NASA Technical Reports Server (NTRS) 20030002519: SORCE: Solar Radiation and Climate Experiment

28 Pages·26.8 MB·English

by  NASA Technical Reports Server (NTRS)

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Key1naiviauals, ssociat.eawit, t,e SORCEMission
....... :
SORCE Principal Investigator:
Gary Rottman, Laboratory for Atmospheric and Space Physics (LASP), University of Colorado, Boulder
SORCE Co-Investigators:
Tom Woods, SORCE Project Scientist, XPS Instrument Scientist, LASP, University of Colorado, Boulder
Jerry Harder, SIM Instrument Scientist, LASP, University of Colorado, Boulder
Greg Kopp, TIM Instrument Scientist, LASP, University of Colorado, Boulder
George Lawrence, Instrument Scientist, LASP, University of Colorado, Boulder
William McClintock, SOLSTICE Instrument Scientist, LASP, University of Colorado, Boulder
Dominique Crommelynck, Royal Meteorological Institute of Belgium, Brussels
Peter Fox, National Center for Atmospheric Research, High Altitude Observatory (HAO), Boulder, Colorado
Claus Fr6hlich, World Radiation Center, Davos, Switzerland
Judith Lean, Naval Research Laboratory, Washington, DC
Julius London, Astrophysical, Planetary, and Atmospheric Sciences, University of Colorado, Boulder
Peter Pilewskie, NASA Ames Research Center, Moffett Field, California
Ray Roble, National Center for Atmospheric Research, HAO, Boulder, Colorado
Paul Simon, Institute d'Aeronomie Spatiale, Brussels, Belgium
Oran White, National Center for Atmospheric Research, HAO, Boulder, Colorado
SORCE Program Management:
Tom Sparn, SORCE Program Manager, LASP, University of Colorado, Boulder
Mike Anfinson, LASP, University of Colorado, Boulder
Brian Boyle, LASP, University of Colorado, Boulder
Tim Holden, LASP, University of Colorado, Boulder
Rick Kohnert, LASP, University of Colorado, Boulder
Chris Pankratz, LASP, University of Colorado, Boulder
Sean Ryan, LASP, University of Colorado, Boulder
Rob Fulton, Spacecraft Program Manager, Orbital Sciences Corporation., Dull.es, Virginia
Philip Burkholder, Orbital Sciences Corporation, Dulles, Virginia
Ed Kozlosky, Orbital Sciences Corporation, Dultes, Virginia
Dave Oberg, Orbital Sciences Corporation, Dulles, Virginia
Iln lers an ling the Sun's
The Sun is the dominant energy source for the ......... .......... ..........
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entire Earth system. It warms the Earth, heating _.._ 0.2 SurfaceTemperature ' ........ i'41370/ ,,i!i_?:!'ii!i_ii!!;!i!!!!!ii!iiiiiii_!i;i!i!i_i!iiiiiiiiiiil
the lands and ocean. It maintains our atmos- _ paleoreconstruction ,' _i?i i_.......i.i.......
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phere, generates clouds, and cycles our planet's n,_ 4_,:_ Reconstructed Total /,,,,',_v V, :_J 1368
water. It,drives the winds and the ocean currents. < _ /,"?....._. ] .a
The Sun senergy sustains and nurtures all life _ o_-_ 3j j
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including plant, animal, and human. _ k_,#!i..i_51:.& /'ii__''_:_:_ _/_/ ..... "./_!_;..., _ _ ¢.1
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1600 1700 1800 1900 2000
_ Year
c_ 0 After accounting for the increase in CO2and other greenhouse gases, the
Earth's surface temperature corresponds with the increase in solar radiation,
o_ except during major volcanic eruptions (arrows).
- There are also many indirect effects that are at least as impor-
m Ultraviolet solarirradiance
...... I....... l....... I........ I....... l.. tant. Other factors entering the climate puzzle, including
0 27 54 81 108 greenhouse gases, aerosols, and atmospheric circulation, are
Days driven by solar radiation, and they influence each other by
complex feedbacks. To establish the impact humans have on
When the Sun isactive its radiation displays a clear variation
related to its 27-day rotation period. Ozone, one of the impor- climate, we must have solid knowledge of any natural factors
tant greenhouse gases in the stratosphere and Earth's biologi- impacting _rtant being the Sun.
cal shield, responds accordingly.
