Sources of Solar Total Irradiance Variations

The daily images and magnetograms acquired by MDI are a rich source of information about the contributions of different types of solar regions to variations in the total solar irradiance (TSI). These data have been used to determine the temporal variation of the MDI irradiance, the mean intensity of the solar disk in the continuum at 676.8 nm. The short-term (days to weeks) variations of the MDI irradiance and TSI are in excellent agreement with rms differences of 0.011%. This indicates that MDI irradiance is an excellent proxy for short-term variations of TSI from the competing irradiance contributions of regions causing irradiance increases, such as plages and bright network, and regions causing irradiance decreases, such as sunspots. However, the long-term or solar cycle variation of the MDI proxy and TSI differ over the 11-year period studied. The results indicate that the primary sources of the long-term (several months or more) variations in TSI are regions with magnetic fields between about 80 and 600 G. The results also suggest that the difference in the long-term variations of the MDI proxy and TSI is due to a component of TSI associated with sectors of the solar spectrum where the contrast in intensity between plages and the quiet Sun is enhanced (e.g., the UV) compared to the MDI proxy. This is evidence that the long-term variation of TSI is due primarily to solar cycle variations of the irradiance from these portions of solar spectrum, a finding consistent with modeling calculations indicating that approximately 60% of the change in TSI between solar minimum and maximum is produced by the UV part of the spectrum shortward of 400 nm (Solanki and Krivova, Space Sci. Rev. 125, 53, 2006).     

ACRIM total solar irradiance satellite composite validation versus TSI proxy models

The satellite total solar irradiance (TSI) database provides a valuable record for investigating models of solar variation used to interpret climate changes. The 35-year ACRIM total solar irradiance (TSI) satellite composite time series has been revised using algorithm updates based on 13 years of accumulated mission experience and corrections to ACRIMSAT/ACRIM3 results for scattering and diffraction derived from recent testing at the Laboratory for Atmospheric and Space Physics/Total solar irradiance Radiometer Facility (LASP/TRF). The net correction lowers the ACRIM3 scale by ～3000 ppm, in closer agreement with the scale of SORCE/TIM results (average total solar irradiance ≈1361.5 W/m²). Differences between the ACRIM and PMOD TSI composites are investigated, particularly the decadal trending during solar cycles 21–22 and the Nimbus7/ERB and ERBS/ERBE results available to bridge the ACRIM Gap (1989–1992), are tested against a set of solar proxy models. Our findings confirm the following ACRIM TSI composite features: (1) The validity of the TSI peak in the originally published ERB results in early 1979 during solar cycle 21; (2) The correctness of originally published ACRIM1 results during the SMM spin mode (1981–1984); (3) The upward trend of originally published ERB results during the ACRIM Gap; (4) The occurrence of a significant upward TSI trend between the minima of solar cycles 21 and 22 and (5) a decreasing trend during solar cycles 22–23. The same analytical approach does not support some important features of the PMOD TSI composite: (1) The downward corrections applied to the originally published ERB and ACRIM1 results during solar cycle 21; (2) The step function sensitivity change in ERB results at the end-of-September 1989; (3) The downward trend of ERBE results during the ACRIM Gap and (4) the use of ERBE results to bridge the ACRIM Gap. Our analysis provides a first order validation of the ACRIM TSI composite approach and its 0.037 %/decade upward trend during solar cycles 21–22. The implications of increasing TSI during the global warming of the last two decades of the 20th century are that solar forcing of climate change may be a significantly larger factor than represented in the CMIP5 general circulation climate models.     

Temporal Variations in Solar Irradiance Since 1947

The study of variations in total solar irradiance (TSI) and spectral irradiance is important for understanding how the Sun affects the Earth’s climate. A data-driven approach is used in this article to analyze and model the temporal variation of the TSI and Mg?ii index back to 1947. In both cases, observed data in the time interval of the satellite era, 1978?–?2013, were used for neural network (NN) model-design and testing. For this particular purpose, the evolution of the solar magnetic field is assumed to be the main driver for the day-to-day irradiance variability. First, we design a model for the Mg?ii index data from F10.7 cm solar radio-flux using the NN approach in the time span of 1978 through 2013. Results of Mg?ii index model were tested using various numbers of hidden nodes. The predicted values of the hidden layer with five nodes correspond well to the composite Mg?ii values. The model reproduces 94% of the variability in the composite Mg?ii index, including the secular decline between the 1996 and 2008 solar cycle minima. Finally, the extrapolation of the Mg?ii index was performed using the developed model from F10.7 cm back to 1947. Similarly, the NN model was designed for TSI variability study over the time span of the satellite era using data from the Physikalisch-Meteorologisches Observatorium Davos (PMOD) as a target, and solar activity indices as model inputs. This model was able to reproduce the daily irradiance variations with a correlation coefficient of 0.937 from sunspot and facular measurements in the time span of 1978?–?2013. Finally, the temporal variation of the TSI was analyzed using the designed NN model back to 1947 from the Photometric Sunspot Index (PSI) and the extrapolated Mg?ii index. The extrapolated TSI result indicates that the amplitudes of Solar Cycles 19 and 21 are closely comparable to each other, and Solar Cycle 20 appears to be of lower irradiance during its maximum.     

