The SORCE (SOlar Radiation and Climate Experiment) Science Working Group Meeting

Steamboat Springs, Colorado

July 17 - 19, 2002


The Solar Radiation and Climate Experiment (SORCE) Working Group Meeting was held July 17-19, 2002, in Steamboat Springs, Colorado. Thirty-five people participated in the two-and-one-half days of oral presentations and poster sessions. SORCE is one element of NASA's Earth Observing System (EOS), and is scheduled for a December 2002 launch.

Tom Woods, the science meeting program chair, began with a welcome and introduction to the Working Group Meeting goals. Scientists studying the Sun and Earth's atmosphere and climate came together to explore the variable Sun and its influence on the terrestrial environment. By focusing on three time domains, the group worked to define our present understanding of solar and climate variations in three sessions as listed in Table 1.

Table 1. SORCE Science Working Group Sessions.

Time Scale
Solar Phenomena
Terrestrial Phenomena


July 17

minutes to ~2 years
  • eruptive events (flares)
  • oscillations
  • sunspots & active regions
  • solar rotation
  • tachocline oscillations
  • eruptive events (volcanoes)
  • diurnal variations
  • 27-day solar-driven events
  • seasonal variations


July 18

1 to 30 years
  • sunspot cycle
  • magnetic (dynamo) cycle
  • QBO
  • ocean circulation
  • 11-year solar-driven events
  • anthropogenic changes


July 19

>30 years
  • cycle-to-cycle variations
  • Maunder minimum
  • stellar evolution
  • droughts
  • ice ages
  • historical records (tree rings, ice cores, etc.)

The group wanted to explore the most important time periods for variations in the Sun and Earth systems. The shorter time periods will be an additional focus for the SORCE science team members, as they analyze the SORCE data.

Figure 1. Tom Woods, science program chair, started with a meeting overview.

Status of SORCE

Gary Rottman, SORCE Principle Investigator, provided an overview of the SORCE mission and updated everyone on the current status (download Gary Rottman's presentation in PDF format). Rottman began by quoting a primary recommendation made in a 1994 National Academy of Sciences Report regarding solar influences on global change. "One activity ranks above all others for determining solar influences on global change: Monitor the total and spectral solar irradiance from an uninterrupted series of spacecraft radiometers employing in-flight sensitivity tracking." This is the main objective for SORCE and it will be accomplished by collecting data from the four instruments onboard. This mission will continue the precise measurements of total solar irradiance (TSI) that began with the Nimbus-7 Earth Radiation Budget (ERB) instrument in 1978 and have continued to the present with the Earth Radiation Budget Satellite (ERBS), Active Cavity Radiometer Irradiance Monitor (ACRIM), and the Variability of Solar Irradiance and Gravity Oscillations (VIRGO) measurements, and it will also provide measurements of the solar spectral irradiance from 1 to 2000 nm.

The SORCE mission, designed for a 5-year duration, will: 1) establish a data set of Total Solar Irradiance (TSI), with an absolute accuracy better than 300 ppm (3s) and a relative accuracy between measurements of 0.001% (10 PPM) per year; 2) establish a data set of solar spectral irradiance from 1 NM to 2 mm with an absolute accuracy of 2% to 5% in the ultraviolet, and 0.1% in the visible to near infrared; and 3) improve understanding and generate new inquiry by studying how the variable solar irradiance affects our atmosphere and climate, and how and why variability occurs at the Sun. This knowledge can then be used to estimate past and future solar climate relations.

The Earth's radiation and energy balance is shown in Figure 2. Because of selective absorption and scattering processes in the Earth's atmosphere, different regions of the solar spectrum affect our environment in distinct ways. Approximately 20-25% of the TSI is absorbed by atmospheric water vapor, clouds, and ozone, impacting convection, cloud formation, and latent heating via processes that are strongly wavelength dependent. To understand the effects solar variability has on Earth's climate, it is important to accurately monitor both the TSI and its spectral dependence.

Figure 2. Radiation Balance of the Earth (Jeffrey T. Kiehl and Kevin Trenberth).

