SORCE Data Processing and Availability
Chris Pankratz
Laboratory for Atmospheric and Space Physics
1234 Innovation Dr
Boulder, CO 80303
chris.pankratz@lasp.colorado.edu
Abstract
The SORCE Science Data System produces Total Solar Irradiance (TSI) and Spectral Solar
Irradiance (SSI) data products on a daily basis, which are formulated using measurements
from the four primary instruments on board the SORCE spacecraft. The TIM instrument
provides measurements of the TSI, whereas the SIM, SOLSTICE, and XPS instruments
collectively provide measurements of the solar irradiance spectrum from 1 nm to 3000 nm
(excluding 31-115nm, which is covered by the TIMED SEE experiment). The Science Data
System utilizes raw spacecraft and instrument telemetry, calibration data, and other
ancillary information to produce a variety of data products that have been corrected for all
known instrumental and operational factors. Since launch of the SORCE spacecraft in
January 2003, science processing algorithms have continued to mature, and “Level 3” data
products are routinely being produced and delivered to the Goddard Earth Sciences (GES)
Distributed Active Archive Center (DAAC) for public release.
Total Solar Irradiance (TSI)
• Product Overview
• The TIM instrument measures the Total Solar Irradiance (TSI), monitoring changes
in incident sunlight to the Earth's atmosphere using an ambient temperature active
cavity radiometer
• The standard “Level 3” TSI science data products consist of daily and 6-hourly
average irradiances, reported at zero relative line-of-sight velocity with respect to the
Sun at both a mean solar distance of 1 astronomical unit (AU) and at the true SunEarth distance.
• Two TSI data products are produced - one containing daily means and the other
containing four 6-hourly means. Each TSI product contains science results for the
entire SORCE mission. Updates to Level 3 TSI data occur monthly.
• To construct these products, high time cadence measurements (every 50 seconds)
from the TIM instrument are combined to produce representative daily and 6-hourly
values of the TSI.
• Shuttered operation of the instrument corrects for thermal background, and stateof-the-art phase sensitive detection algorithms applied to the data at the shutter
fundamental reduce noise and sensitivity to drifts. The TIM shutter period is 100
seconds; four such cycles are weighted to give a single irradiance measurement at a
50 second cadence. These so-called “Level 2” data are averaged over an entire day
as well as over each 6-hour interval to formulate the two primary TSI data products.
This poster provides an overview of the SORCE data processing system, summarizes the
present state of the processing algorithms and the quality of the current SORCE data
products, and provides details on how to access SORCE science data.
Overview
• All data processing is performed by the SORCE Science Data System (SDS), located at the
SORCE Science Operations Center (SOC) at the Laboratory for Atmospheric and Space Physics
in Boulder, Colorado, USA
• Algorithms are implemented as described in the SORCE Algorithm Theoretical Basis Document
(ATBD) to produce science data products. See http://lasp.colorado.edu/sorce/science_algorithm.html
• Data Quality / Status
• On-orbit instrument characterization is an ongoing effort, although only minor
corrections are anticipated at this phase in the SORCE/TIM mission.
• The TIM instrument is proving very stable with on-orbit solar exposure, and longterm relative uncertainties are estimated to be less than 0.014 W/m^2/yr (10 ppm/yr).
• Present absolute accuracy is estimated to be approximately 0.4 W/m^2 (300 ppm),
largely determined by the agreement between all four TIM cones.
• There remains an unresolved 4.5 W/m^2 difference between the TIM and other
radiometers, and this is being studied by the TSI and radiometry community. The
refined calibrations in the version 4 TIM data processing system further support the
lower TSI value reported in the SORCE data products.
