iN.

ersity

form

launc

adian

entire electromagnetic spectrum of the Sun. The SORCE spectral observations extend from 0.1 to 2700 nm with only a portion of

the EUV (35115 nm) missing. Fortunately, the TIMED SEE instrument observes this missing EUV, and therefore, for the rst

ature and structure of the atmosphere, its compositionand dynamics. The very shortest wavelengths X-rays

by the atmosphere, do reach the Earth surface where

incoming solar radiation is a fundamental requirement.It is important to know the amount of solar irradiance,

extends back to the beginning of the seventeenth cen-

tury. In addition to the dark sunspots, there are

accompanying bright features, called faculae as they

appear in the photosphere, that are also directly con-

nected to magnetic eld intensity. These features, both

reserved.

* Corresponding author.

E-mail address: gary.rottman@lasp.colorado.edu (G.J. Rottman).

Advances in Space Research 370273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rightsand ultraviolet out to a wavelength of about 300 nm

are completely absorbed by the atmosphere resulting

in photodissociation and photoionization. These pro-

cesses lead to key photochemistry including ozone crea-

tion and destruction in the stratosphere and

establishment of the ionospheric layers above 60 km.

Meanwhile, the longer wavelengths the visible andinfrared although scattered and partially absorbed

but perhaps it is even more important to understand

the solar variations. The Sun is known to vary on

many time scales from seconds, minutes and days to

years and decades. The Sun has a dominant 11-year

mode of variation that is related to the appearance

and disappearance of magnetic features as seen at the

solar surface, or photosphere. In fact, this 11-year solarcycle was rst recognized in the record of sunspots thattime, the solar irradiance both total and spectral is being reported on a daily basis. This paper presents an overview of the

SORCE spectral measurements with special emphasis on the UV and EUV (k < 200 nm). These SORCE data are produced bythe Solar Stellar Irradiance Comparison Experiment, SOLSTICE, and the XUV Photometer System, XPS, that are improved ver-

sions of rst generation instruments aboard UARS and TIMED, respectively. The SORCE UV and EUV records of solar variations

are presented and discussed. These data include important multi-wavelength observations taken during the solar storms in October

November 2003.

2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Solar variability; Solar ultraviolet irradiation; Atmospheric photochemistry; Atmospheric photoionization; Atmospheric heating

1. Introduction

Solar radiation is the dominant direct energy input to

the Earth system, and this energy establishes the temper-

they heat the lands and ocean, generate clouds, and cy-

cle the planets water.In order to fully understand and model the Earths

atmosphere, detailed and precise knowledge of theSORCE solar UV

Gary J. Rottman *, Thomas

Laboratory for Atmospheric and Space Physics (LASP), Univ

Received 22 December 2004; received in revised

Abstract

The Solar Radiation and Climate Experiment, SORCE, was

measurement of solar irradiance including both Total Solar Irrdoi:10.1016/j.asr.2005.02.072rradiance results

Woods, William McClintock

of Colorado, 1234 Innovation Drive, Boulder CO 80303, USA

22 February 2005; accepted 25 February 2005

hed in January 2003 and is now making the rst comprehensive

ce, TSI, together with spectral irradiance covering almost the

www.elsevier.com/locate/asr

(2006) 201208

The Solar Radiation and Climate Experiment,

gov/upperatm/sorce/) for subsequent distribution to

the scientic and research communities.

After orbit insertion SORCE quickly acquired Sun

pointing and proceeded with initial checkout of all

spacecraft subsystems. Roughly one week later the ve

science instruments were turned on and commissioningtests were completed. In order to minimize optical deg-

radation, the instruments were all sealed and lled with

approximately one atmosphere of pure, dry argon. This

insured that contamination outgassed from the space-

craft, instruments, solar panels, and thermal blankets

would not enter and deposit on the optics. Moreover,

the instruments were not pointed directly at the Sun

for several weeks to minimize solar exposure and furtherinsure that the optics would not degrade.

After several weeks on orbit the instrument doors

were opened and performance verication was com-

Space Research 37 (2006) 201208SORCE, is a small spacecraft with four instruments

measuring the solar irradiance the total irradiance,

or TSI, and the spectral irradiance at most wavelengths

from X-rays to the infrared (Woods et al., 2000).