After reviewing basic radiometric quantities, why the solar irradiance varies, current TSI measurements, and concepts of electrical substitution radiometers, Rottman provided an overview of each of the four SORCE instruments. The Total Irradiance Monitor (TIM) measures TSI introducing modern, state-of-the-art technologies to the electrical substitution radiometer (ESR), and at the same time taking full advantage of the best heritage of previous radiometers. The Spectral Irradiance Monitor (SIM) instrument measures spectral irradiance from 200 to 2000 NM with a newly developed prism spectrometer, also including a miniaturized ESR. There are two Solar Stellar Irradiance Comparison Experiments (SOLSTICE) instruments on SORCE to measure spectral irradiance from 120 to 320 NM These instruments are an evolution and refinement of the Upper Atmosphere Research Satellite's (UARS) SOLSTICE, and they observe bright blue stars as a long-term calibration standard. The XUV Photometer System (XPS) instrument measures broadband spectral irradiance from 1 to 34 NM, and is designed similar to the XPS on the (Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics) TIMED satellite.

The Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado has full programmatic responsibility for the SORCE mission. Under subcontract with LASP, Orbital Sciences Corporation in Dulles, Virginia has provided a highly capable free flying, and low-risk spacecraft bus. After integration (see Figure 2), the launch will occur in late 2002 from the Kennedy Space Flight Center in Cape Canaveral, Florida on a Pegasus XL. LASP will operate SORCE from its Mission Operations Center in Boulder, Colorado for a period of 5 years or more. The LASP facility will process, analyze, validate, and distribute all irradiance data collected.

Figure 3. The SORCE spacecraft with the Solar Arrays deployed at Orbital during Integration and Test (I&T).

To summarize, Rottman emphasized how important this research and data collection is for the future EOS science. There is much to learn about the Earth's radiation and energy balance through continuous TSI measurements. Global energy balance considerations may not provide the entire story, and how TSI variations are distributed in wavelength is critically important in understanding the Earth's response to solar variations. Recalling the earlier National Academy of Sciences Report, Rottman stressed that it must be a priority to continue monitoring the total and spectral solar irradiance after the SORCE mission is complete, and that we should now be planning for future measurement programs.

Figure 4. SORCE Science Working Group Meeting attendees - Front row sitting left to right: Devendra Lal, Judith Lean, David Rind, Dominique Crommelynck, George Lawrence, Bob Cahalan, Jerry Harder; Second row standing left to right: Vanessa George, Judit Pap, Rashid Akmaev, Lon Hood, George Reid, Matt DeLand, Matthew (Geoff) McHarg, Guoyong Wen, Jeff Kuhn, Greg Kopp, Marty Snow; Back row left to right: Bill McClintock, Bill Ochs, Randy Meisner, Linton Floyd, Frank Eparvier, Jesper Schou, Jeffrey Hall, Claus Fröhlich, Peter Fox, Chris Pankratz, Tom Woods, Gary Rottman, Oran White, Tom Sparn.

Session I: Short-term Variations (Minutes to ~2 years)
Chair - Greg Kopp, LASP

Jesper Schou from Stanford University began Session I with a talk on Solar Changes from Helioseismology, which has become an invaluable tool for studying the solar interior and how it varies with time. He gave a brief overview of helioseismology, and then went on to describe some of the recent results, with an emphasis on temporal changes. In particular, the near surface changes and the so-called tachocline oscillations were discussed, and how the various changes relate to the solar dynamo. Schou also touched on the current ability to detect changes on the backside of the Sun and to predict short-term variability.

Figure 5. Left to right: Jesper Schou, Jeff Kuhn, and Judith Lean.

Jeffrey Kuhn from the Institute for Astronomy at the University of Hawaii explored the question, Solar Irradiance Variability: How well do we really understand how it works? While there are no consistent models of the solar cycle that account for the solar irradiance variability, there is much more information available about solar variability than magnetic field observations tell. Kuhn discussed aspects of a consistent physical model while exploring the implications of past and future variability observations.

Lon Hood from the University of Arizona spoke on the Effects of 27-Day Solar Ultraviolet Variations on the Stratosphere. Solar ultraviolet radiation at wavelengths near 200 NM photodissociates molecular oxygen, leading to the formation of ozone in the upper stratosphere. Although a trace constituent (parts in 106), ozone is important both for radiative heating of the stratosphere and for shielding the biosphere from biologically harmful solar UV radiation at wavelengths in the 250 to 300 NM range. Under solar maximum conditions, solar UV variations near 200 NM are as large as 6% on the 27-day time scale. Measured ozone responses were as large as ~3% in the tropics near 40 km altitude. Current work focuses on observational and theoretical studies of mechanisms by which 27-day solar UV variations may produce secondary dynamical effects in the tropical and extra-tropical stratosphere.

