more stable and accurate long‐term measurements from space and the ground.

Microwave proxies for sunspot blocking and total irradiance

The microwave flux of the Sun is responsive to the same conditions that produce magnetically structured radiation at visible and X ray wavelengths, and so the solar flux at high radio frequencies such as 2800 MHz (10.7 cm) has been used as a proxy for solar optical variations. We have previously found that the the microwave flux time series show spectral variations that provide useful proxy information for total irradiance, and we have extended our analysis of the daily solar fluxes from Toyokawa Observatory at 1000, 2000, 3750, and 9400 MHz, in addition to the Ottawa 2800‐MHz flux, for the years 1980‐1989. An essential ingredient in our analysis is the extraction of the rotationally‐modulated microwave component, which differs from the “S component” as recently defined in the literature. The rotationally‐modulated fraction of the emission contains a significant, often dominant, contribution from gyroresonance emission, whereas the S component, as defined by the excess above the cycle minimum level, usually does not. This allows us to distinguish plage‐associated emission from spot‐associated emission in the time series of microwave flux. We show that in combination, the microwave fluxes for 1000–9400 MHz, which span the spectral peak of spot‐associated emission, provide a very good proxy for both the active cavity radiometer irradiance monitor (ACRIM) total irradiance and the sunspot‐blocked component of the irradiance, even without optical sunspot observations. Over the 1984–1989 period, this proxy has a weighted correlation with ACRIM of 95%, and an RMS deviation from the total irradiance of 0.27 w/m², slightly better than the deviation (0.35 w/m²) found using optical data.

The time‐delayed solar cycle luminosity modulation by sub‐surface magnetic flux tubes

AbstractIn order to explore the mechanism of the solar cycle luminosity change observed by the Active Cavity Radiometer Irradiance Monitor (ACRIM) I experiment on board of the spacecraft Solar Maximum Mission, we examined running mean time profiles of the daily ACRIM data from the declining phase of solar cycle 21 to the rising phase of solar cycle 22. By comparing them with those of the daily sunspot number, integrated surface magnetic field flux, integrated He I 10830 Å line equivalent width data, and two kinds of data sets of the daily integrated Ca II K line index as indices of the surface magnetic activities, we found (i) that the running mean time profiles of the six independent data sets have several peaks and valleys in common in one solar cycle with time intervals on the order of a few hundreds of days, and (ii) that the peaks and valleys of the ACRIM data profiles followed the peaks and valleys of all the other five indices of the surface activities by 40 to 60 days. This time delay phenomenon suggests (i) that the luminosity modulation was not directly caused by dark and bright features of the surface magnetic activities that the other five indices represent, and (ii) that the missing sunspot radiative flux which was blocked by sub‐surface magnetic flux tubes of sunspots and sunspot groups should be re‐radiated 40 to 60 days after the surface emergence of the magnetic flux tubes. The concept of the time delay resolves the enigma of the missing sunspot radiative flux and the enigma of the ACRIM experiment that the luminosity dropped when a sunspot or a sunspot group appeared on the surface while the yearly mean of the luminosity decreased and increased along with the decrease and increase of the yearly sunspot number of the 11‐year solar cycle. A model of the mechanism to understand these phenomena is presented and its application to other stars is suggested.

Prior to surface appearance of sunspot flux tubes and magneto‐thermal pulsation of the Sun

AbstractWe found an evidence that the luminosity of the Sun systematically decreased about 20 days before sunspot surface appearance by analysing time‐lag correlation of time derivatives of running mean time profiles of the data of the Active Cavity Radiometer Irradiance Monitor (ACRIM) I experiment on board of Solar Maximum Mission (SMM) and of the data of the daily sunspot number. This indicates that sunspot flux tube cooling and heat transport blocking by the flux tubes start to take place in the interior of the solar convection zone well before the sunspot surface appearance. From this finding and our previous finding that the luminosity of the Sun systematically increased and the blocked heat appeared on the surface about 50 days after the sunspot surface appearance, a new view of sunspot formation and dynamics and a new view of the luminosity modulation emerged. (i) Sunspots of a solar cycle are formed from clusters of flux tubes which can be seen in the running mean time profile of the sunspot number as a peak with duration on the order of 100 to 200 days. (ii) Heat flow is blocked by the cluster of sunspot flux tubes inside the convection zone to decrease the luminosity about 20 days before the surface emergence of the sunspot cluster. (iii) The blocked heat appears on the surface about 50 days after the surface emergence of the cluster of sunspot flux tubes to heat up the surface. This appears as a thermal pulse in the running mean time profile of the ACRIM dat in between the peaks of the sunspot running mean time profile. This process of heating the surface makes the temperature gradient less steep and weakens the buoyancy of sunspot flux tubes below the surface. (vi) The radiative cooling of the surface layer by the excess heat release steepens the temperature gradient so that the buoyancy of the sub‐surface magnetic flux tubes becomes stronger to cause the next surge of emergence of a cluster of sunspots and other magnetic activities, which creates a peak in the time profile of the sunspot number. We call this peak a magnetic pulse of the Sun and the coupled process of alternating pulsed appearance of heat and sunspots the magneto‐thermal pulsation of the Sun.

