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[CIDCFTP Data] [Solar IrradIDC Data on FTP] Data Access To tal Solar Irradiance Data [rule] Readme Contents Data Set Overview Sponsor Original Archive Future Updates The Data Characteristics Source The Files Format Name and Directory Information Companion Software The Science Theoretical Basis of Data Processing Sequence and Algorithms Scientific Potential of Data Validation of Data Contacts Points of Contact References [rule] Data Set Overview Total solar irradiances are presented for the period November 16, 1978 through December 1997. The measurement program is continuing and additional years will be added as they become available. This irradiance provides the energy that powers the Earth's climate and biosphere. It had long been suspected that the solar energy emitted towards the Earth varied with time but this was not definitely demonstrated until accurate, self-calibrating pyrheliometers flown on satellites began to regularly monitor the Sun (Hickey et al., 1980). The measured solar variations are of the order of fractions of a percent and atmospheric transmission problems had previously limited the accuracy of ground based measurements (Willson, 1984). Data from four experiments are included here: The Nimbus-7 Earth Radiation Budget (ERB) measurements (November 1978--December 1993), the Active Cavity Radiometer Irradiance Monitor I (ACRIM I) measurements (February 1980--July 1989) on the Solar Maximum Mission (SMM), the solar monitor measurements (October 1984--June 1996) on the Earth Radiation Budget Satellite (ERBS), and the ACRIM II measurements (October 1991--December 1996) on the Upper Atmosphere Research Satellite (UARS). In May 1997 the preliminary ACRIM II data set (1991-1993) was replaced by the final version which now runs through December 1997. Both daily and monthly mean values are given. For ease of comparison all the measurements are converted to the value that would be obtained at the mean annual Earth to Sun distance. Sponsor The production and distribution of this data set are being funded by NASA's Earth Science enterprise. The data are not copyrighted, however, we request that when you publish data or results using these data please acknowledge as follows: The authors wish to thank John R. Hickey and the Nimbus-7 ERB Experiment Team for their solar data; Richard C. Willson for the ACRIM I & II solar data; Robert B. Lee III and the ERBE Science Team for the ERBS solar data, and the Distributed Active Archive Center (code 902.2) at the Goddard Space Flight Center, Greenbelt, MD, 20771, for putting these data in their present format and distributing them. The production and distribution of these data were sponsored by NASA's Earth Science enterprise. Original Archive The solar irradiance data, in the Goddard DAAC's Inter-Discipline Data Collection, was acquired from the original experiment teams. The data is also held at other sites such as the Solar-Terrestrial Physics division of the National Geophysical Data Center. Future Updates The ARCIM II and ERBS data sets, on board the UARS and ERBS satellites will be updated as new data are acquired. The Data [Irradiance chart] Characteristics Parameters: Total solar irradiance Units: Watts/m^2 Range: ACRIM I 1364.48 to 1369.71 ACRIM II 1363.75 to 1367.14 ERB 1368.50 to 1374.80 ERBS 1363.10 to 1367.60 Temporal Coverage: ACRIM I February 16, 1980 through July 14, 1989 ACRIM II October 4, 1991 through December 31, 1997 ERB November 16, 1978 through December 13, 1993 ERBS October 25, 1984 through June 19. 1996 Temporal Resolution: Records are available in both daily and monthly temporal resolution. Spatial Coverage: This is satellite orbital data Spatial Resolution: Not applicable Source The ACRIM experiment is part of an ongoing program that involves identical instruments. The instrument flew on Spacelab 1 in 1983, ATLAS 1 in 1992 and aboard the Solar Maximum Mission (SMM) Satellite from 1980 to 1989. Another ACRIM instrument is currently, aboard the Upper Atmosphere Research Satellite (UARS), which was launched on September 12, 1991 and is still operating. The ACRIM data available at this FTP site are from the instruments on board the SMM (ACRIM I) and UARS (ACRIM II) instruments. The ACRIM contains four cylindrical bays. Three of the bays house independent heat detectors, called pyrheliometers, which are independently shuttered, self calibrating, automatically controlled, and which are uniformly sensitive from the extreme UV to the far infrared. Each pyrheliometer consists of two cavities, and temperature differences between the two are used to determine the total solar flux. One cavity is maintained at a constant reference temperature, while the other is heated 0.5 K higher than the reference cavity and is exposed to