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Total Solar Irradiance Data

Total Irradiance Monitor Status

(updated 10 Apr. 2014)

TIM daily solar measurements resumed on 24 Feb. 2014 in a new operations mode. Daily TSI data from this new mode are available beginning on 5 March, after which time most of the pointing issues with the new operations mode were addressed.

Dec. 2013 Campaign: The TIM, along with all other SORCE instruments, ceased collecting solar measurements after a battery cell failure on 30 July 2013. They were turned back on for a 1-week campaign to overlap with the newly commissioned TCTE in late Dec. 2013. During this campaign, enabled by very short spacecraft eclipse durations which allowed continual operation of the spacecraft’s main on-board computer, normal operations resumed and the TIM demonstrated full functionality and good thermal stability.

New Operations Mode: On 24 Feb. 2014 the LASP SORCE Operations Team implemented a new means of operating the spacecraft to acquire continued TSI measurements in the present power-limited mode. Time-dependent uncertainties in the released data files reflect the lower quality of these measurements compared to those of the primary mission phase due to this mode’s intermittent and limited duration observations subsampling short-period solar variations, large thermal fluctuations in the instrument (and a lack of “dark,” or thermal background, measurements), and spacecraft pointing issues. Despite these issues, the SORCE/TIM TSI measurements in this new mode are expected to continue into the foreseeable future to help extend the 35-year spaceborne TSI data record.

Daily measurements are updated daily about 7 days after acquisition, as during the primary phase of the mission, although 6-hourly values may not be available as regularly as they were before.

Available TSI Data Summary Table/Data Access

Full mission downloadable data files updated nightly.

Instrument
Time Cadence
Spectral
Coverage
Spectral
Resolution
Level
Version
Data
Info
Full Mission Download
Interactive Access
File Readers
(Listed by Language)
TIM Daily All N/A 3 16
514 KB
TIM 6-Hourly All N/A 3 16
2.03 MB
LISIRD
– text file – Interactive

Reading SORCE TSI Data Product Files

If you use the IDL programming environment, the IDL routine read_lasp_ascii_file.pro may be used to read the above text files into IDL.

SORCE TSI Plots (Updated Daily)

Plots of the spaceborne TSI record and other TSI science nuggets are updated monthly here.

See more SORCE TSI plots.

Historical TSI Reconstruction

 

This historical reconstruction of TSI is a model that needs to be credited to other researchers as it has limited direct contributions from SORCE measurements. These estimated solar irradiances for the last 400 years are based on an historical TSI reconstruction by N. Krivova et al. (see references below), which is used in the IPCC AR5 Working Group I’s Assessment Report. The values from this group’s SATIRE model have been offset a small amount (-0.30 W/m2) for agreement with SORCE/TIM values and replaced by SORCE/TIM annual averages from 2003 onward. The historical reconstruction provided here was computed using TIM V.15 data in February 2014. It is updated annually as new TIM data are available or as improved historical reconstructions are created.

References

1) N.A. Krivova, L.E.A. Vieira, S.K. Solanki (2010).
Journal of Geophysical Research: Space Physics, Volume 115, Issue A12, CiteID A12112. DOI:10.1029/2010JA015431

2) Ball, W. T.; Unruh, Y. C.; Krivova, N. A.; Solanki, S.; Wenzler, T.; Mortlock, D. J.; Jaffe, A. H. (2012).
Astronomy & Astrophysics, 541, id.A27. DOI:10.1051/0004-6361/201118702

3) K. L. Yeo, N. A. Krivova, S. K. Solanki, and K. H. Glassmeier.
Astronomy & Astrophysics, “Reconstruction of total and spectral solar irradiance since 1974 based on KPVT, SoHO/MDI and SDO/HMI observations,” 2014, submitted

Download the ASCII data file

Explore the data interactively with LISIRD

Data Product Description

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 to an absolute accuracy of 350 parts per million (ppm, 1 ppm=0.0001%) (1-sigma) and a precision and long-term relative accuracy of 10 ppm per year. The standard Level 3 TSI data products produced by the SORCE program consist of daily and 6-hourly average irradiances, reported at a mean solar distance of 1 astronomical unit (AU) and zero relative line-of-sight velocity with respect to the Sun. Two TSI data products are produced – one containing the daily means and the other containing the four 6-hourly means. Due to the small size (< 300 KB) of the daily data and to maximize ease of use to end-users, each delivered TSI product contains science results for the entire mission. Updates to Level 3 TSI data occur daily. Lower-level data products (e.g. Level 2) are primarily considered diagnostic with limited scientific value, and are therefore not delivered to the GES DISC. These data are, however, available upon request from the SORCE science team.

