angie.cookson@csun.edu; dora.preminger@csun.edu;
gary.chapman@csun.edu
San Fernando Observatory (SFO), Cal State Northridge
18111 Nordhoff St, Northridge, CA 91330-8268
This work is supported in part by NSF grant ATM0848518
Choosing a TSI composite for examining solar minima: re­
visiting a time period that could make a difference
Angie Cookson, Dora Preminger, and Gary Chapman
San Fernando Observatory
California State University, Northridge
Abstract
Total Solar Irradiance (TSI) composites are based on satellite measurements beginning in 1978 to the present. These composites, extrapolated backward in time, allow for TSI reconstructions spanning several hundred years; the results should give insight into the Sun's behavior over long time periods, including several solar minima, and help inform today's climate models. Currently, three TSI composites dominate: PMOD, ACRIM, and IRMB. The methodologies applied to the satellite data that are used in each TSI reconstruction determine the TSI differences and affect every study that uses them. Which composite to use? Several in­depth explanations of these methodologies are available in the literature, e.g., Scafetta (2009), not be repeated here. However, we re­examine one of the more controversial periods – September 1989 – in order that the end­
user may have additional information on which to base her/his choice of TSI composite. The choice of composite could make a difference in our representation and understanding of solar minima.
Objective
NOT TO rehash what we know about the development of each TSI composite. See the literature, e.g., Scafetta (2009), Chapman, et al. (1996), Lee, et al. (1995), and others.
TO use two sometimes over­looked documents to shed light on the controversial Nimbus­7/ERB satellite measurements of September 1989, the handling of which informs each TSI composite and the subsequent understanding of solar cycle TSI behavior, including the latest minimum.
Documentation
NASA Reference Publication 1316, Nimbus­7 Earth Radiation Budget Calibration History – Part I: The Solar Channels, 1993 ­­ a final report on the instrument calibration and anomalies, analysis, corrections to data, etc., for the Nimbus­7/ERB satellite, upon which a good portion of today's TSI composites are based.
Orbital Anomalies in Goddard Spacecraft for CY 1989 – a record of all instrument anomalies occurring on all Goddard spacecraft during 1989, including Nimbus­7.
The September 1989 problem
From 25­27 September 1989, the channel 10c radiometer was turned off because the output signal was saturated (Lee, et al., 1995, Kyle, et al., 1993). After returning to normal operating status on 27 Sep 1989 the irradiance appeared to increase abruptly by 0.4 Wm­2. (Subsequent analysis using ground­based photometry (Chapman, et al, 1996) and other methods (Scafetta, 2009) attempted to refine this increase.)
The jump in the data can be seen below, but the question of whether or not it is a real increase in the TSI or an artifact of an instrumental problem remains. No matter what other methodologies are employed to create a given composite, this underlying real/instrument question makes a difference in the outcome.
Mid­September through Mid­October 1989 Nimbus­
7/ERB TSI
19890916 1372.26
19890917 1372.30
19890918 1372.30
19890919 1371.87
19890920 1371.91
19890921 1371.73
19890922 1371.59
19890923 1371.62
19890924 1371.98
19890925 ­­­­­­­­­­
19890926 ­­­­­­­­­­ 19890927 ­­­­­­­­­­ 19890928 1372.76
19890929 1373.07
19890930 1372.99
19891001 1372.79
19891002 1372.80
19891003 1373.01
19891004 1373.27
19891005 1373.39
19891006 1373.37
19891007 1373.20
19891008 1373.34
19891009 1373.27
19891010 1373.14
19891011 1373.07
19891012 1373.27
19891013 1372.91
19891014 1372.55
19891015 1372.34
Could a Nimbus­7/ERB instrument anomaly cause a jump in irradiance values?
It happened several times prior to 1989. Abrupt shifts were noted, analyzed, and corrected for in the data set after comparison with SMM, ACRIM, and ERBS solar irradiance. The size of the increases was about the same as the September 1989 increases.
One shift, an apparent calibration change on 26 Sep 1987, resulted in an “upward step function­like discontinuity in irradiance” which translated to an increase of 0.4 Wm­2 that persisted after that date (Hoyt, et al., 1992). Hoyt, et al., believe it was caused by a change in one of the electronic components. The data set was corrected.