• Data are now available through late September 2004
• Data Product Summary
• Total Solar Irradiance (TSI), W/m²
• Daily and 6-Hourly Mean Irradiances
• Measurements provided by the TIM Instrument
• Spectral Solar Irradiance (SSI), W/m²/nm
• Daily Irradiance spectra from 115-2000 nm, normalized to 1 AU
• Measurements from the SIM, SOLSTICE, and XPS instruments
• XPS:
0.1 - 34 nm at 5-10 nm intervals, 6-Channels
• SOLSTICE: 115 - 310 nm at 1 nm intervals
• SIM:
310 - 2000 nm on a fixed wavelength scale
(1-34 nm variable resolution)
Data Availability
SORCE TSI Data Record
• The SORCE SOLSTICE, SIM, and XPS instruments together provide measurements of the full-disk Solar Spectral Irradiance (SSI)
from ~0.1 nm to 2000 nm (excluding 34-115 nm, which is not covered by the SORCE instruments)
• The standard “Level 3” science data products consist of daily average irradiances, reported at a mean solar distance of 1 astronomical
unit (AU).
• Data products are available from the GES DAAC and from the SORCE Web site.
SIM Irradiance Time Series
• Each SSI product contains science results for one UT calendar day.
767 nm
SIM
Recent TSI Data Measurements
o SIM is a Fèry Prism Spectrometer which provides irradiance measurements from
200 nm to 3000 nm with a spectral resolution varying from 1 to 34 nm
o The SIM ESR detector provides the primary absolute calibration for the SIM
instrument, and operates on the same principal and uses the same detector
methodology as the active cavity radiometer in the SORCE TIM instrument.
o SIM contains four focal plane photodiodes, which are used to acquire the solar
spectrum in a "fast-scan" mode, which provides the requisite time cadence and the
spectral sampling that is not possible with the much “slower” ESR detector.
o Data Quality / Status
o On-orbit instrument characterization is still in progress. Certain instrumental
artifacts are present within the data and a few on-orbit instrument calibrations (e.g.
diode degradation) have not yet been applied.
o SIM Data are presently available from 310-1600 nm. Additional wavelength
coverage will be available in the future.
o Version 4 data are now available from 21 April 2004 through present. These
data are available from the SORCE web site (see below) and will be delivered to
the DAAC (with new SOLSTICE data) in the future.
o Version 3 data prior to 21 April 2004 are available from the DAAC
o Absolute Accuracy: An important instrument correction factor is still being
characterized and is not yet being applied to the SIM data. As a result, the
absolute accuracy of SIM data at visible wavelengths is approximately 3%. In the
infared, SIM data are estimated to be low by about 10%. We expect to complete
this missing correction in the future.
o Relative Accuracy: The uncertainties reported with the data provide an
indicator of relative precision. Above 500 nm, relative accuracy is better than
approximately 300 ppm. Below 500 nm, this decreases to approximately 0.7% at
310 nm.
o Wavelength Scale: SIM wavelengths (310-1600 nm) are presently estimated to
be accurate to between 0.5 and 5 nm, depending on wavelength (more accurate at
shorter wavelengths). Relative (day-to-day) accuracy is about a factor of 50 better.
o Known Issues
o Data prior to 21 April 2004 (version 3) exhibit discrete “jumps” or
discontinuities at the ~0.1% level from day to day. These artifacts are caused by
shifts in the SIM wavelength scale, and will be corrected in the next SIM data
version.
o Certain SIM diodes exhibit measurable degradation. A correction is being
developed.
o Characterization of SIM ESR absorptivity is incomplete, hence the increased
absolute irradiance uncertainty.
1501 nm
SIM Solar Spectrum
21 April 2004
Data Access and Services
SORCE standard data products are available via the SORCE web site and from the
Goddard Earth Sciences DAAC free of charge
− SORCE Web Site (http://lasp.colorado.edu/sorce/data_access.html)
− Official Gateway to SORCE Information and Data Products
DAAC Data Pool: http://daac.gsfc.nasa.gov/data/datapool/SORCE/
The ECS data pool maintained at the GES DAAC is a large (50 TB capacity) online anonymous ftp server that allows
users quick access to specific data holdings. Due to its relatively small size, the entire set of SORCE level 3 data
products will always be available from the data pool to download.