Tom Woods, with LASP at the University of Colorado in Boulder, reviewed the Early Results of the Solar Extreme Ultraviolet Irradiances from the TIMED Solar EUV Experiment. The Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite was launched in December 2001 and normal operations began on January 22, 2002. The Solar EUV Experiment (SEE) onboard TIMED measures the solar spectral irradiance from 0.1 NM to 195 NM This presentation focused on the 27-day variations, caused by the solar rotation, and the large X class flare on April 21, 2002 (download Tom Wood's presentation in PDF format).

Matthew DeLand came from Science Systems and Applications, Inc. (SSAI) presented a talk on the Short-term Solar Irradiance and Mg II Index Variations. Previous work has shown a close correlation between ozone abundance at 30-50 km and 205 NM irradiance variations on solar rotational time scales. The Mg II index of solar UV activity has often been used as a proxy for 205 NM irradiance. Excellent correlations are typically observed for solar cycle (~11-year) and rotational (~27-day) time scales. On some occasions, 13-day periodicity is present in 205 NM irradiance data, but is weaker or absent in coincident Mg II index data. DeLand discussed these variations and the potential impact for modeling of solar irradiance variations.

Session II: Medium-Term Variations (1 to 30 years)
Chair - Tom Woods, LASP

Claus Fröhlich traveled from the World Radiation Center in Davos, Switzerland to present a talk on Solar Irradiance Variability. Fröhlich discussed the TSI measurements that are available from space beginning in 1978 and extending over a time period of more than 23 years. From measurements made by different space radiometers (HF on NIMBUS~7, ACRIM~I on SMM, ACRIM~II on UARS, and VIRGO on SOHO) a composite record of TSI has been compiled. This time series is compared to an empirical model based on sunspot darkening and brightening due to faculae and network (download Claus Fröhlich's presentation in PDF format).

Oran R. White from the High Altitude Observatory at the National Center for Atmospheric Research presented A Current Picture of Irradiance Measurements. White reviewed the solar irradiance measurements from 1978 to 2002 that give the solar cycle variability for three solar maxima and two minima. He summarized these data with emphasis on the most recent measurements in solar cycle 23. This provides a basis for estimating values of the solar irradiances to be obtained with SORCE's TIM and SIM instruments. When SORCE launches, solar cycle 23 should be past its maximum and into its declining phase. The next critical base-line measurement will be at solar minimum in about 2007 (download Oran White's presentation in PDF format).

Robert Cahalan from the NASA Goddard Space Flight Center in Greenbelt, Maryland, spoke on Earth, the Water Planet. Cahalan, NASA's SORCE Project Scientist, began with a discussion on recent multi-year and multi-decadal changes in Earth's climate, emphasizing the dominant energy control mechanisms of the 3 phases of water. Regional ocean basins and land masses are associated with characteristic cloud types that mainly cool the climate system, but are likely to produce less cooling in a globally warmed climate. Fresh water, stored primarily in polar ice, and secondarily in large subsurface aquifers, exhibits decadal trends that are largely, but not completely, consistent with global warming. Cahalan concluded by summarizing modes of natural variability such as El Niño - Southern Oscillation (ENSO) and it's cousins, contrasting these with anthropogenic forcing due to greenhouse gases and aerosols (download Bob Cahalan's presentation in PDF format).

Figure 6. Claus Fröhlich and Bob Cahalan.

Rashid A. Akmaev with the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado and the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, gave a talk on Upper-Atmospheric Structure Response to CO2 Increases. The atmospheric response to CO2 increases in recent decades has been studied with the Spectral Mesosphere/Lower Thermosphere Model. The model predicts colder temperatures in the thermosphere with an increase in CO2 and the thermospheric response is much stronger than in the lower atmosphere (download Rashid Akmaev's presentation in PDF format).