the Sun periodically. When the shutter covering the second cavity is open, sunlight enters, creating an even greater difference in cavity temperatures. The power supplied to the second cavity by the ACRIM electronics decreases automatically to maintain the 0.5 K temperature difference between the two cavities. This decrease in the amount of electricity is proportional to the solar irradiance entering the cavity. Additional details about the individual sensors is given by Willson (1979 & 1980) and of the instrument by Willson (1981). The fourth bay holds a sensor that measures the relative angle between the instrument and the Sun. To guarantee precision, the ACRIM cavities have mirror-like black surfaces that reflect light toward the apex of the cavity, where 99.99998 percent of the Sun's incoming energy in the 180 to 3,000-nm wavelength range is absorbed. In normal operation the ACRIM is on a platform which tracks the Sun. One of its detector channels makes regular measurements while the other two are kept shuttered to reduce possible degradation by solar UV radiation, atmospheric or satellite outgased gases, etc. Readings are taken at 1.024 second intervals. About once a month the second channel, B, is opened for comparison measurements; while at longer intervals the third channel, C, is also compared. This triple detector arrangement proved valuable. On the SMM Satellite channel A degraded about 600 parts per million compared to channel C during the 9.75 year mission. Channel B, opened roughly once a month, also showed a slight degradation compared to channel C by 1989. This degradation was allowed for in the calibration equation (Willson and Hudson, 1991). The SMM spacecraft was in a circular orbit of 33-degree inclination to the equator, and the ACRIM I observed the Sun for about 65 minutes of each 96 minute orbit. In normal operation the satellite has precision solar pointing, and the shutter over the active sensors open or close about every 65 seconds (131.072 seconds per cycle), giving a solar observation followed by a reference comparison. During the reference phase the sensor views the internal surface of the shutter which compared to the Sun is a near-zero irradiance source. The difference between the electrical power dissipated in the cavity during the reference and the observation phases, adjusted for the shutter temperature, is equivalent to the amount of radiative energy absorbed by the cavity. The data consists of averages of 32 individual readings per shutter cycle that for the SMM were averaged again into an orbital mean that consists of as many as 28 shutter cycles (Willson et al., 1986). Measurements can be made with the shutter continuously open but this is not normally done. The individual readings are digitized on a (0-8191) quantization scale. In December 1980 the solar-pointing system of the SMM failed, and the spacecraft was placed into a spin-stabilized mode until its repair by the crew of the NASA space shuttle in April 1984. During the spin-stabilized period of operation the shutter of ACRIM I channel A was opened at orbit sunrise and closed at orbit sunset. In this mode an average of 100 solar observations were made per day. This measurement mode produced a systematic bias of 0.12% compared to the usual sun pointing mode. This effect was removed from the published data (Willson et al., 1986). In September 1991 the UARS was placed in a 585 km altitude, circular orbit which is inclined 57 degrees to the Equator. The orbit period is 97 minutes. The ACRIM II instrument is on a Sun tracking arm. It started its measurement program on October 4, 1991. Since then the measurement program has been continuous except for a few periods. The longest gap runs from June 3 through July 21, 1992. This was caused by satellite system problems. Measurements (1991-1993) using a preliminary calibration routine were released (Willson, 1994). These were available on this site for some time. In the Spring of 1997 Willson issued a new measurement set (1991-1996) which was updated through December 31, 1997 in January 1998. These measurements are calculated using the final calibration equation, and now replace the preliminary measurement set. The measurements are reported on the ACRIM II native scale defined by the operation of sensor B, the full-time monitoring sensor. The results are reconciled to the mean Earth to Sun distance and are fully corrected for sensor degradation (Willson, 1997). The Earth Radiation Budget Experiment (ERBE) solar sensors have a basic design rather similar to that of the ACRIM but its instrument package and mode of operation are quite different (Lee et al., 1987 & 1991). There is only one sensor, instead of three, and the shutter is normally opened and closed every 32 seconds, instead of every 65 seconds. The