− Permits customized selection, plotting, and access to SORCE data (Publicly Available Soon)
DAAC-Provided Tools and Services:
− Permits direct download of ASCII TSI Product Files
The GES DAAC provides C, Fortran and IDL tools for reading and displaying the SORCE HDF5 data files. Online
documentation, guides and FAQs, describing these data will also be available. For information describing the HDF5 file
format, please see the web site http://hdf.ncsa.uiuc.edu/hdf5. HDF5 is maintained by the National Center for
Supercomputing Applications (NCSA), located at the University of Illinois, at Champaign.
Users with questions concerning data orders or DAAC holdings should contact the user help desk. Users should also
contact the help desk when requesting hardcopy documentation or posters. Contact information for the GES DAAC help
desk is listed below:
e-mail: help@daac.gsfc.nasa.gov
telephone: 1-301-614-5121 (local), 1-877-422-1222 (toll free)
− GES DAAC – via the DAAC SORCE Gateway - http://daac.gsfc.nasa.gov/upperatm/sorce/
• GES DAAC FTP Data Pool (Preferred)
• GES DAAC Search and Order
• NASA Earth Observing System (EOS) Data Gateway (EDG)
SORCE Data Accessors: http://lasp.colorado.edu/sorce/sorce_data_access/
• Managed by the SORCE Science Team
• Provides fast, customizable access to
SORCE data products, including spectra,
time-series, and TSI
• Product Overview
Solar Spectral Irradiance (SSI)
DAAC Data Pool: http://daac.gsfc.nasa.gov/data/datapool/SORCE/
• The GES DAAC interface utilizes a simple point-and-click
approach for searching and ordering both on-line and near-line
data products that are archived locally. The data are organized
using a hierarchical design into tables according to different
“Views” (e.g. by data set, data product, parameter).
• Provides the most streamlined WWW interface to the SORCE
Data
• Fastest response time of WWW-based interfaces
SOLSTICE
o SOLSTICE provides precise daily measurements of solar spectral irradiance
at ultraviolet wavelengths from 115 nm to 310 nm with a spectral resolution of
approximately 0.1 nm
o SORCE SOLSTICE is a follow-on to the very successful SOLSTICE
launched aboard the Upper Atmospheric Research Satellite (UARS) in 1991.
SORCE SOLSTICE makes daily solar irradiance measurements and compares
them to the irradiance from an ensemble of 18 stable early-type stars. This
approach provides an accurate monitor of instrument in-flight performance and
provides a basis for solar-stellar irradiance comparison for future generations.
o Data Quality / Status
o Absolute Accuracy: Absolute accuracy is presently dominated by the lack
of an instrument degradation correction. The SOLSTICE degradation is being
characterized, and should be ready in the next data version. The current
version 4 data are low by 5-8 % from 115-180 nm, and low by < 3% from 180
-310 nm.
o Relative Accuracy: estimated to be about 0.1%, probably a bit higher at
shorter wavelengths.
o Present (Version 4) data are suitable for detecting relative irradiance changes
over short periods of time (a few solar rotations) but not for long-term studies.
o Known Issues
o Instrument Degradation - the most significant correction that still needs to be
applied. The SORCE science team expects to have a degradation correction in
place for the next version of SOLSTICE data (version 5), expected to become
available in late 2004.
o A scattered light correction needs to be applied to the data, which will
provide a ~1% effect at some wavelengths.
o Temperature Gain – In-flight characterization data is being processed now to
further reduce the instrument’s susceptibility to detector temperature changes.
XPS
• The XPS measures the solar soft x-ray (XUV) irradiance from 1 to 34 nm (in 5
discrete bandpasses) and the bright hydrogen emission at 121.6 (H I Lyman-alpha).
• 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.
• Data Quality / Status
• The current data available via the SORCE web site include version 6 XPS data.
These data are known to be of good quality and are appropriate for research and/or
publication.
SOLSTICE Lyman Alpha
SOLSTICE Spectrum
21 April 2004