Comparison of TSI from SORCE TIM with SFO Ground-Based Photometry

Total solar irradiance (TSI) measurements have been available from the TIM instrument on the SORCE spacecraft since 2003. We compare TSI data, both 24-h and 6-h averages, with photometric indices from red and K-line images obtained on a daily basis at the San Fernando Observatory (SFO). For 1253 days of data from 2 March 2003 to 5 May 2010 we compare the data in linear multiple regression analyses. The best results come from using two photometric indices, the red and K-line photometric sums, and SORCE TSI 6-h averages interpolated to the SFO time of observation. For this case, we obtain a coefficient of multiple determination, R ², of 0.9495 and a quiet-Sun irradiance S ₀?=?1360.810?±?0.004?W?m^?2. These results provide further support for the hypothesis that the quiet Sun is constant over time.     

Solar X-Ray Spectral Irradiance Variability

Solar spectral irradiance at X-ray wavelengths show large variations over a period of solar cycle. We use X-ray irradiance data in three narrow spectral regimes deduced from Yohkoh SXT measurements to study coronal irradiance and their possible association with the activity in the lower atmosphere. Time variation of the X-ray irradiance is important in understanding the emergence of magnetic flux and the effects of such variation on the upper atmosphere of the Earth. We note that about 66% of the total (2 – 30 Å) X-ray irradiance arise from 10 to 20 Å spectral range, while 2 – 10 Å contribute only about 3% of the total. The time variation in 2 – 10 and 10 – 20 Å ranges follow each other closely. Further they follow closely the solar indices such as sunspot, F _10.7, and plage indices, although similarity in the variation of 10 – 20 Å is quite apparent. However, the variation in the other spectral band (20 – 30 Å) differ to a large extent except for the solar cycle dependent variation. We infer that in addition to the active regions, the remnants of active regions contribute considerably to the emission in this spectral range.     

Solar Spectral Irradiance Variability in November/December 2012: Comparison of Observations by Instruments on the International Space Station and Models

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.     

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.     

Extreme Ultraviolet Variability Experiment (EVE) on?the?Solar Dynamics Observatory (SDO): Overview?of?Science Objectives, Instrument Design, Data?Products, and Model Developments

The highly variable solar extreme ultraviolet (EUV) radiation is the major energy input to the Earth’s upper atmosphere, strongly impacting the geospace environment, affecting satellite operations, communications, and navigation. The Extreme ultraviolet Variability Experiment (EVE) onboard the NASA Solar Dynamics Observatory (SDO) will measure the solar EUV irradiance from 0.1 to 105?nm with unprecedented spectral resolution (0.1?nm), temporal cadence (ten seconds), and accuracy (20%). EVE includes several irradiance instruments: The Multiple EUV Grating Spectrographs (MEGS)-A is a grazing-incidence spectrograph that measures the solar EUV irradiance in the 5 to 37?nm range with 0.1-nm resolution, and the MEGS-B is a normal-incidence, dual-pass spectrograph that measures the solar EUV irradiance in the 35 to 105?nm range with 0.1-nm resolution. To provide MEGS in-flight calibration, the EUV SpectroPhotometer (ESP) measures the solar EUV irradiance in broadbands between 0.1 and 39?nm, and a MEGS-Photometer measures the Sun’s bright hydrogen emission at 121.6?nm. The EVE data products include a near real-time space-weather product (Level?0C), which provides the solar EUV irradiance in specific bands and also spectra in 0.1-nm intervals with a cadence of one minute and with a time delay of less than 15?minutes. The EVE higher-level products are Level?2 with the solar EUV irradiance at higher time cadence (0.25?seconds for photometers and ten seconds for spectrographs) and Level?3 with averages of the solar irradiance over a day and over each one-hour period. The EVE team also plans to advance existing models of solar EUV irradiance and to operationally use the EVE measurements in models of Earth’s ionosphere and thermosphere. Improved understanding of the evolution of solar flares and extending the various models to incorporate solar flare events are high priorities for the EVE team.     