sensor does not normally point at the Sun. About once every 14 days the satellite is turned so that the sensor can view the Sun during a single orbit for a 128 to 640 second period. During the 32 second measurement periods the Sun drifts through the unobstructed field of view of the monitor which is + or - 4.6 angular degrees. The angular position of the Sun with respect to the optical axis is considered since the response of the monitor varies as the cosine of the angular position. A sensor reading is taken every 0.9 seconds but the sensor time constant is 3.3 seconds and it takes 28 seconds (8.5 time constants) for the output signal to reach 99.98% of its full-scale value. The solar reading is taken by averaging over the last 4 seconds, or last five data points of a phase. The individual readings are digitized on a (0-8191) quantization scale (Mecherikunnel et al., 1988). Similar ERBE instrument packages were placed on three satellites: the NASA Earth Radiation Budget Satellite (ERBS) and two NOAA operational weather satellites, NOAA-9 & NOAA-10. The solar measurements from the NOAA-9 & -10 were noisier than those from the ERBS (Barkstrom et al., 1990), and only the ERBS solar measurements are included in this data collection. The Nimbus-7 Earth Radiation Budget (ERB) solar sensor differs both in design and operating mode from the other two. There were two Nimbus ERB instruments built in the early 1970s. The first was launched in June 1975 (Smith et al., 1977). The second instrument was somewhat modified and then launched on the Nimbus-7 in October 1978. One important change was the replacement in the solar telescope of the solar channel 10s with a cavity pyrheliometer (channel 10c). Both the sensor size and data system were thus constrained. The sensor is non symmetric with a toroidal plated thermopile in the back. A cavity receiver is affixed to its front. The cavity is composed of an inverted cone within a cylinder, the interior of which is coated with a specularly reflecting black paint. A calibration heater is wound mostly on the cone (about 94%) and partially on the lower cylinder (about 6%). This distribution is to achieve the best match to where radiation heating will occur for direct beam measurements. A precision aperture of 0.5 cm^2 is mounted in front of the cavity. The cavity has a larger diameter than the aperture so that all of the direct beam energy falls on the cone. The radiometer has a 10-degree field of view which allow the Sun to fully irradiate the cavity for about three minutes of each 104-minute orbit (Hickey et al., 1988). The Sun drifts through the field of view. The channel 10c time constant is 0.4 seconds. There is one reading per second with a signal integration time of 0.8 seconds and a read out and reset time of 0.2 seconds. Each reading is digitized to a (0-2047) quantization scale. The readings from the sensor vary as the cosine of the Sun's off-axis angle. The essentially flat peak, 40 central "on Sun" readings are averaged to obtain a mean value for each orbit (Hoyt et al., 1992). The on-Sun counts are corrected to a deep-space reference, by applying the average offset of the radiometer when viewing deep space 13-minutes before the solar reading. Channel 10c is calibrated at 12-day intervals by introducing a measured amount of electrical resistance heat into the cavity. The Files Format * File Size: range in size from 0.4 kB to 123 kB * Data Format: ASCII tables * Headers: none * Column Order: The column order for each data file is as follows o ACRIM I Daily: year, month, day, solar irradiance, standard deviation o ACRIM I Monthly: year, month, solar irradiance, standard deviation, number of values used to calculate monthly mean o ACRIM II Daily: year, month, day, solar irradiance, standard deviation o ACRIM II Monthly: year, month, solar irradiance, standard deviation, number of values used to calculate monthly mean o ERB Daily: year, month, day, solar irradiance, standard deviation, number of values used to calculate daily mean o ERB Monthly: year, month, solar irradiance, standard deviation, number of values used to calculate monthly mean o ERBS Daily: year, month, day, time, solar irradiance, standard deviation o ERBS Monthly: year, month, solar irradiance, standard deviation, number of values used to calculate monthly mean * Delimiters: space * Missing or no value: -99, -9.9, -9.999, -9999.9, or -9999.999 Name and Directory Information Naming Convention The file naming convention for the Total Solar Irradiance data files is ddddddd.ppppp.t.ascii where ddddddd is the instrument acrimi = Active Cavity Radiometer Irradiance Monitor (ACRIM I) on board the Solar Maximum Mission (SMM) spacecraft acrimii = Active Cavity Radiometer Irradiance Monitor (ACRIM II), on board the Upper Atmospheric Research