..... ...................H...o..w......D..o...e.. .....t.h..e......$..u..a... A Climate?
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By How Much Does fhe
Sun's Racliafion Vary?
Energy
from the Sun
makes life onEarth
possible. Solar energy also
drives the Earth's climate, and slight variations in solar radiance could offset
(orincrease) global warming. (Photograph courtesy of Philip Greenspun.)
The 23.5 ° angle between Earth's spin axis and its orbit about the Sun gives rise to the seasonal cycle, causing the
length of the day and the sunlight angle to vary during the year. As aresult, summer is much warmer than winter,
and polar regions are dramatically colder than the tropics. Also, Earth's orbit about the Sun is an ellipse, not a circle,
with the Earth slightly closer to the Sun in early January than it is in July. This results in about 7% more sunlight
reaching the Earth in January than in July. Both of these seasonal effects, the tilt of the Earth's axis and the orbital
distance to the Sun, are stable and very predictable, and the resulting annual cycle in sunlight remains essentially
the same year after year during recent centuries, though the Earth's orbit and the Sun's brightness (TSI) have both
evolved over much longer time scales.
A more important and difficult question is whether the Sun's radiation itself varies -- are there intrinsic changes in
the total energy output of the Sun m the so-called "total solar irradiance" (TSI)? Following the development of the
telescope in about 1609, Galileo Galilei, Thomas Harriot, and others independently studied small blemishes or
"spots" on the surface of the Sun. Since that time observers have compiled meticulous records of the number and
character of these sunspots. Sunspots visible from the Earth change from day-to, day, and in 1843 S. Heinrich
Schwabe recognized a distinctive 11-year sunspot cycle. In the 17th century astronomers observed almost no
sunspots for a period of almost 50 years. During this period, referred to as the Maunder Minimum, winters in Europe
were much longer and colder than today. Since then, the trend suggests a gradual increase in solar output.
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This radiation balance picture shows how incoming solar energy is
distributed among parts of the Earth's atmosphere and surface.
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SORCE Will Monitor Solar Irrat iance Daily
"One activity ranks above all others for determining solar influences on
global change: Monitor the total and spectral solar irradiance from an
uninterrupted series ofspacecraft radiometers employing in-flight
sensitivity tracking." -- 1994 NRC Report
History of Solar Irratliance Observations
Only a portion of the Sun's radiation penetrates the Earth's atmos-
phere to its surface, and at some wavelengths the radiation is
absorbed entirely. It is therefore not possible to use ground-based
observations of the Sun to estimate the top-of-the,atmosphere
radiation. Although there was an ambitious ground observing
program during most of the 20th century, it provided only
The evolution of active regions on the Sun is the
ambiguous estimates of irradiance, and little or no information on primary cause for solar variability. The active regions
whether the Sun varied. change over all time scales - seconds to centuries.
Observations of solar irradiance from space began in the 1960s and reached some level of maturity and reliability in
the late 1970s, providing the first view of the full solar spectrum. The more energetic ultraviolet part of the spectrum
was a discovery that provided new insight into the complexities of solar physics and improved our understanding
of the temperature and structure of the Earth's upper atmosphere. By the mid-1970s observations were indicating
that the ultraviolet varied by large amounts -perhaps factors of 2 near 120 nm and factors approaching an order of
magnitude for much of the extreme ultraviolet (EUV) at wavelengths below 30 nm.
today's Record of Total Solar Irra liance
Total Solar Irradiance monitoring using electrical substitution radiometers (ESRs) from the vantage point of space
began with the launch of the Nimbus 7 satellite in November 1978. This was soon followed by an Active Cavity
Radiometer Irradiance Monitor (ACRIM) instrument on the Solar Maximum Mission and by the Earth Radiation
Budget Experiment (ERBE). More recently, second and third ACRIM instruments have been launched, in addition to
the launch of two instruments on the
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The SORCE Mission
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Schematic of the solar spectrum and areference used to identify the role of
each of the SORCE instruments. SOLSTICE and SIM overlap from 200 nm
to 320 nm.