SOVAP/Picard,a Spaceborne Radiometer to Measure the Total Solar Irradiance

The Picard spacecraft was successfully launched on 15 June 2010, into a Sun-synchronous orbit. The mission represents one of the European contributions to solar observations and Essential Climate Variables (ECVs) measurements. The payload is composed of a Solar Diameter Imager and Surface Mapper (SODISM) and two radiometers: SOlar VAriability Picard (SOVAP) and PREcision MOnitor Sensor (PREMOS). SOVAP, a dual side-by-side cavity radiometer, measures the total solar irradiance (TSI). It is the sixth of a series of differential absolute-radiometer-type instruments developed and operated in space by the Royal Meteorological Institute of Belgium. The measurements of SOVAP in the summer of 2010 yielded a TSI value of 1362.1 W?m^?2 with an uncertainty of ±?2.4 W?m^?2 (k=1). During the periods of November 2010 and January 2013, the amplitude of the changes in TSI has been on the order of 0.18 %, corresponding to a range of about 2.4 W?m^?2.     

The Solar Ultraviolet Spectrum Estimated Using the Mg <Emphasis Type="SmallCaps">ii</Emphasis> Index and Ca <Emphasis Type="SmallCaps">ii</Emphasis> K Disk Activity

As part of a program to estimate the solar spectrum back to the early twentieth century, we have generated fits to UV spectral irradiance measurements from 1 – 410 nm. The longer wavelength spectra (150 – 410 nm) were fit as a function of two solar activity proxies, the Mg ii core-to-wing ratio, or Mg ii index, and the total Ca ii K disk activity derived from ground based observations. Irradiance spectra at shorter wavelengths (1 – 150 nm) where used to generate fits to the Mg ii core-to-wing ratio alone. Two sets of spectra were used in these fitting procedures. The fits at longer wavelengths (150 to 410 nm) were derived from the high-resolution spectra taken by the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on the Upper Atmospheric Research Satellite (UARS). Spectra measured by the Solar EUV Experiment (SEE) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite were used for the fits at wavelengths from 1 to 150 nm. To generate fits between solar irradiance and solar proxies, this study uses the above irradiance data, the NOAA composite Mg ii index, and daily Ca ii K disk activity determined from images measured by Big Bear Solar Observatory (BBSO). In addition to the fitting coefficients between irradiance and solar proxies, other results from this study include an estimated relationship between the fraction of the disk with enhanced Ca ii K activity and the Mg ii index, an upper bound of the average solar UV spectral irradiance during periods where the solar disk contains only regions of the quiet Sun, as was believed to be present during the Maunder Minimum, as well as results indicating that slightly more than 60% of the total solar irradiance (TSI) variability occurs between 150 and 400 nm.     

Modulations of the surface magnetic field on the intra-cycle variability of total solar irradiance

Solar photospheric magnetic field plays a dominant role in the variability of total solar irradiance (TSI). The modulation of magnetic flux at six specific ranges on TSI is characterized for the first time. The daily flux values of magnetic field at four ranges are extracted from MDI/SOHO, together with daily flux of active regions (MF\(_{\text{ar}}\)) and quiet regions (MF\(_{\text{qr}}\)); the first four ranges (MF\(_{1\mbox{--}4}\)) are: 1.5–2.9, 2.9–32.0, 32.0–42.7, and 42.7–380.1 (\(\times 10^{18}\) Mx per element), respectively. Cross-correlograms show that MF₄, MF\(_{\text{qr}}\), and MF\(_{ \text{ar}}\) are positively correlated with TSI, while MF₂ is negatively correlated with TSI; the correlations between MF₁, MF₃ and TSI are insignificant. The bootstrapping tests confirm that the impact of MF₄ on TSI is more significant than that of MF\(_{\text{ar}}\) and MF\(_{\text{qr}}\), and MF\(_{\text{ar}}\) leads TSI by one rotational period. By extracting the rotational variations in the MFs and TSI, the modulations of the former on the latter at the solar rotational timescale are clearly illustrated and compared during solar maximum and minimum times, respectively. Comparison of the relative amplitudes of the long-term variation show that TSI is in good agreement with the variation of MF₄ and MF\(_{\text{ar}}\); besides, MF₂ is in antiphase with TSI, and it lags the latter by about 1.5 years.     