SORCE was launched on a Pegasus XL rocket from

the Kennedy Space Flight Center on January 25, 2003

and has an anticipated lifetime of at least ve years.

The launch proceeded south across the Atlantic, andthe spacecraft was inserted into a near-circular orbit

with a mean altitude of 640 km and an inclination of

40. Data were returned from a rst ground station passover South Africa, and all spacecraft systems have been

operating nominally ever since. Data from SORCE are

relayed through the NASA ground data system to a con-

trol center at LASP, University of Colorado, Boulder.

The Mission Operations Center (MOC) and ScienceOperations Center (SOC) at LASP carry out the instru-

ment and spacecraft planning and scheduling on a daily

basis, they upload all commands, and they download all

data twice per day. At LASP the data are entered into a

data base system from which they are processed to pro-

gressively higher levels. The Level 3 science data prod-

ucts are made available on the LASP web site

(http:lasp.colorado.edu/sorce/) and they are also trans-ferred to the Goddard Space Flight Center Distributivedark and bright, give rise to irradiance variations when

they reach the Earth. Seen from the Earth, the Sun ro-

tates with a period of about 27-days (dependent on so-

lar latitude) and as the magnetic features move across

the solar disk they produce a striking 27-day variation

in the solar signal. As the number and size of these ac-tive regions increase and then decrease every ve to six

years, from maximum activity to minimum activity,

they likewise produce a striking 11-year solar-cycle

irradiance variation. The active regions are also centers

for the generation of intense transient events ares,

prominence eruptions, and coronal mass ejections

(CMEs). Flares are phenomena that occur primarily

in the solar corona and give rise to irradiance changes,especially for X-rays and other coronal emission. The

direct eect of ares on the Earths lower atmosphereis small, but of major importance throughout the upper

atmosphere.

The Earths atmosphere processes the solar radiationin a very wavelength dependent way, and at the same

time the emission from the Sun varies dramatically with

wavelength. The requirement for measuring solar irradi-ance and its variation, therefore, becomes a requirement

for making simultaneous spectral measurements at most

or all wavelengths.

2. SORCE

202 G.J. Rottman et al. / Advances inActive Archive Center, DAAC (http://daac.gsfc.nasa.pleted. The instruments began routine solar irradiance

observations in early March 2003. The SORCE instru-

ments are described in the following section.

2.1. Instruments

Fig. 1 is a loglog plot of the solar spectrum from the

very shortest X-ray wavelengths to 2 lm in the nearinfrared. The dark bars dene the wavelength range of

the three spectral devices on SORCE, and the fourth

SORCE instrument measures total solar irradiance,

TSI, which is the integrated ux over all wavelengths

extending even further out into the infrared beyond10 lm. The SORCE instruments and their capabilitiesare considered in the following subsections.

2.1.1. The total irradiance monitor, TIM

TIM is a four-channel electrical substitution radiom-

eter, ESR (Lawrence et al., 2000, 2003; Kopp and Law-

rence, in press). Each channel has a thin-wall conical

bolometer with an integral heater and thermister. The

Fig. 1. The solar irradiance spectrum from 1 nm to 2 lm whichrepresents about 95% of the Total Solar Irradiance, TSI. The bars

indicate the spectral coverage of the three spectral instruments onSORCE, and also of the EGS in NASAs TIMED spacecraft.

n Spainterior of each cone is extremely black using nickel

phosphorous (NiP) black coating as an ecient absor-

ber of radiation. The cones are used in pairs, one arbi-

trarily called the active cone and the other the

reference cone. Electronics provide Joule heat (known

voltage across the resistance of the cones heater) to bal-ance the two cones at a temperature slightly elevated to

their surroundings. The pair of cones is pointed at the

Sun, and although both have shutters over a precise

aperture, at any time only one (the active cone) is open,

with the result that solar radiation entering that cone is

completely absorbed in its interior. The balancing cir-

cuit immediately reduces the Joule heat to that ac-

tive cone in order to maintain its temperaturebalance to the reference cone, and the amount of hea-

ter power removed is precisely equivalent to the radiant

power (Watts) entering the shutter/aperture. Knowing

the size of the aperture and the amount of power re-

moved from the active cone provides a precise measure-

ment of the solar radiant ux density (W/m2) or

irradiance.