Satellite (UARS) erb = Earth Radiation Budget instrument (ERB), on board the NIMBUS-7 satellite erbs = Solar monitor data on board the Earth Radiation Budget Satellite (ERBS) ppppp is the parameter, irrad = solar irradiance t is the temporal resolution d = daily m = monthly ascii is the file format type Directory Path /data/inter_disc/radiation_clouds/solar_irrad Companion Software Not available at this revision. The Science Theoretical Basis of Data The radiant energy received from the Sun at satellite altitude is absorbed in a cavity and thus converted into heat energy. This in turn is converted into an electrical voltage which is measured. The sensors are calibrated by inserting into the cavity carefully measured amounts of electrical resistance heat and measuring the voltage generated. The calibration heating is done by having a known current pass through a wire of known resistance wound inside the cavity. The measurement is thus basically a calibration problem. Adjustments are also made to account for the direction of the Sun with respect to the sensor axis and for the Earth to Sun distance. The absolute accuracy of each instrument depends on how accurately the calibration terms are know. These include the resistor value, the accuracy of the current and voltage measurements, the size of the sensor aperture, and the ratio of electrical heat to radiant heat signals. Any changes during the life time of the experiment must also be monitored. Exposing the sensors to the space environment and the Solar UV radiation causes some small changes on the surface of the cavities which may affect the measurements. The ACRIM instrument monitors this type of problem by carrying three similar sensors, two of which are normally covered. At times these are opened for comparison purposes. The precision, or repeatability, of the measurements for all four instruments is about a factor of ten greater than the absolute accuracy. Thus while all the data sets show about the same variation in the solar signal, there is a bias separation between the separate data sets because of absolute calibration problems. Processing Sequence and Algorithms The voltage signal measured at the sensor is immediately changed to quantized digital counts to prevent possible bias shifts occurring during the transmission to the analysis facility. The various experiment teams than transform the counts into solar irradiances by applying calibration equations. These equations correct for a number of problems including: changes in the satellite to Sun distance, sensor temperature variations, off-axis measurements, changes in the sensor operating mode, and sensor degradation. Additional information concerning the calibration of the ACRIM instrument is given by Willson (1980) and for the ERBS solar data by Lee et al. (1987). The Nimbus-7 calibration coefficients were revised in 1990 and the earlier data recalibrated (Hoyt et al. ,1992). Scientific Potential of Data The variation of the total solar irradiance is an important study area both from the point of view of solar physics and because of the possible effect on the Earth's climate. During the active Sun periods the daily measurements clearly show variations on solar rotational and active region time scales. The large, short-term decreases are caused by the total solar irradiance (TSI) blocking effect of sunspots in magnetically active regions as they rotate through our view from Earth. The peaks of TSI preceding and following these sunspot dips are caused by the faculae of solar active regions whose larger areal extent causes them to be seen first as the region rotates onto our side of the sun and last as they rotate over the opposite solar limb (see for instance, Lean, 1991). The downward trend through the 1991-1996 period is similar in slope and amplitude to that observed by ACRIM I during the declining activity phase of solar cycle 21. From the peak of solar cycle 21 to its minimum the TSI, measured by the ACRIM I, decreased by about 0.08 %. The ACRIM II results through 1997 demonstrate a TSI minimum in early 1996, a flat period with high variability due to solar magnetic activity between early 1996 and early 1997, and increasing TSI beginning in early 1997 leading to the maximum of solar cycle 23. (Willson 1997) reports that the results of successive Active Cavity Radiometer Irradiance Monitor (ACRIM) experiments have been related with sufficient precision to resolve a multi-decadal, upward trend in total solar irradiance of 0.036 percent per decade between the minima of solar cycles 21 and 22. The measurements have shown that the Sun is a slightly variable star with a period of approximately eleven years. The variability is associated with changes in the Sun's magnetic field (Lean 1991). Such variability is fairly common among