The Global Context of Solar Activity During the Whole Heliosphere Interval Campaign

The Whole Heliosphere Interval (WHI) was an international observing and modeling effort to characterize the 3-D interconnected ??heliophysical?? system during this solar minimum, centered on Carrington Rotation 2068, March 20??C?April 16, 2008. During the latter half of the WHI period, the Sun presented a sunspot-free, deep solar minimum type face. But during the first half of CR 2068 three solar active regions flanked by two opposite-polarity, low-latitude coronal holes were present. These departures from the quiet Sun led to both eruptive activity and solar wind structure. Most of the eruptive activity, i.e., flares, filament eruptions and coronal mass ejections (CMEs), occurred during this first, active half of the interval. We determined the source locations of the CMEs and the type of associated region, such as active region, or quiet sun or active region prominence. To analyze the evolution of the events in the context of the global solar magnetic field and its evolution during the three rotations centered on CR 2068, we plotted the CME source locations onto synoptic maps of the photospheric magnetic field, of the magnetic and chromospheric structure, of the white light corona, and of helioseismological subsurface flows. Most of the CME sources were associated with the three dominant active regions on CR 2068, particularly AR 10989. Most of the other sources on all three CRs appear to have been associated with either isolated filaments or filaments in the north polar crown filament channel. Although calculations of the flux balance and helicity of the surface magnetic features did not clearly identify a dominance of one region over the others, helioseismological subsurface flows beneath these active regions did reveal a pronounced difference among them. These preliminary results suggest that the ??twistedness?? (i.e., vorticity and helicity) of subsurface flows and its temporal variation might be related to the CME productivity of active regions, similar to the relationship between flares and subsurface flows.     

The Solar Dynamo and Its Phase Transitions during the Last Millennium

Variations in total solar irradiance (TSI) correlate well with changes in projected area of photospheric magnetic flux tubes associated with dark sunspots and bright faculae in active regions and network. This correlation does not, however, rule out possible TSI contributions from photospheric brightness inhomogeneities located outside flux tubes and spatially correlated with them. Previous reconstructions of TSI report agreement with radiometry that seems to rule out significant “extra-flux-tube” contributions. We show that these reconstructions are more sensitive to the facular contrasts used than has been generally recognized. Measurements with the Solar Bolometric Imager (SBI) provide the first reliable support for the relatively high, wide-band, disk-center contrasts required to produce 10% rms agreement. Longer term bolometric imaging will be required to determine whether the small but systematic TSI residuals we see here are caused by remaining errors in spot and facular areas and contrasts or by extra-flux-tube brightness structures such as bright rings around sunspots or “convective stirring” around active regions.     

The Whole Heliosphere Interval in the Context of a Long and Structured Solar Minimum: An Overview from Sun to Earth

Throughout months of extremely low solar activity during the recent extended solar-cycle minimum, structural evolution continued to be observed from the Sun through the solar wind and to the Earth. In 2008, the presence of long-lived and large low-latitude coronal holes meant that geospace was periodically impacted by high-speed streams, even though solar irradiance, activity, and interplanetary magnetic fields had reached levels as low as, or lower than, observed in past minima. This time period, which includes the first Whole Heliosphere Interval (WHI 1: Carrington Rotation (CR) 2068), illustrates the effects of fast solar-wind streams on the Earth in an otherwise quiet heliosphere. By the end of 2008, sunspots and solar irradiance had reached their lowest levels for this minimum (e.g., WHI 2: CR 2078), and continued solar magnetic-flux evolution had led to a flattening of the heliospheric current sheet and the decay of the low-latitude coronal holes and associated Earth-intersecting high-speed solar-wind streams. As the new solar cycle slowly began, solar-wind and geospace observables stayed low or continued to decline, reaching very low levels by June??C?July 2009. At this point (e.g., WHI 3: CR 2085) the Sun?CEarth system, taken as a whole, was at its quietest. In this article we present an overview of observations that span the period 2008??C?2009, with highlighted discussion of CRs 2068, 2078, and 2085. We show side-by-side observables from the Sun??s interior through its surface and atmosphere, through the solar wind and heliosphere and to the Earth??s space environment and upper atmosphere, and reference detailed studies of these various regimes within this topical issue and elsewhere.     

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.