Devices similar to TIM have operated on a number ofspace missions since about 1978 (Willson, 1984, 1994;

Lee et al., 1987; Hoyt et al., 1992; Frohlich et al.,

1997) and since that time they have observed almost

three complete 11-year solar cycles. For these three cy-

cles TSI values show a clear solar cycle variability of

about 0.1%, with the higher levels coinciding with the

maximum levels of sunspots. In fact, these observations

seem to show quite conclusively that the dominant solarvariability over the 11-year cycle is due to magnetic

activity in the photosphere with a positive contribution

originating in the bright faculae and a negative contribu-

tion arising from the dark sunspots (Foukal and Lean,

1988). The best t to the TSI data is achieved with a fac-

ulae contribution roughly twice the sunspot darkening,

and a net variation of about 0.1% (Frohlich and Lean,

1998).Shorter-term variations of TSI are also apparent in

the observational record, and the major cause is the pas-

sage of dark sunspots across the disk of the Sun. These

appear as dips in TSI data of about 0.1% and last for

several days as the sunspots and sunspot groups traverse

the center of the solar disk. Since the associated faculae

are more evenly spread across the solar disk, they do not

typically produce intermediate- and short-term increasesto TSI as striking as the sunspot dips.

Fig. 1 illustrates that some 95% of the Suns radiationis in the visible and infrared, and therefore the TSI mea-

sured by TIM is heavily weighted by these longer wave-

lengths. Since TSI varies at about the 0.1% level, it

should be expected that the visible/infrared wavelengths

vary in a comparable fashion. The ultraviolet, and espe-

cially the shorter wavelength extreme ultraviolet and X-rays, comprise only a small fraction of TSI (k < 300 nm

G.J. Rottman et al. / Advances iabout 1% of TSI, and k < 100 nm about 0.01% of TSI)and therefore factors of 2 and even larger variations at

these short wavelengths are still compatible with the

small variation of TSI.

2.1.2. The spectral irradiance monitor, SIM

SIM is a newly developed prism spectrometer de-signed to measure solar irradiance throughout the visi-

ble and near infrared. The science objective of SIM is

to make these measurements with a combined standard

uncertainty of less than 0.1% and precision and long-

term relative accuracy of 0.03%. Although SIMs spec-tral coverage extends to ultraviolet wavelengths as short

as 200 nm, this region is only a secondary objective. As

mentioned above, the small TSI variations indicate thatthe solar variations in the visible and near infrared do

not exceed a fraction of 1%, and the SIM measurements

now conrm this level of variability (Fontenla et al., in

press).

It is indeed a challenge for a space-based spectrome-

ter to provide a stable responsivity over many years on

orbit and to be able to establish solar variability at the

level of 0.1%. SIM achieves this using only a single opti-cal element a suprasil fused-silica prism with a concave

front face and a convex rear surface that is aluminized

for high reectivity. The solar radiation enters an en-

trance slit and then is dispersed and refocused by the

prism back to a set of exit slits where the solar spectrum

is recorded. SIM uses four photodiodes a combination

of silicon and InGAs to cover the spectral region

200 nm to 1 lm, but its most important detector is aminiaturized version of the ESR used in TIM and de-

scribed above. The SIM ESR is an absolute detector

operating over the entire spectral range 250 nm to

2.7 lm, and because it is a stable and absolute detectorit is used to continually recalibrate the diodes. Harder

et al. (in press) provide a complete description of SIM

and its operation.

There are two completely independent optical chan-nels in SIM. One is used on a daily basis and the second

is used infrequently (1% duty cycle). With the assump-tion that instrument degradation is dependent on solar

exposure, the cross-calibration of the two channels is

used to estimate degradation of the primary channel.

In addition, there is a small, periscope device that can

direct monochromatic radiation from either of the two

instruments to the other. The receiving channel has adiode to measure this radiation and then move out of

the beam to allow the light to pass to the test

prism. The prism refracts, transmits, and returns the

light to the same diode, thereby determining the prism

transmission. These prism calibrations are conducted

routinely at many wavelengths, providing reliable

knowledge of changes in SIMs responsivity.The SIM data product in the ultraviolet,

200 nm < k < 300 nm, overlaps with the SOLSTICE

ce Research 37 (2006) 201208 203measurements that are described below. The SIM data

have very high signal-to-noise, although with somewhat

lower spectral resolution than SOLSTICE.