Precise ground‐based solar photometry and variations of total irradiance

Variations in the total solar irradiance measured by the active cavity radiometer irradiance monitor (ACRIM) on SMM have been correlated with measures of magnetic activity on the solar disk. Quantitative indices of magnetic activity were derived from ground‐based, full‐disk, photometric images of the Sun at red (6723 Å) and violet (3934‐Å K line) wavelengths. The red images have been obtained on a daily basis at the San Fernando Observatory since 1985, and the K line images since 1988. Sunspot irradiance deficits are calculated directly from the red images while proxy measures of facular irradiance excesses are derived from the K line images. The images analyzed here were made during 21 days between June 20 and July 14, 1988, a period centered on the disk passage of a large sunspot group. The best two‐parameter multiple correlation coefficient between the ACRIM data and the photometric data is R² = 0.97 (21 data points, 18 degrees of freedom). The zero point S0 = 1367.27 W m−2 agrees well with the solar irradiance measured by ACRIM/SMM during the 1986 activity minimum; the residual standard deviation was 0.13 W m−2 (about 100 ppm). The multiple correlations were extended to include measures of the irradiance contribution of “network” magnetic fields, unassociated with active regions. NOAA 9 spacecraft observations of UV Mg II lines at 2800 Å gave R² = 0.99 (17 degrees of freedom) with S0 = 1366.68 + 0.08 W m−2. The index of 10.7‐cm microwave flux gave R² = 0.98, with S0 = 1366.43 + 0.11 W m−2. We can thus model short‐term irradiance changes to within 100 ppm relative precision from ground‐based data.

The Nimbus 7 solar total irradiance: A new algorithm for its derivation

The Nimbus 7 satellite has measured the solar total irradiance from November 1978 to July 1991 (153 months). These measurements are important both in solar physics and for climate change. To insure that the Nimbus 7 measurements are capturing the true behavior of the Sun, it is essential that the properties of the radiometer and its changes over time be understood. The calibration of the radiometer can be viewed as a process of removing instrumental influences from the raw measurements, leaving the experimenter with an estimate of solar variability. In this paper the changing radiometer pointing, the zero offsets, the stability of the gain, the temperature sensitivity, and the influences of other platform instruments are all examined and their effects on the measurements considered. Only the question of relative accuracy (not absolute) is examined. The resulting derived solar irradiances are compared to previous analyses of the Nimbus 7 radiometer and to the Solar Maximum Mission (SMM) measurements. Compared to previous analyses, the newly derived values are higher and somewhat less variable than the older values. Removal of the off‐axis pointing errors and a new treatment of the zero offsets are the major reasons for the changes. Compared to the SMM measurements, both agree quite well so long as any solar activity is present. When the Sun becomes quiet, so its irradiance variability is less than the Nimbus 7 radiometer resolution, the comparison to the SMM results breaks down. Between 1980 and 1988 the correlation of the daily values is 0.83, compared to 0.62 using previously published values from both satellites. The monthly means have a correlation of 0.90, indicating that about 80% of the longer‐term variance is in common. In 1980 when both satellites were operating without problems, Nimbus 7 was 0.3155% higher on average. For May 1984 to December 1988, Nimbus 7 was 0.3255% higher. Therefore if the Nimbus 7 satellite continues to operate properly until a period of about 1 year after the launch of UARS with its active cavity radiometer irradiance monitor (ACRIM), it appears that the differences between Nimbus 7, SMM ACRIM, and UARS ACRIM can be measured to within a few hundredths of a percent. A self‐consistent set of solar irradiance measurements from several satellites over nearly two solar cycles appears feasible.

The Sun's luminosity over a complete solar cycle

THE Active Cavity Radiometer Irradiance Monitor (ACRIM I), an instrument carried on NASA's Solar Maximum Mission satellite, measured the Sun's luminosity (total power outflow) from early 1980 to late 19891–5. Here we present the first account of the complete ACRIM I data set, and give evidence confirming our previous suggestion that solar luminosity varies with the 11-year solar cycle6. As previously reported, this slow variation closely follows statistical measures of the distribution of magnetic and photospheric features on the Sun's surface4–8. But there was an exception to this correlation in the form of a remarkable irradiance excess during 1980, at about the time of the sunspot maximum of solar cycle 21. The linkage, over a whole cycle, of luminosity variation to photospheric activity suggests the existence of an unknown physical mechanism other than the thermal diffusion model that explains luminosity deficits due to sunspots. Luminosity models connecting total irradiance to global indicators of solar activity, such as the equivalent width of the 1,083-nm helium line, are consistent with the gross features of the variability, but fail to account for the 1980 irradiance excess.