Data Quality Description

On-orbit instrument characterization is an on-going effort, as the TIM team regularly tracks instrument degradation and calibrates the instrument servo system on-orbit, periodically updating the data processing system with new calibration values. Only minor corrections are anticipated at this phase in the SORCE/TIM mission. To date the TIM is proving very stable with usage and solar exposure, and long-term relative uncertainties are estimated to be less than 0.014 W/m2/yr (10 ppm/yr). Present absolute accuracy is estimated to be 0.48 W/m^2 (350 ppm), largely determined by the agreement between all four TIM radiometers. The 4.5 W/m^2 by which the TIM reads lower than prior instruments has been resolved as being largely due to internal instrument scatter in those prior instruments causing erroneously high readings (see Kopp & Lean, GRL, 38, L01706, 2011).

Measurement Objectives

The primary objective of the SORCE Total Irradiance Monitor (TIM) instrument is to make precise and accurate measurements of Total Solar Irradiance (TSI), adding to previous TSI measurements in order to continue the long-term climate record. Once on-orbit instrument characterization is complete, these TSI measurements will be provided with a relative standard uncertainty (absolute accuracy) of approximately 0.01% (100 parts per million, ppm) based on SI units and with a long-term precision (relative accuracy) of 0.001%/yr (10 ppm).

Data Product Summary

The Total Irradiance Monitor (TIM) measures the total solar irradiance (TSI), a measure of the absolute intensity of solar radiation, integrated over the entire solar irradiance spectrum. To construct this product, high time cadence measurements (approximately every 50 seconds during sunlit portions of the SORCE spacecraft orbit) from the TIM instrument are combined to produce representative daily and 6-hourly values of the Total Solar Irradiance. Four TIM channels track on-orbit degradation in the primary channel and additionally provide a limited degree of redundancy. Shuttered operation of the instrument corrects for thermal background, and state-of-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 cadence of 50 seconds. 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 SORCE TSI data products that are released to the GES DISC. The TSI value and the time stamp reported are means of the TSI measurements acquired during the interval.

Research and Applications

Measurements of total solar irradiance (TSI) are known to be linked to Earth climate and temperature. Proxies of the TSI based on sunspot observations, tree ring records, ice cores, and cosmogenic isotopes have given estimates of the solar influence on the Earth that extend back thousands of years, and correlate with major climatic events on the Earth. These estimates extrapolate many recent detailed observations to long-term observations of fewer (or even one) measurement. For example, accurate TSI measurements from the last 25 years are correlated with solar measurements of sunspots and faculae; these correlations can then be used to extrapolate the TSI to time periods prior to accurate space-borne measurements, since the solar records extend back 100 years for faculae and 400 years for sunspots. Over this extended time range, the extrapolated TSI record can be compared with longer term records, such as tree rings or ice cores, and correlation with these allows extension of the estimated TSI to more distant times, albeit with decreasing certainty. This extrapolation is important for understanding the relationship between TSI and the Earth’s climate; yet the extrapolation begins with the comparison of solar surface features to accurate TSI measurements, a record which is currently only 25 years long.

Data Set Evolution

Attempts to measure the TSI began in earnest in the 1830’s, with independent measurements by Claude Pouillet and John Herschel, yet were nearly a factor of two low because of atmospheric absorption. Even balloon-borne measurements in the 1900’s lacked the instrumental accuracy to detect the ~0.1% changes in the TSI. It was not until long-duration measurements from space were available that changes in TSI were accurately measured and the misconception of a “solar constant” changed. Total Solar Irradiance (TSI) monitoring using electrical substitution radiometers (ESRs) from the vantage point of space began with the launch of the Nimbus 7 satellite in November 1978. This was soon followed by an Active Cavity Radiometer Irradiance Monitor (ACRIM) instrument on the Solar Maximum Mission and by the Earth Radiation Budget Experiment (ERBE). More recently, second and third ACRIM instruments have been launched, in addition to the launch of two instruments on the NASA/ESA Solar and Heliospheric Observatory (SOHO). The various data sets are in basic agreement and show conclusively that variations of TSI track the passage of sunspots across the solar disk with an amplitude of about 0.2%, and that long-term solar cycle variations are only on the order of 0.1%. The SORCE TSI data set continues these important observations with improved accuracy on the order of ±0.01%.