2.1.3. The solar stellar irradiance comparison experiment,

SOLSTICE

SOLSTICE is a grating spectrometer that measuressolar spectral irradiance in the ultraviolet,

115 < k < 320 nm. Fig. 2 is an illustration of the UVirradiance as measured by SOLSTICE at instrument res-

olution of about 0.1 nm. These measurements have a

combined standard uncertainty of better than 5% (wave-

length dependent), and a precision and long-term rela-

tive accuracy of better than 0.5%. McClintock et al.

(in press) provide a complete description of the SORCESOLSTICE instrument.

204 G.J. Rottman et al. / Advances in SpaThis instrument is a second generation of the SOL-

STICE (Rottman et al., 1993) that ies on the Upper

Atmosphere Research Satellite, UARS, which launched

in 1991 and is still making solar measurements today.

SOLSTICE observes the Sun during daylight portions

of the satellite orbit, and then during nighttime portions

it uses the very same optics and detectors to observebright blue stars. (Unfortunately, due to spacecraft con-

straints the stellar observations were discontinued in

2001.) The large dynamic range between the stellar

and solar ux is accommodated by changing only aper-

tures (factor of 2 105) and integration times (factor of103), both parameters in the measurement equation that

are well calibrated and do not change during the mis-

sion. The repeated observation of the stars accomplishestwo things; rst, the stellar ux from main-sequence B

and A stars should not vary (Mihalas and Binney,

1981) and any changes in the SOLSTICE signal while

observing the stars are unambiguously interpreted as

changes in the instrument reponsivity, which is corrected

accordingly. Second, both UARS and SORCE SOL-

STICE establish the ratio of the solar to stellar ux that

is independent of instrument responsivity. Future obser-vations (up to thousands of years) can repeat these ratio

measurements. Assuming that the stars do not vary, the

Fig. 2. The ultraviolet irradiance spectrum, 115 < k < 320 nm, as

measured by the SORCE SOLSTICE.ratios from the dierent SOLSTICE observers can be

directly related to establish variations in the Suns ultra-violet radiance over the arbitrary time base.

The UARS SOLSTICE has three channels, 115

185 nm, 170320 nm, and 280430 nm (Rottman et al.,

1993). The measurements from all three channels havea combined standard uncertainty of 610%, and long-

term relative accuracy of 12%. The rst two channels

achieve their objective and record solar variations on

all time scales from days, to weeks, and even to the

11-year solar cycle. (These observations are discussed

below in Section 3.) However, the longest wavelength

channel found that solar variations, especially over time

periods of months to years, were smaller than its detec-tion limit, and the observations could only provide an

upper limit of about 1% for the solar variations at wave-

lengths longer than 300 nm. The SORCE SOLSTICE

therefore abandoned the third, long wavelength UARS

channel and uses only two channels to concentrate on

the spectral regions 115180 nm and 170320 nm.

The SOLSTICE II has a single optical path, but the

nal optic, the camera mirror, can be rotated to oneof two positions to select either a photomultiplier tube

with cesiumiodide photocathode (115180 nm) or ce-

siumtelluride photocathode (170320 nm). To com-

plete a full spectral scan a single instrument would

rst use one channel and then switch to the other. For

this reason, SORCE carries two completely redundant

SOLSTICE units, and operationally uses one to rou-

tinely observe the short wavelength range while theother observes the long wavelengths.

2.1.4. The XUV photometer system, XPS

XPS is a combination of lter photometers that mea-

sure solar irradiance from 0.1 to 34 nm with an addi-

tional channel at the important Lyman-a line at121.6 nm. A very similar instrument ies on NASAsTIMED mission that was launched in 2001 (Woods etal., 1998). In total there are twelve silicon diodes, eight

with metal lms directly deposited on them, one with

a 121 nm interference lter in front, and the remaining

three are bare photodiodes (Woods and Rottman, in

press). The lter material, either metal coating or inter-

ference, establishes the wavelength sensitivity (band-

pass) and also blocks the long wavelength solar

radiation that would overwhelm the relatively weak sig-nal at these short X-ray wavelengths. The various lter

choices are discussed by Powell et al. (1990), and the

particular materials used for the SORCE XPS are

shown in Fig. 3 (there are two of the Ti/C lters).