Matt DeLand from Science Systems and Applications, Inc. (SSAI) in Lanham, Maryland, presented the SBUV Observations of PMCs over Two Solar Cycles. The appearance of cloud-like structures at high altitudes during local summer at polar latitudes has been documented for more than 100 years. These phenomena are called noctilucent clouds (NLC) by ground-based observers, and polar mesospheric clouds (PMC) when observed from space. DeLand presented PMC results from five Solar Backscatter Ultraviolet (SBUV) and SBUV/2 instruments covering 23+ years (1978-2002). The overlapping data sets from nearly identical instruments give an accurate picture of long-term variations. PMC occurrence frequency is anti-correlated with solar Lyman a irradiance.

Linton Floyd from Interferometrics, Inc. and the Naval Research Laboratory reviewed the Measurements of Solar UV Irradiance over Solar Cycle Time Scales. The experience and prospects for long-term solar UV irradiance measurements are explored in the context of the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) experiment aboard the Upper Atmosphere Research Satellite (UARS). SUSIM has measured the solar UV irradiance (115-410 NM) for nearly 11 years, about the duration of a solar sunspot cycle. Like its SOLSTICE counterpart on UARS, SUSIM is able to measure changes in its own responsivity. This is made possible through analysis of measurements of the UV output of 4 stable deuterium lamps and by solar and lamp measurements using redundant optical elements. Floyd reviewed the correspondence of solar UV irradiance time series in various wavelength ranges to that of the MgII index.

Judit Pap from the Goddard Earth Sciences and Technology Center and the University of Maryland, Baltimore County, presented the group with a talk on Solar Magnetic Activity and Irradiance Variations: Agreements and Discrepancies. Total solar irradiance and UV irradiances have been measured from space for almost three decades. Correlative studies indicate that a major portion of irradiance variations is explained by the Sun's surface magnetic activity features. However, there is growing evidence that empirical models based on sunspots, faculae and the magnetic network cannot explain all aspects of the observed irradiance changes, especially during solar cycle 23. These results underscore the necessity of continuous space irradiance monitoring and improvement of irradiance models used for climate studies.

Figure 7. Oran White and Jerry Harder.

Session III: Long-term Variations (>30 years)
Chair - Jerry Harder, LASP

Jeffrey Hall came from the Lowell Observatory in Flagstaff, Arizona, to share information on Long-term Echelle Spectroscopy of the Sun and Solar Analogs. In the 40 years since Olin Wilson began long-term monitoring of Sun-like stars, stellar activity cycles research has been presented with increasing attention to its relevance to analogous observations of the Sun. Hall reviewed the important data sets, in particular the Mount Wilson Observatory time series that span 30 years or more, and presented what is currently known about variations of Sun-like stars on 30-year timescales. Dr. Hall discussed the current best candidate solar "twin", the G2 star 18 Sco (or HR6060) which not only closely matches the Sun in mass, luminosity, and spin, but also has a magnetic activity cycle apparently in phase with the Sun (download Jeffrey Hall's presentation in PDF format).

Judith Lean from the E. O. Hulburt Center for Space Research at the Naval Research Laboratory, reported on the Relationships between the Sun and Climate on Time Scales of Decades to Centuries. There are numerous empirical associations between solar variability and climate change throughout the Holocene, on multiple time scales. Recent examples include surface temperatures in the past 1000 years, solar-like 22- and 210-year cycles in drought, and 1500-year solar-related fluctuations in North Atlantic ice drifts. The nature of long-term solar forcing of climate remains ambiguous. Also uncertain are the geophysical mechanisms responsible for climate response to solar forcing. Speculated processes include direct heating of land and ocean surfaces by changes in visible and near infrared solar radiation and indirect influences of solar UV radiation on ozone concentrations and stratospheric heating, with subsequent radiative and dynamical coupling to climate. Lean summarized recent Sun-climate relationships and possible mechanisms on time scales of decades to centuries, and described current understanding of long-term solar irradiance changes, and how they may - or may not - relate to cosmogenic isotope proxies (download Judith Lean's presentation in PDF format).

