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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