The twelve photodiodes/lters are packaged in a sin-

gle unit with a lter wheel mechanism in front. As the

wheel turns it places an open aperture, a blocked posi-

tion, or a window (fused silica) in front of each diode.For the nine lter diodes the open aperture allows the

ce Research 37 (2006) 201208solar irradiance measurement, while in turn the blocked

G.J. Rottman et al. / Advances in Spaposition provides a reading of the diode dark signal, andthe window position provides a measure of the long

wavelength leakage or background through the lter.

2.2. Spacecraft and operations

SORCE has an orbit period of about 97 min and

completes 15 orbits per day. Once per day commands

for the spacecraft and instruments are relayed to the sa-tellite from the Mission Operations Center (MOC) at

LASP, and on one or two ground station passes per

day all data are transferred back to the MOC and from

there into the Science Operations Center for data pro-

cessing. Although real-time data are examined as they

arrive at the MOC, the vast majority of the data is in

the playback mode and is processed roughly 24 h after

the solar observation. Subsequently the data are exam-ined, validated, and made available in a preliminary

form to the scientic community (from the LASP

Fig. 3. Identication of the seven dierent metal lters used with the

silicon diodes in the SORCE XPS.SORCE website: http://lasp.colorado.edu/sorce/). Sub-

sequent examination, validation, and correction for

instrument degradation is undertaken on an instru-

ment-by-instrument basis, and this process can take

from only a few days (as in the case of TIM and XPS)

to several weeks (as in the case of SIM and SOLSTICE).Information about the data version, quality, and level of

validation accompanies each data le as header infor-

mation. The data user is cautioned to pay careful atten-

tion to the quality, appropriateness, and reliability of

each data set as expressed in these metadata les.

3. Observations

3.1. Solar spectral irradiance

Fig. 1 shows the solar irradiance spectrum from 1 nm

out to 2 lm. This is a loglog plot and illustrates thatthe visible to near ultraviolet contributes the vast major-

ity of the solar ux. Nevertheless the EUV and X-rays,

although contributing only about 104 of the total areextremely important because of their large energy per

photon and because of the large cross-section for

absorption by atmospheric gases they therefore domi-nate the energetics of the Earths upper atmosphere.

Several instruments measure the total solar irradi-

ance, TSI, including TIM on SORCE. The visible to

near infrared spectral irradiance is only observed by

SIM on SORCE, and these new measurements are pro-

viding exciting and unique information on solar vari-

ability at these long wavelengths (e.g., Fontenla et al.,

in press).The ultraviolet, the extreme ultraviolet, and the soft

X-rays are measured by the SORCE XPS and SOL-

STICE and by the TIMED XPS and EGS (Eparvier

et al., 2001). The UARS SOLSTICE (Rottman et al.,

1993) and SUSIM (Brueckner et al., 1993; Floyd et al.,

1998) also continue to make daily observations of the

UV irradiance at wavelengths longer than 115 nm. In

combination these instruments, especially the SORCEXPS, SORCE SOLSTICE, and TIMED EGS, provide

near simultaneous observations of solar spectral irradi-

ance with a time cadence of daily, hourly, and some-

times by the minute. These complementary data

continue to provide new and unique information about

the Suns variability. They improve our understandingof the Sun and at the same time provide the accurate

knowledge of the varying solar energy that drives theEarths atmosphere and climate system.

3.2. Solar variability

Fig. 4 is a sample time record including four dierent

solar irradiance data sets, and clearly illustrates the va-

lue of having simultaneous observations from several

instruments, and at several wavelengths. This particulartime period is discussed in detail by Woods et al. (2004)

and covers most of a 27-day solar rotation period. It is a

somewhat unique event in that most activity was local-

ized on one hemisphere of the Sun while the other was

relatively quiet. Moreover, some of the participating ac-

tive regions contained sunspots and groups of sunspots

that were unusually large and produced solar ares of

truly phenomenal intensity (the largest and fourth larg-est are ever recorded by the GOES satellite occurred on

November 4 and October 28, 2003, respectively).

