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Laboratory for Atmospheric and Space Physics


The XPS measures the solar soft x-ray (XUV) irradiance from 1 to 34 nm and the bright hydrogen emission at 121.6 (H I Lyman-alpha). The solar XUV radiation is mostly emitted from the hot, highly-variable corona of the Sun, and these high-energy photons are a primary energy source for heating and ionizing Earth’s upper atmosphere. Of all the SORCE instruments, the XPS is most sensitive to flare events on the Sun as the solar XUV radiation often changes by a factor of 2 to 10, or more, during flares.

Table 1: XPS Properties

Instrument Type Filter Photometer
Wavelength Range 1-27 nm, 121-122 nm
Wavelength Resolution 1-10 nm
Optics Thin Film Filters (deposited on Si photodiodes) and Interference Filter for 121.5nm channel
Detector 12 Si photodiodes: 8 XUV, 1 Ly-alpha, 3 bare
Absolute Accuracy 12-24%
Long-term Accuracy 1%/year
Field-of-View 4° cone
Dimensions (H×W×D) (without GCI) 15.6 cm × 18.7 cm × 17.2 cm
Mass (with GCI) 3.6 kg
Orbit Average Power 9 W
Peak Power 14 W peak (~30 sec/orbit)
Orbit Average Data Rate 0.3 kbits/s
Redundancy 3 redundant XUV diodes
Heritage TIMED SEE, SNOE, and rocket XPS
In-flight Cal Redundant Channels, TIMED SEE claibration rockets
Pre-flight Cal. Std. NIST SURF-III, ref. Si diode


The Sun is the dominant energy source driving the Earth’s climate system and has both direct and indirect influences on the terrestrial system. The TIM’s measurement of the total solar irradiance arriving at the top of the Earth’s atmosphere is 1361 W/m2, providing a globally averaged value of 340 W/m2 (see Earth Energy Balance Figure below based on Kiehl and Trenberth [1997] and adapted from a presentation by Bill Collins, NCAR, at the 2006 SORCE Science Team Meeting). This external energy input, minus losses caused by the Earth’s planetary albedo, is balanced by a global infrared emittance corresponding to an average Earth atmosphere temperature of 254 K (-19° C). The external energy input in combination with surface and atmospheric processes, most notably the effects of water vapor and other greenhouse gases, equilibrate to a global mean surface temperature of 288 K (15° C).

Schematic depiction of global energy flows in the Sun-Climate system

Schematic depiction of global energy flows in the Sun-Climate system by Kiehl and Trenberth (Bull. Am. Meteor. Soc., 1997).

Quantities in parenthesis indicate changes in global heat flows in 10 years of Community Climate System Model (CCSM) research. The -2 associated with incoming solar radiation is based on SORCE TIM results of 1361 Wm-2. This figure is adapted from a presentation by Bill Collins, NCAR, given at the 2006 SORCE Science Team Meeting.

Precise space measurements obtained during the past 3 decades imply that the TSI varies on the order of 0.1% over the 11-year solar cycle, but with greater variations on day-to-month scales due to solar rotation and the passage of sunspots and facular regions across the solar disk. Variations in TSI occur over time scales from minutes to 11-year solar cycles and longer. Climate models including a sensitivity to solar forcing estimate a global climate change of up to 0.2° C due to solar variations over the last 150 years.

To determine long-term changes in the Sun’s output, which may have time scales extending much longer than the 11-year solar cycle, the TSI climate record requires either very good absolute accuracy or very good instrument stability and continuous measurements. To date, no TSI instrument has achieved the necessary absolute accuracy, and the TSI record relies on measurement continuity from overlapping spacecraft instruments (see TSI Database Figure below). The SORCE/TIM instrument continues the 3-decade long climate record of spaceborne TSI measurements. The TIM has an estimated absolute accuracy of 350 ppm, where 1 ppm = 0.0001%, and a long-term repeatability for detecting relative changes in solar irradiance of 10 ppm per year.

TSI Database

TSI Database

For more details, see Kopp, G., Lawrence, G., and Rottman, G., “The Total Irradiance Monitor (TIM): Science Results,” Solar Physics, 230, 1, Aug. 2005, pp. 129-140.