Line-blanketing variations in the irradiance spectrum of the sun from maximum to minimum of the solar cycle

Solar irradiance spectra obtained at or near the maximum and minimum phases of the solar magnetic activity cycle are compared. The characteristics of these spectra are related to the irradiance measurements available from the Active Cavity Radiometer Irradiance Monitor (ACRIM) of the SMM. An interpretation based on sunspots and faculae being added to the atmosphere at solar maximum can account for less than 4 percent of the ACRIM decrease to be arising in the 500-560 nm range. The UV, with its much greater line-blanketing, is more important, in agreement with the finding of Lean (1989).

Helioseismology with the ACRIM instrument on the solar maximum mission

The Active Cavity Radiometer Irradiance Monitor (ACRIM) instrument on board SMM pioneered high-precision solar photometry from space, and provided the first detection of solar p-mode oscillations at low degree by this technique. The observations extended from February, 1980, until December, 1989, with a hiatus of low sampling rate in 1981–1984. During summer 1989, the instrument operated in a “no-shutter” mode with continuous viewing between the orbital gaps. This resulted in a fourfold increase of the duty cycle, and an effective increase in the Nyquist frequency from 3.815 mHz to some tens of mHz. This review discusses the initial results from this campaign along with a general review of the analyses to date of the entire ACRIM data set.

Application of solar max ACRIM data to analysis of solar-driven climatic variability on Earth

Climatic change caused by solar variability has been proposed for at least a century, but could not be assessed reliably in the past because the uncertainty in solar irradiance measured from the Earth's surface is too large. Now satellite measurements by such instruments as the Active Cavity Radiometer Irradiance Monitor (ACRIM) permit a preliminary assessment. The satellite data exhibit irradiance variations over a spectrum of shorter timescales, but the first 5-yr overall trend indicates slightly decreasing luminosity. The global temperature response to monthly-mean ACRIM-measured fluctuations from 1980–1984 was computed from the NYU 1D transient climate model - which includes thermal inertia effects of the world oceans - starting from an assumed pre-existing steady state, and the results compared with observations of recent global temperature trends. The modeled surface temperature evolution exhibited a complex history-dependent behavior whose fluctuations were an order of magnitude smaller than observed, primarily owing to oceanic thermal damping. Thus solar variability appears unlikely to have been an important factor in global-scale climate change over this period. The possibility of using the measurements to develop simple correlations for irradiance with longer term solar activity observable from the surface, and therefore to analyze historical effects, was considered, but is not supported by the satellite data. However, we have used a model of solar irradiance variation with time (Schatten, 1988), covering the period 1976–1997 in order to assess our model's response to forcing whose fluctuation timescale is comparable to the thermal relaxation time of the upper ocean. Continuous monitoring of solar flux by space-based instruments over timescales of 20 yr or more, comparable to timescales for thermal relaxation of the oceans, and of the solar cycle itself, is probably needed to resolve issues of long-term solar variation effects on climate.

Intercomparison of solar total irradiance data from recent spacecraft measurements

Solar total irradiance data obtained by the solar monitors of the Earth Radiation Budget Experiment (ERBE) on board the Earth Radiation Budget Satellite (ERBS) and on the National Oceanic and Atmospheric Administration spacecraft NOAA 9 and NOAA 10 are compared with the solar total irradiance data obtained from the Solar Maximum Mission (SMM) Active Cavity Radiometer Irradiance Monitor 1 (ACRIM 1), the sounding rockets, and the Nimbus 7 Earth radiation budget (ERB) channel 10 C. For the first 2 years of solar total irradiance measurements in space, the ERBE solar monitors on board ERBS and NOAA 9 spacecraft yielded the same mean irradiance value of 1364.9 W m−2 for the solar constant. This value is 0.2% lower than the mean value of the solar constant derived from the SMM ACRIM 1 and the sounding rockets, and it is 0.4% lower than the Nimbus 7 ERB measurements. For the first 6 months of solar irradiance, data available from the ERBE solar monitor on board NOAA 10 spacecraft yielded a mean value of 1363.2 W m−2 for the solar constant. The current solar monitoring and ERB measurement programs have yielded solar total irradiance data of high accuracy and precision. The disparity between the solar total irradiance values obtained in near‐simultaneous measurements with absolute pyrheliometers of similar design and mode of operation is an indication of the uncertainty associated with absolute pyrheliometers available at present. This disparity points out the need for intercalibration of electrically self‐calibrating pyrheliometers from spacecraft to derive an absolute pyrheliometric scale, valid for solar total irradiance measurements from space.

Spectrum Line Intensity as a Surrogate for Solar Irradiance Variations

Active Cavity Radiometer Irradiance Monitor (ACRIM) solar constant measurements from 1980 to 1986 are compared with ground-based, irradiance spectrophotometry of selected Fraunhofer lines. Both data sets were identically sampled and smoothed with an 85-day running mean, and the ACRIM total solar irradiance (S) values were corrected for sunspot blocking (S(c)). The strength of the mid-photospheric manganese 539.4-nanometer line tracks almost perfectly with ACRIM S(e), Other spectral features formed high in the photosphere and chromosphere also track well. These comparisons independently confirm the variability in the ACRIM S(e), signal, indicate that the source of irradiance is faculae, and indicate that ACRIM S(e), follows the 11-year activity cycle.