The top panel in Fig. 4 is the TSI as measured by

SORCEs TIM and is comprised mostly of photosphericemission from the visible and near infrared. This light

curve is dominated by the passage of the large sunspots.

The resulting decrease of 0.3% is the largest short-termdecrease in TSI ever recorded. The second panel is dom-inated by the strong Lyman-a emission at 121.6 nm, and

ce Research 37 (2006) 201208 205this emission originates in the solar chromosphere and

206 G.J. Rottman et al. / Advances in Space Research 37 (2006) 201208Fig. 4. An overlay comparing the solar variation in TSI (top panel), in

Lyman-a (2nd panel), a short wavelength channel of the XPS (3rdpanel) and in the GOES X-ray data (bottom panel). The top two

panels are linear in irradiance and the bottom panels are logarithmic.4transition region. Here a modest increase of about 12%marks the passage of the active centers across the disk,

and the ares become apparent at the few percent level.

These are daily irradiance values and the instantaneous

are increases are signicantly larger. The bottom two

panels are from very energetic photons originating in

the solar corona. For these panels the intensity scale is

adjusted to a logarithmic scale because the increases

are factors of ten and larger, and the are enhancementsexceed a factor of 1000.

Fig. 4 is a primary example of the value of multiple

data sets that are reliable and simultaneous, providing

a study of the complex solar excitation and emission.

At the same time these data illustrate the necessity for

making observations at those specic wavelengths tai-

lored to their impact and inuence on the Earthsatmosphere.

Fig. 5 is the result of an analysis of the multi-year

data set from the UARS mission. Although this gure

is derived from SOLSTICE I observations, quite similar

results are obtained from the SUSIM observations.

From the extended data set numerous periods of 27-

day variability stand out, especially near the maximum

of solar activity. If a local maximum is compared to

the neighboring minimum, and ratioed wavelength-by-wavelength the top panel is obtained. The particular

rotation period used in this analysis occurred in Febru-

Variations range from 0.3% for TSI to a factor of 10 for the X-rays.

The rotation period is in October 2003.ary 1992, and if a dierent rotation period is selected the

main features of the curve remain the same although the

amplitude may vary. The features in this variability

curve are recognizable and easily identied with the

emissions and absorption features (lines and edges) in

the solar spectrum (see Fig. 2).

The bottom panel of Fig. 5 is a ratio (again wave-length-by-wavelength) of the mean value of the solar

irradiance when the Sun is very active to when it is quiet.

In this case it is a period in early 1992 ratioed to a period

in 1996 and, although 1996 is the minimum between so-

lar cycle 22 and 23, 1992 is slightly after the peak of so-

lar cycle 22. This bottom panel represents about 90% of

the full solar cycle swing for cycle 22 (Floyd et al., 1998).

It is interesting to note the similarities, and perhapssmall dierences, between the 27-day variations and

the solar cycle variations. It is also apparent that the

noise in the ratio is signicantly larger in the solar cycle

determination. This is due to the fact that measurements

separated by about ve years are used for the solar cycle

estimate and unaccounted trends and drifts in the instru-

ment responsivity enter these ratios at 12% level. The

Fig. 5. Solar variability of the UV irradiance as measured by the

UARS SOLSTICE. The top panel is variation seen during a typical

27-day rotation period. The bottom panel is the variation from near

the maximum of cycle 22 in early 1992 to the minimum in 1996.

the irradiance data sets. NASA continues to explore op-

tions to ll the gap and insure continuity of the data

mation, including SORCE data, is available at http://

Phys. (in press).

n Space Research 37 (2006) 201208 207sets, but so far NASA has only identied a possible

ight opportunity for TIM. NASAs GLORY Mission,if it ies, will launch no sooner than 2008 and may carrySORCE SIM and SOLSTICE provide enhanced capa-

bility over UARS, and their new measurements will pro-

vide meaningful improvements.

