Modeling Solar Irradiance With the PSPT Solar Disk Observations
and RISE Solar Spectrum Synthesis
Sean M. Davis1, Juan Fontenla1, Jerald W. Harder1, Gary Rottman1, and Randy Meisner2
1Laboratory for Atmospheric and Space Physics University of Colorado, 1234 Innovation Drive, Boulder, CO 80303
2High Altitude Observatory, National Center for Atmospheric Research PO BOX 3000, Boulder, CO 80307
PSPT/RISE Image Analysis Algorithm
A. Preprocessing (Both Images):
The PSPT (Precision Solar Photometric Telescope) at the Mauna Loa Solar Observatory
produces full disk solar images in the Ca II-K spectral line (393 nm ± 0.15 nm), blue (409.3 nm ±
0.15 nm) continuum, and red (607.2 nm ± 0.15 nm) continuum, with ~0.1% photometric precision
and 1 arc sec pixels. The RISE (Radiative Inputs of the Sun to Earth) spectral synthesis calculation is
based on 7 solar atmosphere models corresponding to quiet and active solar features, and currently
calculates the emitted intensity as a function of wavelength from 0.5 to 10 mm for 10 positions in the
solar disk.
We use the RISE-derived center-to-limb variation functions for each surface feature in the
instrument spectral band to extract the feature distribution on the solar surface corresponding to each
PSPT image. The average disk intensity as a function of wavelength is then constructed using the
RISE calculated spectra for each feature and position on the disk. We present the results of a
preliminary study of solar irradiance calculations and comparisons with measurements from the Solar
Radiation and Climate Experiment (SORCE) satellite. These comparisons are made with the Spectral
Irradiance Monitor (SIM), which measures spectral irradiance (200 nm - 2700 nm).
Two Images are processed to remove effects of the CCD flat field,
linear gradients, and quadrant readout; the images are then co-aligned.
Ca II K
Red Band (607.1 nm)
TSI and Visible-Band Irradiance Plot:
578.5
m = cos(q)
RISE Model Overview
m=
0.8
-0
.9
1360
577.5
1359
1358
577.0
C. Red Image:
B. CA II K Image:
Divide image into 10 annuli of 0.1-m.
For each annulus, make a histogram of
contrast intensities (relative to bin with max
number of pixels).
The intensity of the bin with the max number
of pixels is assigned model C at the m value
of the annulus.
Using model contrast intensities, and
interpolating in m, assign a model to each
Use Legendre polynomial fit to remove
limb-darkening.
Use threshold to identify sunspots and
sunspot penumbras.
Assign sunspot pixels to model S,
penumbras to model A.
vCompute Irradiance or Brightness Temperature
= Plage
= Sunspot
where:
hn
k
Tb =
4 1
ln 2hn
+1
2
c l Ia
= TIM TSI
)
(
W = Solid Angle of the Sun at 1 AU (6.7995 x 10-5 sr)
R = Irradiance at 1AU (Wm -2 nm -1)
Tb= Brightness Temperature (K)
= SIM (vis 2)
= RISE/PSPT
1 image chosen for RISE analysis
Mask Image:
-Each pixel assigned to a model
Tables:
- Relative areas for each model in each annulus
z
Masked Image
Model
Solar Structure Type
Rel. Area
(5 Feb. 2003)
6000
Brightness Temperature (K)
SIM Overview
Measure 4 absolute solar irradiance spectra per day
Broad wavelength coverage: 200 to >2000 nm
High wavelength accuracy:1.3 mm knowledge in focal plane or Dl/l < 150 ppm
High measurement accuracy: Goal of 300 ppm (1 s)
High measurement precision
- SNR=500 @ 300 nm
- SNR=20000 @ 800 nm
In-Flight recalibration
- Prism transmission calibration
- Duty cycling 2 independent spectrometers
Sample RISE-Synthesized Spectrum
D. Output
Images used are 30-exposure
composites
5800
5600
Faint supergranule cell interior
Average supergranule cell interior
Average network
Bright network/faint plage
Average plage
Bright plage
Sunspot umbra
0.0287
0.8057
0.1219
0.0312
0.0110
0.0008
0.0007
5200
5000
4600
400
1 Jul
1 Sep
600
l (nm)
800
1000
= RISE
= RISE convolved with SIM instrument function
1 Nov
0.0100
0.0010
0.0001
1.0010
1.0005
1.0000
0.9995
l = 642.1 nm
1.0010
1.0005
1.0000
0.9995
l = 762.6 nm
1.0010
1.0005
1.0000
0.9995
1.0010
l = 885.7 nm
1.0005
1.0000
0.9995
1 Jun
5400
4800
A
C
E
F
M
P
S
Relative Area
1 May
~10 image sets taken daily
u
v
w
x
y
578.0
1,000 ppm
Fractional Change from Mean
2048 x 2048 CCD Imager
3 wavelength bands:
Ca II K: 393 ± .15 nm
Blue: 409.3 ± .15 nm
Red: 607.2 ± .25 nm
0.1% relative photometric precision
Im,i = Intensity of model m feature at weightedcenter of the ith annulus.
Am,i = Relative area of model m features in ith annulus
Ia = Average Disk Intensity (Wm -2 nm -1 sr -1)
Sunspots enhanced
and shown in red
1361
RISE/PSPT Visible Irradiance (W/ m 2)
579.0
1362
Images from February 05, 2003
where:
PSPT Overview
- 500nm - 929nm "Visible" Integrated Irradiance from PSPT/RISE
- Both data sets plotted -3,000 ppm to 1,000 ppm from their means
- Data show good general agreement, but much room for improvement
uCompute Area-Weighted Average Disk Intensity
where q= heliocentric angle
~Studies the physical origins of solar variability arising form variations of surface structures.
~ Uses semi-empirical models of the solar atmosphere to synthesize the intensity spectrum from
features on the solar surface.
~ Determines the distribution and evolution of surface features that contribute to irradiance
variations.
~ Combines the above with spectral synthesis models of lines and broad spectral bands to
compute a spectral irradiance for a particular state of the sun.
Comparison of PSPT/RISE and
SORCE Irradiances
Solar Irradiance Synthesis with the RISE
Model
TIM TSI
ABSTRACT
5 Jun
10 Jun
15 Jun
20 Jun