The ERBS, ACRIM-III, and VIRGO continue to make observations. Willson [1997] combined the ACRIM-I and ACRIM-II data sets using their overlap with the ERB data, and his analysis suggests a net increase of solar radiation between solar minima in 1986 and 1996. (Note: only two solar minima have actually been observed thus far). The estimated increase of 0.04% would induce appreciable climate change if it persists for a sufficient number of solar cycles and if the climate system feedbacks reached their full equilibrium response to the forcing.

Instrument Description

For a description of the TIM instrument go to the SORCE TIM overview page.

Scientific Contact

Dr. Greg Kopp, LASP, CU
(303) 735-0934
Email: (use firstname.lastname@lasp.colorado.edu)

References

  • Kopp, G. and Lean, J.L., A New, Lower Value of Total Solar Irradiance: Evidence and Climate Significance, Geophys. Res. Letters Frontier article, Vol. 38, L01706, doi:10.1029/2010GL045777, 2011.
  • Kopp, G., Heuerman, K., Harber, D., and Drake, V., The TSI Radiometer Facility – Absolute Calibrations for Total Solar Irradiance Instruments, SPIE Proc. 6677-09, 26-28 Aug. 2007.
  • Kopp, Greg, George Lawrence, and Gary Rottman. The Total Irradiance Monitor (TIM): Science Results. Submitted to Solar Physics 2005.
  • Kopp, Greg, and George Lawrence. The Total Irradiance Monitor (TIM): Instrument Design. Submitted to Solar Physics 2005.
  • Kopp, Greg, Karl Heurerman, George Lawrence. The Total Irradiance Monitor (TIM): Instrument Calibration. Submitted to Solar Physics 2005.
  • Kopp, G., G. Lawrence, and G. Rottman. Total Irradiance Monitor Design and On-Orbit Functionality, SPIE Proc. 5171-4, 2003.
  • Lawrence, G.M., G. Kopp, G. Rottman, J. Harder, T. Woods, and H. Loui. Calibration of the Total Irradiance Monitor. Metrologia 40, 2003, S78-S80.
  • Lawrence, G. M., G. Rottman, G. Kopp, J. Harder, W. McClintock,and T. Woods. The Total Irradiance Monitor (TIM) for the EOS SORCE Mission. SPIE Proceedings, 4135, 215-224, 2000.
  • Lawrence, G. M., G. Rottman, J. Harder, and T. Woods. Solar Total Irradiance Monitor: TIM. Metrologia, 37, 407-410, 2000.
  • Woods, Tom, Gary Rottman, Jerry Harder, George Lawrence, Bill McClintock,Greg Kopp, and Chris Pankratz. Overview of the EOS SORCE Mission. SPIE Proceedings, 4135, 192-203, 2000.
  • Willson, R.C. and R.S. Helizon. SPIE Proceedings (Earth Observing Systems IV Conference, Denver, CO), 3750, 233-242, 1999.
  • Fröhlich C., B. N. Anderson, T. Appourchaux, G. Berthomieu, D. A. Crommelynck, V. Domingo, A. Fichot, M. F. Finsterle, M. F. Gómez, D. Gough, A. Jiménez, T. Leifsen, M. Lombaerts, J. M. Pap, J. Provost, T. Roca Cortés, J. Romero, H. Roth, T. Sekii, U. TellJohann, T. Toutain, and C. Wehrli. The First Results from SOHO (Edited by B. Fleck and Z. Svestka), Dordrecht/Boston/London, Kluwer Academic Publishers, 267-86, 1997; see also Solar Phys., 170, 175, 1997.
  • Willson, R. C. Total Solar Irradiance Trend in Solar Cycles 21 and 22. Science, 277, 1963-1965, 1997.
  • Fröhlich, Claus. First Results from the VIRGO Experiment. Transactions AGU, Spring Meeting, 1996.
  • Lee, R. B., M. A. Gibson, R. S. Wilson, and S. Thomas. Long-term total solar irradiance variability during sunspot cycle 22. J. Geophys. Res., 100, 1667-1675, 1995.
  • Fröhlich, Claus. The Sun as a Variable Star. Eds.: J. Pap, C. Fröhlich, H. Hudson, and S. Solanki, Cambridge University Press, New York, 28-36, 1994.
  • Willson, R. C. Irradiance observations of SMM, Spacelab-1, UARS, and ATLAS Experiments. The Sun as a Variable Star. Eds.: J. Pap, C. Fröhlich, H. Hudson, and S. Solanki, Cambridge University Press, New York, 54-62, 1994.
  • Kyle, H.L., D.V. Hoyt, J.R. Hickey, R.H. Maschoff, and G.J. Vallette. Nimbus-7 Earth Radiation Budget Calibration History. Part 1: The Solar Channels. NASA Reference Publication 1316, 1993.