In-Flight Performance of SORCE SOLSTICE
Marty Snow, William McClintock, Gary Rottman, and Tom Woods
Laboratory for Atmospheric and Space Physics
University of Colorado, Boulder, CO 80303
snow@lasp.colorado.edu
The SOLar-STellar Irradiance Comparison Experiment (SOLSTICE)
observes an ensemble of bright, early-type stars to track the loss of
sensitivity over time. The stars were chosen to be stable over
timescales of centuries, and by using multiple stars, we remove the
effect of any intrinsic stellar variability.
Irradiance (photons/cm2/s/nm)
Measurements:
Wavelength Coverage:
Solar Spectral Resolution:
Stellar Spectral Resolution:
115-320 nm
0.1 nm (FUV)
0.2 nm (MUV)
1.1 nm (FUV)
2.2 nm (MUV)
1013
Each stellar observation is of a single
wavelength (averaged over the stellar
bandpass), so no spectral information is
present in the measured flux. To
determine the actual wavelength, we
measure the position of zeroth order
before and after each stellar
observation.
Eta UMa Irradiance (×109)
11
10
1010
108
150
200
Wavelength (nm)
250
300
Wavelength Corrections
The majority of SOLSTICE stellar
observations are at a single wavelength, but
we also regularly do a spectral scan of each of
our program stars. A typical example of a
single spectrum (taken on March 6, 2003) is
shown at right. These scans have a shorter
integration time, so the signal-to-noise ratio is
lower and the uncertainty of each point in the
spectrum is roughly 10% (cf. standard fixedwavelength observations have an uncertainty
of 1%).
This correction has been applied to
the airglow region, but has not been
included in the other wavelength
regions shown below.
SORCE SOLSTICE Stellar Flux for eta_uma
1.2•104
1.0•10
In order to combine stellar
observations at nearby wavelengths,
we need to account for each star’s
intrinsic spectral shape. In the
airglow region, we are currently
relying on archived IUE spectra of
our stars reduced to SOLSTICE’s
spectral resolution. Only the relative
shape of the spectrum is required
from the IUE spectrum, not the
absolute irradiance.
4
8.0•103
6.0•103
4.0•103
2.0•103
IUE
SOLSTICE A
SOLSTICE B
Co-adding multiple scans reduces the signalto-noise ratio to an acceptable level and the
measured shape of the spectrum at
SOLSTICE’s resolution is used to correct for
the spread in observed wavelength for the
fixed-wavelength observations.
0
110
115
120
125
Wavelength (nm)
130
135
140
FUV (130-180 nm) and MUV (180-320 nm)
Typical FUV and MUV stellar fixed-wavelength
observations are shown at left. The fractional uncertainty
in the mean irradiance for each observation is roughly
1%.
Geocoronal Lyman alpha emission complicates
the stellar observations below 130 nm.
Emission from Hydrogen in the earth’s
atmosphere (aka airglow) is nearly as bright as
the stellar flux that we are trying to measure.
We fit a polynomial to the two dark area
observations to model the background
contribution.
Stars are observed during the eclipse portion of every orbit. Both
instruments simultaneously observe in the same mode, alternating
between FUV and MUV on a daily basis. On average, we make
about 50 stellar fixed-wavelength observations per day, so each the
responsivity at each wavelength is measured roughly once per day.
109
Airglow Region (115-130 nm)
We remove this background contamination by
adjusting our standard observing technique.
We first observe a region of the sky free of
bright stars (“dark area”) to measure the signal
from the airglow. Then we point the
instrument at the star and measure the
combined star+background flux. We reobserve the dark area to measure the changes
in the airglow. We also measure the position
of zeroth order before and after the star to
determine the true wavelength.
Changes in the responsivity are monitored at discrete wavelengths in
each channel (MUV and FUV) for each instrument (Solstice A and
Solstice B) using the stars. We have chosen 18 wavelengths in the
FUV channel and 22 in the MUV channel. These wavelengths are
shown on the plot at left as squares or diamonds.
1012
The measured wavelength is
affected by a number of factors,
primarily spacecraft pointing and
grating drive offsets.
The pointing accuracy of the
spacecraft is normally better than
a few arc seconds, but before
August, 2003, there was an error
in the precession software. This
error resulted in stellar pointing
that was off by up to 2 arc
minutes, translating into
wavelength shifts of up to 0.3
nm.
Before launch, the absolute responsivity of the instrument is measured
at SURF (Synchrotron Ultraviolet Radiation Facility) to an accuracy
of ~5%. This preflight calibration is transferred to the stars during the
early orbit campaign.
Solar Irradiance
1014
SOLSTICE observes both the sun and stars with the same optical path.
A factor of 108 in dynamic range is achieved by changing apertures
(~104), bandpass (~101), and exposure time (~103).
Preflight Calibration at SURF
Solar-Stellar Irradiance Comparison
1015
We aim for at least 104 total counts in each stellar
observation so that counting statistics are kept to 1%.
We use at least 200 integrations per observation, so the
uncertainty of the Gaussian mean is
σ=
2050
2000
SOLSTICE_A MUV ALF_CRU at 191.7 nm
Each such stellar observation at a given
wavelength is combined using a simple
exponential model.
1950
1900
1850
1800
1750
11:06:43
30
25
uncertainty per integration
700 700
=
≈ 0.5%
number of integrations
200
11:07:26
11:08:09
11:08:52
Time on 3/16/2003
11:09:35
11:10:19
Histogram of Flux Measurements with Model Fit
D = Astar ((1 − β ) + β ⋅ e − t /τ )
20
15
Finally, we derive the change in stellar
Irradiance which is due to loss of sensitivity in
the instrument.
10
5
0
3.80
3.90
4.00
4.10
4.20
Uncorrected Flux (x10^4 photons/s/cm^2/nm)
4.30
Where Astar is a fit normalization factor (I.e.
the relative brightness at that wavelength), τ
is the timescale factor, and β is the
asymptote.
SORCE Science Meeting