4. Future of SORCE and other observing programs

SORCE launched in January 2003 and has a nominal

lifetime of ve years, implying an end-of-mission near the

end of 2007. There are no expendables in the spacecraft

nor in any of the instruments, so an extended mission

is certainly feasible. If SORCE continues to meet its sci-

ence objectives, requests will be made to NASA to extend

the mission probably on a year-to-year basis. The valueof long-term data sets, especially of the solar irradiance

type, far exceeds the incremental costs of extending the

operations and data analyses. For example, the UARS

observations now extend over more than one complete

solar cycle, although this great accomplishment could

never have even been suggested in the original UARS

planning phase. That is, the initial design and fabrication

costs of a space mission increase dramatically if the re-quired lifetime is extended beyond about ve years.

In addition to SORCE there are several other observ-

ing programs of TSI including ACRIMSAT (Willson,

2005), the SOHO VIRGO sensors (Frohlich et al.,

1997), and the Earth Radiation Budget System, ERBS

(Lee et al., 1987). For spectral measurements the UARS

SOLSTICE (Rottman et al., 1993) and SUSIM (Brueck-

ner et al., 1993) observations between 115 and 400 nmoverlap the UV observations of the SORCE SOLSTICE

and SIM, but it is unlikely that UARSwill continuemuch

longer. There are no UV (115250 nm) observations

planned after SORCE. The SORCE XPS observations

presently overlap similar observations of the TIMED So-

lar EUV Experiment, SEE (Woods et al., 1998), and in

the future NASAs Solar Dynamics Observatorys EUVVariability Experiment, EVE, will also provide overlapand continuity following its launch in 2008.

The TSI as well as the visible and near infrared

irradiance as measured by SIM comprise one of the Envi-

ronmental Data Records (EDRs) to be measured by the

National Polar Orbiting Operational Satellite System,

NPOESS, and may be operational after about 2010.

NPOESS carries an instrument package called the Total

and Spectral Irradiance Sensor, TSIS, and this is plannedto include a second generation TIM and SIM.

More than likely there will be a gap between the end-

of-mission for SORCE and the launch of NPOESS, and

this delay would introduce a very unfortunate break in

G.J. Rottman et al. / Advances ia TIM instrument. Likewise PICARD is a EuropeanFoukal, P., Lean, J. Magnetic modulation of solar luminosity by

photospheric activity. Ap. J. 328, 347357, 1988.

Frohlich, C., Crommelynck, D., Wehrli, C., Anklin, M., Dewitte, S.,

Fichot, A., Frosterle, W., Jimenez, A., Chevalier, A., Roth, H.J. In-

ight performance of VIRGO solar irradiance instruments on

SOHO. Solar Phys. 175, 267286, 1997.

Frohlich, C., Lean, J. Total solar irradiance variations: the construc-

tion of a composite and its comparison with models. In: Deubner,

F.L., Christensen-Dalsgaard, J., Kuntz, D. (Eds.), IAU Sympo-

sium 185: New Eyes to See Inside the Sun and Stars. Kluwer

Academic, Dordrecht, The Netherlands, pp. 89102, 1998.

Harder, J., Lawrence, G., Fontenla, J., Rottman, G., Woods, T. Solar

Phys. (in press).

Hoyt, D.V., Hickey, H.L., Maschho, R.H. The NIMBUS-7 solar

total irradiance: a new algorithm for its derivation. J. Geophys.lasp.colorado.edu/sorce/ and also from the GSFC

DAAC at http://daac.gsfc.nasa.gov/upperatm/sorce/.

References

Brueckner, G.E., Edlow, K.L., Floyd, L.E., Lean, J.L., Van Hoosier,

M.E. The solar ultraviolet spectral irradiance monitor (SUSIM)

experiment on board the upper atmosphere research satellite

(UARS). J. Geophys. Res. 98, 1069510711, 1993.

Eparvier, F.G., Woods, T.N., Ucker, G., Woodraska, D.L. TIMED

Solar EUV experiment: pre-ight calibration results for the EUV

grating spectrograph. SPIE Proc. 4498, 91100, 2001.

Floyd, L.E., Reiser, P.A., Crane, P.C., Herring, L.C., Prinz, D.K.,

Brueckner, G.E. Solar cycle 22 UV spectral irradiance variability:

current measurements by SUSIM UARS. Solar Phys. 177, 7987,

1998.