The Sun has both direct and indirect influence on the terrestrial system; and appropriate measurements of both total and spectral solar irradiance provide the requisite understanding of one of the primary climate system variables. SOLSTICE provides precise daily measurements of solar spectral irradiance at ultraviolet wavelengths. Even small variations in the Sun’s radiation at these short wavelengths lead to changes in atmospheric chemistry. Although the ultraviolet radiation from the Sun varies by as much as a factor of 2, its measurement requires access to space since this radiation does not penetrate the atmosphere. Also, the variability of solar ultraviolet radiation over an 11-year solar cycle is a strong function of wavelength, leading to varying requirements for different portions of the ultraviolet spectrum. The SOLSTICE measurements provide coverage from 115 nm to 320 nm with a spectral resolution of 0.1 nm, an absolute accuracy of better than 5%, and a relative accuracy of 0.5% per year.

Table 1: SOLSTICE Properties

Instrument Type Modified Monk-Gilleison spectrometers
Detector Type photomultiplier tubes
Wavelength Range 115 – 310 nm
Resolution 1 nm
Absolute Accuracy 1.2-6%
Relative Accuracy 0.2-2.6%/year
Dimensions (H×W×D) 18.3 × 38.7 × 84.6 (×2) cm
Mass 36.0 kg (total)
Power 33.2 watts (total)
Nominal Data Rate 738 bps (total)
Field-of-View 1.5° × 1.5° (uncalculated) and 0.75° × 0.75° (calculated)



A summary of the SIM instrument’s properties is provided below, and is described in detail in the text.

SIM Properties
Instrument Type Dual Fèry Prism Spectrometer
Wavelength Range 310-2400 primary, 200-300 secondary
Wavelength Resolution 0.25-33 nm
Optics Suprasil 300 prism
Detectors ESR, n-on-p and p-on-n silicon, InGaAs (5 detectors)
Absolute Accuracy 2%
Long-term Accuracy 0.03%/year
Field of View 1.7° x 2.5° (half power points)
Dimensions (H×W×D) 25.4 × 17.8 × 76.2 cm
Mass 22 kg
Orbit Average Power 25.3 W @ 28 V
Peak Power 35.4 W @ 25 V
Orbit Average Data Rate 1.6 kbytes command, 0.65 mbytes
Redundancy 2 redundant spectrometers
Heritage New Design
Pre-flight Cal. Std. Component unit level test + NIST cryogenic radiometer
In-flight Cal. Prism transmission cal + redundant channel comparison


The SIM instrument is making the first continuous record of the solar spectral irradiance in the visible/near infrared region of the spectrum at the top of the Earth’s atmosphere. To date, the time history of spectral irradiance variability in this part of the spectrum is not nearly as complete as the total solar irradiance and the solar ultraviolet radiation records.

Absorption of Solar Radiation

Absorption of solar radiation at the top of the atmosphere, at the Earth (click image to enlarge)

Measurements of spectral irradiance in the visible/near infrared spectral region are important because absorption of solar radiation in the atmosphere, on land surfaces, and in ocean water is strongly wavelength dependent. Therefore, an understanding of solar variability as a function of wavelength is important to climate studies.

Solar variability models, with the additional constraint of Total Solar Irradiance (TSI) observations, predict very small fractional changes on the order of 0.1-0.01% in the solar output over the visible/near infrared spectral range so the instrument requires very high precision.

Fractional 11-year solar cycle graph

Fractional 11-year solar cycle graph (click image to enlarge)

The graph to the left shows the fractional 11-year solar cycle variability as a function of wavelength over much of SIM’s measurement band pass. The y-axis of the graph is logarithmic between 104 and 100, and linear below that value. For most wavelengths, the light intensity increases at solar max, but in the infrared, especially in the neighborhood of the 1.6 µm H- opacity region, the variability is very small and is out of phase with the solar cycle. The graph also shows the spectral regions of importance to the Earth climate system. Solar variability in the 174 to 300 nm spectral region is critical to both the dynamics and chemistry of ozone in the middle atmosphere (L. L. Hood, 1999). However, visible and infrared radiation penetrates deeper into the Earth’s atmosphere and therefore contributes an important component to solar forcing of climate.

The fractional variability in the visible/near infrared region is on the order of 0.1%, but in terms of energy flux the visible spectrum varies by about 1.3 W/m2. This is greater than the more variable UV spectrum by an order of magnitude. SIM detects changes in the 27-day solar rotation period, which is greater than or equal to the solar cycle change.



Why Study the Sun and How does SORCE Help?