stars of the same type (Radick et al. 1990; Zhang et al. 1994), and is only partially understood (Hathaway 1994). For the observation period, 1979 to present, the direct radiative forcing effect on the Earth's climate is thought to be small (Hansen and Lacis 1990; Ardanuy et al., 1992) partially because the variation is cyclic. It is in phase with .the Sun spot cycle which presently has about a ten year period. Research is being carried out concerning regional and or/phase lagged effects such as variations in the stratosphere and upper troposphere (Labitzke and van Loon 1992), regional variations in the sea surface temperature (Reid 1991), and precipitation in the western part of the USA (Perry 1994). Perry has developed a Web site which discuses his research on the effects of climate variations on floods and droughts. If the small long term trend reported by Willson (1997) is sustained over several cycles (decades to centuries) the long term changes in the solar irradiance should have a very noticeably effect on the climate through radiative forcing (Lean 1991; Hoyt and Schatten 1993). It is suspected that some climate variations in the past have been due to solar variations. Validation of Data The experiment teams validated the date by careful and continuous review of the original and inflight calibration data, by intercomparison of the several independent measurements, and by comparison with empirical models of how the irradiance is expected to vary. The absolute calibration accuracy claimed by the experiment teams was: Nimbus-7 ERB (+ or - 0.5%), ACRIM I & II (+ or - 0.1%), and ERBS (+ or - 0.2%). There are observable biases between the four data sets but these biases are less than the respective claimed absolute accuracies. The long term stability (precession) of each data set is considered to be at least an order of magnitude better than the absolute accuracy. Hoyt et al. (1992) state that for the Nimbus-7 solar data the worst case error in the calibration stability amounts to (+ or - 0.04%). However for the years 1980-1988 the Nimbus-7 measurements drifted relative to SMM ACRIM measurements by only 0.13 W/m^2 or 0.01% which indicates that the tracking of the long term trends may be of this order. Shorter term shifts larger than 0.13W/m^2 do occur between the four data sets. During the period (December 1980-Spring 1984) when the SMM had no solar pointing capability the bias between the ACRIM I and the Nimbus-7 decreased by some (0.3 to 0.4 W/m^2).Willson et al. (1986) state that they applied a bias correction of 0.12% to the ACRIM data of this period to bring it into line with the Sun pointing ACRIM I data measured before and after this period. Hoyt et al. (1992) speculate that a slight correction to this bias shift is needed. It should also be noted that the ACRIM I data was noisier during this period (December 1980-Spring 1984) than during the solar pointing intervals. Several investigators have used proxy solar signals to estimate what the changes in the total solar irradiance is. This is done both to check the consistency of the total irradiance measurements and more importantly to estimate what the irradiance variations were in the past before accurate irradiance measurements started in November 1978. These proxies include sunspot measurements, Calcium plage data, 10.7-cm solar radio flux, etc. (Lean 1991). These models are empirical models which are tuned (fitted) to the accurate measurements. Some of these models indicate that the Nimbus-7 does not locate the irradiance peaks in solar cycles 21 and 22 in the proper years. In cycle 21 Nimbus-7 locates the peak in 1979, while some models locate it at about the end of 1981. Willson and Hudson (1991) point out that the ACRIM I showed a signal that was slowly decreasing through out 1980 while the SMM still had sun pointing capability. This suggests that the irradiance peak may have been in 1979 or 1980. Mecherikunnel (1994) and Lee et al. (1995) compare the ERBS and Nimbus-7 data during the peak of cycle 22. The ERBS and some models show the Irradiance peak towards the end of 1989. The Nimbus-7 shows it in 1991 or early 1992. Kyle et al. (1994) shows that for the period 1984-1991 the yearly mean bias between the Nimbus-7 and the ERBS measurements varies over a range of 0.5W/m^2. Contacts Points of Contact For Science information concerning the ACRIM experiments, contact Dr. Richard C. Willson Center for Climate Systems Research Columbia University 2845 Windfall Ave., Altadena, CA, 91001 Phone: 818-398-9803 Fax: 818-398-6334 E-mailacrim@primenet.com For information about or assistance in using any DAAC data, contact EOS Distributed Active Archive Center(DAAC) Code 902.2 NASA Goddard Space Flight Center Greenbelt, Maryland 20771 Internet: daacuso@daac.gsfc.nasa.gov 301-614-5224 (voice) 