David Rind from the Goddard Institute for Space Studies (GISS) in New York addressed the question, Was there a Maunder Minimum cooling, and should there have been? Some controversy has arisen over the actual magnitude of cooling during the Maunder Minimum (MM) time period (approximately 1650-1715). A recent reconstruction indicated that the global cooling was only about 0.6°C, actually less than during the late 1800s, primarily due to a muted tropical response. Climate model simulations for this time period are reviewed, comparing the importance of solar, volcanic, and trace gas plus tropospheric aerosol forcing. The results show that anthropogenic effects by themselves should have produced close to 1°C cooling for the Little Ice Age in general (including the MM). Reconstructions of volcanic aerosols imply a potentially large contribution to the MM. The solar contribution has been estimated as anywhere between <0.1% to ~0.4% change. Whether large cooling did occur in the MM, and whether it should have, remain open questions.

Devendra Lal from the Scripps Institution of Oceanography, Geosciences Research Division, La Jolla, California shared information on Recent Lessons in Solar Physics Based on Studies of Surface Lunar Soils and Ice Cores from Summit, Greenland. It is now well recognized that solar phenomena produce appreciable changes in the radiations we receive in the interplanetary space and on the Earth. Professor Lal discussed the composition of the solar wind on radionuclides in lunar soil and ice cores. Lal reviewed the experimental methods and the reasons which lead us to the two conclusions which suggest that, (1) the solar wind contains relatively short lived radionuclides which must be concurrently produced in the outer photosphere or coronal regions, and (2) Sun could exhibit rather long periods (~1000 years) of very low solar-activity, and therefore very weak solar modulation of the galactic cosmic ray flux, mimicking the historically recorded 70 years duration (solar) Maunder Minimum epoch during 1645-1715.

Matthew (Geoff) McHarg from the U.S. Air Force Academy in Colorado Springs wrapped-up Session III by reporting on Long-term Variations in Ionospheric Joule Heating in Response to the Interplanetary Magnetic Field. Joule heating is caused by currents flowing in the Earths ionosphere, and is one of the three major sources of thermospheric heating. McHarg reported on a study of the effects of the Interplanetary Magnetic Field (IMF) on ionospheric joule heating, and the long-term variations in the joule heating as derived from a proxy magnetic index. IMF causes a generally linear increase in integrated joule heating over the polar cap, with greater heating for southerly IMF conditions.

Poster Session

Natural Variability of the Atmosphere: Limitation of Ground-based Estimates of Solar Irradiance, by Guoyong Wen from the Joint Center for Earth Systems Technology [JCET]/UMBC, and NASA's Goddard Space Flight Center.

The San Fernando Observatory Program of Full-Disk Photometry, by Gary Chapman with the San Fernando Observatory, California State University in Northridge.

Variations in Solar EUV Flux as Measured by SOHO / CELIAS / SEM, by Judit Pap at Goddard Earth Sciences and Technology Center and the University of Maryland.

Solar Irradiance Variability - Comparison of Models and Observations, by Peter Fox with the High Altitude Observatory at the National Center for Atmospheric Research in Boulder, Colorado. (Download Peter Fox's poster in PDF format).

Stellar Observations with the UARS SOLSTICE, by Marty Snow with the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado. (Download Marty Snow's poster in PDF format).

SORCE Data Processing and Distribution, by Chris Pankratz with LASP at the University of Colorado. (Download Chris Pankratz's poster in PDF format).

Total Irradiance Monitor (TIM) Instrument, by Greg Kopp with LASP at the University of Colorado. (Download Greg Kopp's poster in PDF format).

Spectral Irradiance Monitor (SIM) Instrument,, by Jerry Harder with LASP at the University of Colorado.

XUV Photometer System (XPS) Instrument, by Tom Woods with LASP at the University of Colorado. (Download Tom Wood's poster in PDF format).

Solar Stellar Irradiance Comparison Experiment (SOLSTICE) Instrument, by William McClintock with LASP at the University of Colorado. (Download Bill McClintock's poster in PDF format).

Conclusions - Connections to SORCE

Tom Woods ended the SORCE Working Group Meeting by summarizing the relevance of the discussions to SORCE. There are three aspects of the discussions during this meeting that are connected to the SORCE program: validation of the solar irradiance time series, solar irradiance modeling, and atmosphere / climate modeling. A summary of these connections as grouped by the SORCE instruments is given in Table 2. The EUV region, which is not measured by the SORCE instruments and thus not listed in Table 2, will be measured during the SORCE mission by the TIMED Solar EUV Experiment (SEE).