Fontenla, J., Harder, J., Lawrence, G., Rottman, G., Woods, T. Solarmission that will include a TSI device and may launch

in the 2008 timeframe. There are no plans for any spec-

tral observations during the gap (20082012), and even

when NPOESS is operational there will be no UV obser-

vations between 100 and 200 nm.

These nal thoughts emphasize that there is a welljustied scientic requirement to measure total and

spectral solar irradiance. There is extreme value, border-

ing on necessity, of insuring that the data sets overlap.

The entire international scientic community needs to

nd a coordinated approach in creating new opportuni-

ties for these measurement programs to continue.

Acknowledgments

SORCE has been a project in the making for many

years. It started with an initial proposal to NASA in

1988 for the EOS program, continued with the TSIM

proposal in 1997, and nally coming to fruition with

the launch of SORCE in 2003. This great achievement

is a tribute to the many, many individuals who have con-tributed professionals and students at LASP, employ-

ees of Orbital, NASA, and numerous other institutions.

SORCE is supported by NASA contract NAS5-97045 to

the University of Colorado, Boulder. Additional infor-Res. 97, 5163, 1992.

Kopp, G., Lawrence, G. Solar Phys. (in press).

Lawrence, G.M., Rottman, G., Harder, J., Woods, T. Solar total

irradiance monitor (TIM). Metrologia 37, 407410, 2000.

Lawrence, G.M., Kopp, G., Rottman, G., Harder, J., Woods, T.,

Loui, H. Calibration of the total irradiance monitor. Metrologia

40, S78S80, 2003.

Lee III, R.B., Barkstrom, B.R., Cess, R.D. Characteristics of the earth

radiation budget experiment solar monitors. Appl. Opt. 26, 3090

3096, 1987.

McClintock, W.E., Rottman, G., Woods, T. Solar Phys. (in press).

Mihalas, D., Binney, J.. Galactic Astronomy Structure and Kinemat-

ics. W.H. Freeman, New York, 1981, pp. 135139.

Powell, F.R., Vedder, P.W., Lindblom, J.F., Powell, S.F. Thin lm

lter performance for extreme ultraviolet and X-ray applications.

Opt. Eng. 26, 614, 1990.

Rottman, G.J., Woods, T.N., Sparn, T.P. Solar stellar irradiance

comparison experiment I: 1 instrument design and operation. J.

Geophys. Res. 98, 1066710677, 1993.

Willson, R.C. Measurement of solar total irradiance and its variability.

Space Sci. Rev. 38, 203242, 1984.

Willson, R.C. Irradiance Observations from SMM, UARS, and

ATLAS Experiments. In: Pap, J., Frohlich, C., Hudson, H.,

Solanki, S. (Eds.), The Sun as a Variable Star, Solar and Stellar

Irradiance Variations. Cambridge University Press, Cambridge,

UK, pp. 5462, 1994.

Willson, R.C. ACRIMSAT, Measuring the Suns Energy. http://acrim.jpl.nasa.gov/, 2005.

Woods, T., Eparvier, F., Bailey, S.C., et al. TIMED Solar EUV

experiment. SPIE Proc. 3442, 180191, 1998.

Woods, T., Rottman, G., Harder, J., Lawrence, G., McClintock, W.,

Kopp, G., Pankratz, C. Overview of the EOS SORCE Mission.

SPIE Proc. 4135, 192, 2000.

Woods, T.N., Rottman, G. Solar Phys. (in press).

Woods, T.N., Eparvier, F.G., Fontenla, J., et al. Solar irradiance

variability during the October 2003 solar storm period. Geophys.

Res. Lett. 31, L10802.1L10802.4, 2004.

208 G.J. Rottman et al. / Advances in Space Research 37 (2006) 201208

SORCE solar UV irradiance resultsIntroductionSORCEInstrumentsThe total irradiance monitor, TIMThe spectral irradiance monitor, SIMThe solar stellar irradiance comparison experiment, SOLSTICEThe XUV photometer system, XPS

Spacecraft and operations

ObservationsSolar spectral irradianceSolar variability

Future of SORCE and other observing programsAcknowledgmentsReferences