301-614-5268 (fax) References Ardanuy, P. E., H. L. Kyle, and D. Hoyt., 1992: Global relationships among the Earth's radiation budget, cloudiness, volcanic aerosols, and surface temperature, J. Climate, 5, 1120-1139. Barkstrom, B. R., E. F. Harrison, and R. B. Lee III., 1990: Earth Radiation Budget Experiment, preliminary seasonal results, EOS Transactions A. G. U., 71, 279. Hathaway, D. H., 1994: Producing the solar dynamo, EOS Transactions A. G. U., 75, 548. Hansen, J. E., and A. A. Lacis, 1990: Sun and dust versus green-house gases: An assessment of their relative roles in global climate change, Nature, 346, 713-719. Hickey, J. R., L. L. Stowe, H. Jacobowitz, P. Pellegrino, R. H. Maschhoff, F. House, and T. H. Vonder Haar, 1980: Initial solar irradiance determination from Nimbus-7 cavity radiometer measurements, Science, 208, 281-283. Hickey, J. R., B. M. Alton, H. L. Kyle, and D. Hoyt, 1988: Total solar irradiance measurements by ERB/Nimbus-7, a review of nine years, Space Science Reviews, 48, 321-342. Hoyt, D. V., H. L. Kyle, J. R. Hickey, and R. H. Maschhoff, 1992: The Nimbus-7 total solar irradiance: A new algorithm for its derivation, J. Geophys. Res., 97, 51-63. Hoyt, D. V., and K. H. Schatten, 1993: A discussion of plausible solar irradiance variations, 1700-1992, J. Geophys. Res., 98, 18,895-18,906. Kyle, H. L., D. V. Hoyt, and J. R. Hickey, 1994: The Nimbus-7 total solar irradiance measurements, November 1978 to May 1992, an article in 'Solar-terrestrial energy program, the initial results from STEP facilities and theory campaigns', edited by D. N. Baker, V. O. Papitashvili and M. J. Teague, COSPAR Colloquia Series, Vol. 5, Pergamon Press Ltd., 69-72. Labitzke, K., and H. van Loon, 1992: Association between the 11-year solar cycle and the Atmosphere. Part V: Summer, J. Climate, 5, 240-251. Lean, J., 1991: Variations in the Sun's radiative output, Rev. Geophys., 29, 505-535. Lee, R. B. III, B. R. Barkstrom, and R. D. Cess, 1987: Characteristics of the earth radiation budget experiment solar monitors, Applied Optics, 26, 3090-3096. Lee, R. B. III, M. A. Gibson, N. Shirakumar, R. Wilson, H. L. Kyle, and A. T. Mecherikunnel, 1991: Solar irradiance measurements: minimum through maximum solar activity, Metrologia, 28, 265-268. Lee, R. B., III, M. A. Gibson, R. S. Wilson, and S. Thomas, 1995: Long-term total solar irradiance variability during sunspot cycle 22, J. Geophys. Res., 100, 1667-1675. Mecherikunnel, A. T., R. B. Lee III, H. L. Kyle, and E. R. Major, 1988: Intercomparison of solar total irradiance data from recent space craft measurements, J. Geophys. Res., 93, 9503-9509. Mecherikunnel, A. T., 1994: A comparison of solar total irradiance observations from spacecraft: 1985-1992, Solar Physics, 155, 211-221. Perry, C. A., 1994: Solar-irradiance variations and regional precipitation fluctuations in the western USA, Int. J. Climatology, 14, 969-983. Radick, R. R., G. W. Lockwood, and S. L. Baliunas, 1990: Stellar activity and brightness variations: A glimpse at the sun's history, Science, 247, 39-44. Reid, G. C., 1991: Solar total irradiance variations and the global sea surface temperature record, J. Geophys. Res., 96, 2835-2844. Willson, R. C., 1979: Active cavity radiometer type IV, Applied Optics, 18, 179-188. Willson, R. C., 1980: Active cavity radiometer type V, Applied Optics, 19, 3256-3257. Willson, R. C., 1981: Solar total irradiance observations by active cavity radiometers, Solar Physics, 74, 217-229. Willson, R. C., 1984: Measurements of solar total irradiance and its variability, Space Science Reviews, 38, 203-242. Willson, R. C., 1994: "Irradiance observations of SMM, Spacelab 1, UARS, and Atlas experiment", in The Sun as a Variable Star, edited by J.M. Pap, C. Frohlich, H.S. Hudson and S.K. Solanki, Cambridge Univ. Press, Cambridge, England, 54-62. Willson, R. C., 1997: Total solar irradiance trend during solar cycles 21 and 22, Science, 277, 1963-1965. Willson, R. C., H. S. Hudson, C. Frohlich, and R. W. Brusa, 1986: Observations of a long term downward trend in total solar irradiance, Science, 234, 1114-1117. Willson, R. C. and H. S. Hudson 1991: The Sun's luminosity over a complete solar cycle, Nature, 351, 42-44. Zhang, Q., W. H. Soon, S. L. Baliunas, G. W. Lockwood, B. A. Skiff, and R. R. Radick, 1994: A method of determining possible brightness variations of the Sun in past centuries from observations of solar-type stars, Astrophys. J. Lett., 427, L111-L114. ------------------------------------------------------------------------ [NASA] [GSFC] [GoddardDAAC] [cidc site] NASA Goddard GDAAC CIDC Last update:Fri Jun 6 15:28:48 EDT 1997 Page Author: Dr. Lee Kyle -- lkyle@daac.gsfc.nasa.gov Web Curator: Daniel Ziskin -- ziskin@daac.gsfc.nasa.gov NASA official: Paul Chan, DAAC Manager -- chan@daac.gsfc.nasa.gov