Solar radiation is the dominant, direct energy input into the terrestrial ecosystem; and it affects all physical, chemical, and biological processes. The Sun provides a natural influence on the Earth’s atmosphere and climate. In order to understand mankind’s roles in climate change, the Sun’s impact must first be understood.

SORCE measures the Sun’s output with the use of state-of-the-art radiometers, spectrometers, photodiodes, detectors, and bolometers engineered into instruments mounted on a satellite observatory. The SORCE satellite orbits around the Earth accumulating solar data. Spectral measurements identify the irradiance of the Sun by characterizing the Sun’s energy and emissions in the form of color that can then be translated into quantities and elements of matter. Data obtained by the SORCE experiment will be used to model the Sun’s output and to explain and predict the effect of the Sun’s radiation on the Earth’s atmosphere and climate.

Detailed Science Introduction

Energy from the Sun makes life on Earth possible. Solar energy also drives the Earth’s climate, and slight variations in solar radiance could offset (or increase) global warming. 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 a result, summer is much warmer than winter, and the polar regions have continuous daylight during their summer and continuous nighttime during their winter. Also, Earth’s orbit about the Sun is an ellipse, and not a precise circle, with the Earth slightly closer 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 the same year after year.

The total solar irradiance, or TSI, along with Earth’s global average albedo, determines Earth’s global average equilibrium temperature. Because of selective absorption and scattering processes in the Earth’s atmosphere, different regions of the solar spectrum affect Earth’s climate in distinct ways. Approximately 20 – 25 % of the TSI is absorbed by atmospheric water vapor, clouds, and ozone, by processes that are strongly wavelength dependent. Ultraviolet radiation at wavelengths below 300 nm is completely absorbed by the Earth’s atmosphere and contributes the dominant energy source in the stratosphere and thermosphere, establishing the upper atmosphere’s temperature, structure, composition, and dynamics. Even small variations in the Sun’s radiation at these short wavelengths will lead to corresponding changes in atmospheric chemistry. Radiation at the longer visible and infrared wavelengths penetrates into the lower atmosphere, where the portion not reflected is partitioned between the troposphere and the Earth’s surface, and becomes a dominant term in the global energy balance and an essential determinant of atmospheric stability and convection. Thus it is important to accurately monitor both the TSI and its spectral dependence.

Variations of the solar radiation field are still largely unknown, but in the visible they are likely far less than one percent. The observations therefore require precision and accuracy that can only be achieved from space. Although the ultraviolet radiation from the Sun varies by much larger factors, its measurement also requires access to space since the radiation does not penetrate the atmosphere. Precise space measurements obtained during the past 20 years imply that TSI varies on the order of 0.1% over the solar cycle (see Figure 1), but with greater variations on a short-term basis. For example, the passage of sunspots over the disk produces 2-4 times that amount. The variation apparently occurs over most time scales, from day-to-day variations up to and including variations over the 11-year solar cycle. How TSI variations are distributed in wavelength is still poorly understood. The largest relative solar variations are factors of two or more at ultraviolet and shorter wavelengths, but the greater total energy available at visible and longer wavelengths makes their small variations of potential importance.

Total Solar Irradiance Coverage

Figure 1: Total Solar Irradiance Coverage


Assuming climate models include a realistic sensitivity to solar forcing, the record of solar variations implies a global surface temperature change on the order of only 0.2° C. However, global energy balance considerations may not provide the entire story. Some recent studies suggest that the cloudy lower atmosphere absorbs more visible and near infrared radiation than previously thought (25% rather than 20%), which impacts convection, clouds, and latent heating. Also, the solar ultraviolet, which varies far more than the TSI, influences stratospheric chemistry and dynamics, which in turn controls the small fraction of ultraviolet radiation that leaks through to the surface.

Figure 2 shows the solar irradiance between 1 nm and 2000 nm where the spectrum is displayed with an effective spectral resolution of 1 nm. This spectrum has the general characteristic of a continuum spectrum throughout, with many absorption features, both lines and absorption edges, and at the shorter wavelengths superposed with emission features. In consideration of the Sun as the source of this emission, the longest wavelengths originate low in the photosphere, and progressing toward shorter wavelengths the emission originates higher and higher in the solar atmosphere, until at the very shortest wavelengths the emission is dominantly from the chromosphere with a few higher temperature lines originating in the solar transition region.

Solar Irradiance Observations

Figure 2: Solar Irradiance: 1 nm to 2000 nm