XPS: XUV Photometer System
Tom Woods, Gary Rottman, Scott Bailey, and Frank Eparvier
Laboratory for Atmospheric and Space Physics, University of Colorado, 1234 Innovation Dr., Boulder, CO 80303, USA, Tom.Woods@lasp.colorado.edu
The XUV Photometer System (XPS) is a set of 12 Si photodiodes designed to measure the solar soft x-ray (XUV) irradiance from 0 to 35 nm with about 7 nm resolution, with an absolute
accuracy of 20%, and with a relative precision of 4%. This instrument is one of four instruments on the SOlar Radiation and Climate Experiment (SORCE), a NASA/EOS satellite
mission scheduled for launch on December 1, 2002. An almost identical version of the XPS is one of two instruments of the Solar EUV Experiment (SEE) on the NASA ThermosphereIonosphere-Mesosphere-Energetics-Dynamics (TIMED) mission, which was launched in December 2001 and has been making daily measurements since January 22, 2002.
SCIENCE: The XPS measures the solar soft X-ray
This false color image of the sun is a measurement of the bright
helium emission at 30.4 nm made by SOHO EIT. This view
indicates how different the sun looks, and behaves, in the XUV than
in the visible: the active regions on the sun appear as dark sun spots
in the visible but as bright plages in the XUV.
SPECTRAL COVERAGE
DESIGN:
XPS is a set of twelve photometers consisting of Si photodiodes with thin -film filters, each having a spectral bandpass of about 7 nm. These
filtered Si photodiodes have proven to be very stable over long time periods and with extended exposure and have been adopted by the National Institute of
Standards and Technology (NIST) as secondary radiometric detector standards. Although little, if any, degradation is expected for XPS over the SORCE mission,
three of the XPS photometers are redundant channels that are used just once a week to track instrument degradation.
Solar
Radiation
12 PhotoDi o d e
Detectors
Amplifiers
&
VFCs
Interface
Board
LVPS
Filter Wheel
Mechanism
Filter Wheel
Control
Electronics
XPS
The assembled XPS instrument prior to
integration with the SORCE spacecraft.
XPS - Generic
Channel Interface
• XPS
– XUV Photometer (XP) converts photocurrent to frequency (VFC)
– Pulses are integrated in XPS Generic Channel Interface (XPS-GCI)
for a command selectable period
– Filter wheel moves windows and clear apertures in front of XP’s
– Windows are used to measure the visible light background
– Bare Si photodiodes are used to track the window transmission
• XPS- GCI provides
– Data integration & housekeeping monitors (telemetry)
– Command decomposition (6 commands)
– Power conditioning and regulation (5V, +15V, -15V)
HISTORICAL BACKGROUND:
Studies of the solar XUV radiation began in the 1950s with spacebased rocket experiments, but the knowledge of the solar XUV
irradiance, both in absolute magnitude and variability, has been
questionable due largely to the very limited number of
observations. With the launch of Solar & Heliospheric
Observatory (SOHO) in 1995, Student Nitric Oxide Explorer
(SNOE) in 1998, and Thermosphere-Ionosphere-MesosphereEnergetics-Dynamics (TIMED) spacecraft in 2001, there is now a
continuous data set of the solar XUV irradiance, and advances in
the understanding of the solar XUV irradiance have begun. The
SORCE XPS, which evolved from earlier versions flown on
SNOE and TIMED, will continue on these solar XUV irradiance
measurements with improvements to accuracy, spectral range, and
temporal coverage.
Solar Min
Max
The XPS -type
measurements from
SNOE and TIMED
indicate that the
historical values for
the XUV below 20
nm is higher by
almost a factor of 4 !
(NN%) = percent solar signal in the bandpass shown
Spectral coverage on SNOE is limited to 2-20 nm
IRRADIANCE EQUATION
A Straightforward Calculation
E=
(I Total
−I Dark − I Vis )• fXUV • D• K E
TXUV • A
Ix = C x • b
MEASUREMENT APPROACH:
Multiple Photometers with Low Spectral Resolution
1
IRD XUV
Photodiode
Thin Film
Coating
Solar Spectrum
2
The signal from each photometer is the total current summed over all wavelengths.
That is, a single photometer does not provide spectral information.
1. The thin film coating on the XUV photodiode is selected to limit the bandpass to
3
Thin film coating deposited directly on
photodiode produces detector with a
definedx -raybandpass .