     

Total Solar Irradiance Variation During Rapid Sunspot Growth

Large sunspot areas correspond to dips in the total solar irradiance (TSI), a phenomenon associated with the local suppression of convective energy transport in the spot region. This results in a strong correlation between sunspot area and TSI. During the growth phase of a sunspot other physics may affect this correlation; if the physical growth of the sunspot resulted in surface flows affecting the temperature, for example, we might expect to see an anomalous variation in TSI. In this paper we study NOAA active region 8179, in which large sunspots suddenly appeared near disk center, at a time (March 1998) when few competing sunspots or plage regions were present on the visible hemisphere. We find that the area/TSI correlation does not significantly differ from the expected pattern of correlation, a result consistent with a large thermal conductivity in solar convection zone. In addition we have searched for a smaller-scale effect by analyzing white-light images from MDI (the Michelson Doppler Imager) on SOHO. A representative upper-limit energy consistent with the images is on the order of 3×10³¹ ergs, assuming the time scale of the actual spot area growth. This is of the same order of magnitude as the buoyant energy of the spot emergence even if it is shallow. We suggest that detailed image analyses of sunspot growth may therefore show `transient bright rings' at a detectable level.     

A New Method for Modeling the Coronal Magnetic Field with STEREO and Submerged Dipoles

Recent magnetic modeling efforts have shown substantial misalignment between theoretical models and observed coronal loop morphology as observed by STEREO/EUVI, regardless of the type of model used. Both potential field and non-linear force-free field (NLFFF) models yielded overall misalignment angles of 20??C?40 degrees, depending on the complexity of the active region (Sandman et al., Solar Phys. 259, 1, 2009; DeRosa et al., Astrophys. J. 696, 1780, 2009) We demonstrate that with new, alternative forward-fitting techniques, we can achieve a significant reduction in the misalignment angles compared with potential field source surface (PFSS) models and NLFFF models. Fitting a series of submerged dipoles to the field directions of stereoscopically triangulated loops in four active regions (30 April, 9 May, 19 May, and 11 December 2007), we find that 3??C?5 dipoles per active region yield misalignment angles of ???11°??C?18°, a factor of two smaller than those given by previously established extrapolation methods. We investigate the spatial and temporal variation of misalignment angles with subsets of loops for each active region, as well as loops observed prior to and following a flare and filament eruption, and find that the spatial variation of median misalignment angles within an active region (up to 75%) exceeds the temporal variation associated with the flare (up to 40%). We also examine estimates of the stereoscopic error of our analysis. The corrected values yield a residual misalignment of 7°??C?13°, which is attributed to the non-potentiality due to currents in the active regions.     

Empirical Modeling of TSI: A Critical View

Solar empirical models based on regression of two variability indices for radiation from the photosphere and chromosphere fit total solar irradiance (TSI) observations with accuracy comparable to the precision reported for the observations themselves. However, the physical meaning of the fitting coefficients and their stability during different phases of the solar cycle has not been examined in detail. We test the stability of the coefficients in regression models of the VIRGO TSI observations over the nine years from the minimum of Cycle 23 in 1996 through the maximum to 2005. We also show how the coefficients converge to the ‘`best fit’' using a search in the coefficient space. Analysis of TSI variability in different phases of this cycle shows little change in regression models as long as the time periods used in the regression are long enough to show the slow solar cycle variation in TSI. We extend our analysis to TSI observations from ERB, ACRIM2, ACRIM3, DIARAD, and TIM. The regression models from these time series show large systematic differences in fitting coefficients for the plage and sunspot indices that we used. These differences are significantly larger than the estimated uncertainties in the coefficients and point to the difficulty of combining observations from different instruments to create an accurate composite TSI record over several solar cycles. Our results clearly demonstrate the improvement in precision of TSI measurements from the Nimbus 7 ERB in Cycle 22 to the latest SORCE TIM data in Cycle 23.     

Quiet Sun Contribution to Variations in the Total Solar Irradiance

An analysis of spatially-resolved measurements of the intensity of the photospheric continuum by the Michelson Doppler Imager (MDI) on the SOHO spacecraft indicates that these data can be used to study variations of the Total Solar Irradiance (TSI). Since the techniques employed depend upon ratios of intensities measured by MDI, they are independent of the absolute photometric calibration of the instrument. The results suggest that, while it is possible to account for short-term (weeks to months) variation in TSI by variations in the irradiance contributions of regions with enhanced magnetic fields (larger than ten G as measured by MDI), the longer-term variations are influenced significantly by variations in the brightness of the quiet Sun, defined here as regions with magnetic field magnitudes smaller than ten G. The latter regions cover a substantial fraction of the solar surface, ranging from approximately 90% of the Sun near solar minimum to 70% near solar maximum. The results provide evidence that a substantial fraction, 50% or more, of the longer term (≥one year) variation in TSI is due to changes in the brightness of the quiet Sun.