The SOlar Radiation and Climate Experiment (SORCE) is a small free-flying
satellite carrying four scientific instruments to measure the solar radiation
incident at the top-of-the-Earth's atmosphere. This mission is one element of
NASA's Earth Observing System (EOS), which is the major observational and
scientific element of the U.S. Global Change Research Program.
How Do the 50RCE Instruments Measure Solar Raaiation?
Scientists and engineers at the University of Colorado's Laboratory for Atmospheric and Space Physics (LASP)
designed, built, calibrated, and tested four science instruments for flight on-board SORCE. These are the Total
Irradiance Monitor (TIM), the Spectral Irradiance Monitor (SIM), two Solar Stellar Irradiance Comparison
.... Experiments (SOLSTICE), and the XUV Photometer System (XPS). The TIM continues the record of total solar
...................................................i.r.r.a..d..i.a..n..c..e. (TSI), but incorporates modern, state-of-the-art technologies of the electrical substitution radiometer
...............................................................................
: (ESR), while at the same time taking full advantage of the best heritage of previous radiometers. The SIM provides
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The TIM will provide a measurement of total solar irradiance (TSI)
with an absolute accuracy of 0.01% and a relative stability of 0.001%
per year. Imperative for climate modeling, this instrument will report
the total radiative input at the top of the Earth's atmosphere four
times each day.
Measurement Concept
This new instrument uses the best heritage of previous radiometers, but is enhanced with modern, state-of-the-art
technologies including phase-sensitive detection, metallic absorptive materials, and digital electronics. Any one of
four electrical substitution radiometers (ESRs) can measure the incident solar power. A precision aperture defines
the area through which sunlight is collected, and the ratio of sunlight power to area gives the solar irradiance
(Watts per square meter - W/m2). The TIM achieves extremely low noise using phase sensitive detection at the
shutter frequency. This detection method reduces sensitivity to noise, thermal drifts, and thermal emission from the
instrument itself, improving the signal/noise ratio compared to traditional time domain analysis. Using this
technique, the TIM has demonstrated noise levels less than 0.0002%.
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Wavelength (nm)
SIM measures spectral
solar irradiance from 200
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Objective .....
This newly developed spectrometer wili provide spectral solar irradiance measurements in the near-ultraviolet, visible
and near infrared four times per day. The wavelength coverage -- from 200 to 2000 nm- overlaps the observations
of SOLSTICE. Solar variability models, with the additional constraint of TSI observations, predict very small frac-
tional changes -- only 0.1 to 0.01% -- in the visible /near infrared spectrum. Understanding the wavelength-depend-
ent solar variability is of primary importance for determining long-term climate change processes.
Historical Backgroun l
Prior to SORCE, the time history.of spectral irradiance variability in the visible part of the spectrum isnot nearly as
complete, or as reliable, as the total solar irradiance and the solar ultraviolet radiation records. SIM will make the
first continuous record of the top-of-the-atmosphere spectral solar irradiance in the visible/near infrared region.
Measurement Concept
SIM is a prism spectrometer and uses.,a sing,.le. optical element-- aprism.--- to both. disperse, and focus solar radia-
tion. This is a slight modification of an optical design first proposed by Charles F6ry in.191.0,The single optical
element minimizes the SIMsusceptibility to deterioration of sensitivity on orbit. In the focal plane of the
substitution radiometer (ESR)to serve as an:absolute detector, In fact, SlIMincludes two completelv., independent., and
calibra tion: capability.: to tracki chan.ges..in prism., transmission.
_,_',',SIM','E,SR'',',i,i6pi_igeg',:,',6_if',h',il'',,'_,',i',', '',,'p,_iheiple _asithe _M sensors;b ut with some m0dificationi_e, lighf Sig_ N',',',',',',',',',',',',',',',','_
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the silver cones used fd_ =M_ ml,ia_re syntVtic diamond strips are used. Like the TIM sensors; these
diamond _nmrs am a!so bla_kened with mckel phosphorous (NIP)tO become highly absorbin_ and aNo _aVe
i{_etmistors and _placement _aters to provide their thermal control, The two identical _ors; _ne deterring
i sunIi_t from Ne sp_trometer, while the reference does not, are b anted.