4
X-raybandpass convolved with solar
spectrum defines range of solar sensitivity.
about 7 nm in the XUV region (0 -35 nm).
2. Reference solar spectra, both solar minimum and maximum values, are used to
verify that the filter bandpass is designed properly for solar measurements and to
estimate the expected signal levels.
3. The sensitivity is calibrated as a function of wavelength at the NIST Synchrotron
Ultraviolet Radiation Facility (SURF) in Gaithersburg, MD. A mo del of the
sensitivity is calculated using the Henke atomic constants in order to extend the
sensitivity calibrations to all wavelengths.
4. The expected signal levels are determined using the reference solar spectra and the
modeled sensitivity.
References
• Bailey, S. M., T. N. Woods, C. A. Barth, S. C. Solomon, L. R. Canfield, and R. Korde, Measurements of the solar soft X-ray irradiance from the Student Nitric Oxide Explorer: first analysis and underflight calibrations,
J. Geophys. Res., 105, 27179-27193, 2000.
• Woods, T., E. Rodgers, S. Bailey, F. Eparvier, and G. Ucker, TIMED Solar EUV Experiment: pre-flight calibration results for the XUV Photometer System, SPIE Proceedings, 3756, 255-264, 1999.
• Woods, T. N., F. G. Eparvier, S. M. Bailey, S. C. Solomon, G. J. Rottman, G. M. Lawrence, R. G. Roble, O. R. White, J. Lean, and W. K. Tobiska , TIMED Solar EUV Experiment, SPIE Proceedings, 3442, 180-191, 1998.
• Woods, T., Rottman, G., Harder, G., Lawrence, G., McClintock, B., Kopp, G., and Pankratz, C., Overview of the EOS SORCE Mission, SPIE Proceedings, 4135, 192-203, 2000.
• SORCE web page: http://lasp.colorado. edu/sorce/
Spectral coverage on SORCE XPS is
very similar to TIMED XPS
+28V Pwr
•Instrument Type: Filter Photometer
•Wavelength Range: 1-34 nm
•Wavelength Resolution: 5-10 nm
•Optics: Thin film filters (deposited on Si diodes)
•Detector: 12 Si photodiodes: 8 XUV, Ly -α, 3 bare
•Absolute Accuracy : 20%
•Long-term Accuracy : 4%
•Field of View: 4° cone
•Mass: 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, rocket XPS
•Pre-flight Cal. Std: NIST SURF-III Ref. Si Diode
•In-flight Cal.: Redundant channels
Cmd / TM
From / To MU
(XUV) irradiance from 1 to 34 nm and the bright hydrogen emission
at 121.6 nm (H I Lyman -α). The solar XUV radiation is mostly
emitted from the hot, highly-variable corona on 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.
T • Em • dλ
TXUV = ∫
∫ E m • dλ
E = Irrad ia nc e [W m -2 ]
I = Current [nA]
f = FOV Fac tor [ unit le ss] from FOV ma ps
D = Degradati on fact or [ unitl ess] from redundan t chann els
K E = En ergy C onve rsion [3.63 x 10 -5 W nA-1 cm2 m-2]
T = Transmissi on [unitl ess]
A = A perture Area [cm2]
C = Co unt Ra t e [cps]
b = Count to Curre nt Conversion [nA/c ps] (varies wit h T)
Key:Gree n = pre-flight ca librati on mea suremen t
Blue = in-flight meas ureme nt
Re d = deriv ati on bas ed o n spe cia l i n-fli ght experiments
Use of Reference Solar Spectra
The transmission parameter, T, is obtained using the pre-flight
sensitivity curve that is weighted with reference solar spectra. The
difference in the transmission between using solar minimum and
maximum spectra is typically less than 5%, and this difference is
applied as an extra uncertainty of the XPS irradiance. This app roach is
desirable because the value for T, that is derived once, is simply a
constant in the routine XPS data processing software.
Another approach for analyzing XUV photometer data, such as for the
SNOE and SOHO SEM measurements, is to scale a model solar
spectrum to match the detector current output. This approach requires
the use of a model solar spectrum in the data processing and raises the
issue on how one knows how the spectral shape changes with time in
the solar XUV spectrum. The above approach, used for TIMED and
SORCE XPS, does rely on reference solar spectra to obtain the
appropriate transmission for solar observations, but this newer
approach has minimum dependence on any spectral shape differences.
Tom Woods, 12 August
2002