Also, two “special operations” periods were identified (Hoyt, et al., 1992) during which the radiometer was seldom in a stable condition, the solar ­2
irradiance values were offset upward by 2.5 Wm
and 0.2 Wm­2 respectively. The data set was corrected.
Spacecraft Instrument Anomalies during the September 1989 Period
Orbital Anomalies in Goddard Spacecraft for CY 1989
page 7: “There were only minor anomalies reported in 1989. Three of them involved the ERB Instrument. One of the anomalies involved temporary loss of the entire instrument. In September there was a failure in the analog multiplexer/digital converter which suspended operation. However four or five days later it started working and has been ok ever since.”
page 20: “9/24/89 (3988) INST­ERB 2 Failure in analog multiplexer/digital converter. ERB use temporarily lost. A01242”
Investigation of Instrument Anomaly
The Nimbus­7 Earth Radiation Budget Calibration History, published by NASA in 1993, shows extensive investigation of the period 1978­1988, including instrument anomalies that affected solar measurements. After 1988, the Nimbus team investigated and dismissed the 25­27 Sep 1989 instrument shutdown as having no effect on the irradiance measurements. Appendix C­5 states that this electronics problem was “short­lived, and left no apparent after affects. The electronics problems were probably caused by energetic particles damaging solid­state components in the electronic circuits. In both September 1989 and January 1992, the instrument was then turned off for a day or so and this apparently allowed the damaged component to heal itself.”
Investigation, continued
The period 1989­1992 does not appear to have been analyzed as thoroughly as the 1978­1988 period, but this may be deceptive since SMM and ACRIM were no longer available for comparison, leaving only ERBS. Some consider the ERBS measurements to be flawed since there were so few of them (approx every 2 weeks) and undetected sensor degradation was also a possibility. The Nimbus­7 report, page 32, reads: “Quantitatively, the Nimbus­7 and SMM comparison indicates that the Nimbus measured trends may be accurate to +/­ 0.07 Wm­2 for the period 1980 through 1988. We know of no reasons why the Nimbus values should be considered less accurate in its other years (1979 and 1989­1992). However, it would be useful to know the cause of the 0.45 Wm­2 jump in the (Nimbus­7 minus ERBS) mean difference from 1989 to 1990.”
[Bolded sentence mine.]
Investigation, continued
The Nimbus­7/ERB experimental team found no evidence for a change in radiometer sensitivity nor any internal evidence to think the increase wasn't real (personal communication with Hoyt in Scafetta and Willson [2009]).
Lee, et al., (1995), referencing Kyle, et al., (1993), believe the increase was instrumental and amounted to approximately 0.4 Wm­2. Chapman, et al., (1996) used a series of multiple regressions based on Nimbus­7/ERB TSI against ground­based CFDT1 (San Fernando Observatory) 512x512 photometric images in the red continuum (672.3 nm) and the Ca II K­line (393.4 m) to refine the increases seen by Lee, supporting the idea of an instrumental problem. Some researchers believe this is a meaningful exercise; others do not.
Conclusion
Know as much as possible about the origins of the TSI composite you choose to use, including instrument and spacecraft anomalies.
Apart from whatever corrections each TSI composite applies to the satellite data, based on the compositor's documented methodologies, the 25­27 September 1989 Nimbus­7/ERB period remains controversial.
In an attempt to fully understand and characterize the Nimbus­7/ERB irradiance increase after 27 Sep 1989, we present the spacecraft history for that time period. We find that an anomaly with the analog multiplexer/digital converter caused the shutdown of the instrument on those dates. After operations were resumed, there was an apparent increase in irradiance measurements that not all researchers agree is real.
Conclusion, continued
We at the San Fernando Observatory used a series of multiple regressions on the our CFDT1 512x512 photometric images in the continuum (672.3nm) and the Ca II K­line (393.4nm) to help refine these jumps. The results have been applied to the PMOD TSI composite but not the ACRIM TSI. Long­term data sets are essential to understanding solar behavior, including past and present solar minima. TSI composites are used not just to interpret the period for which satellite measurements are available, but also, combined with other parameters such as sunspots, to extrapolate backwards in time, in order to help understand the present. The composite we use is important; we should understand it. References
Chapman G. A., A. M. Cookson and J. J. Dobias, 1996, “Variations in total solar irradiance during solar cycle 22”, J Geophys Res, v, 101, 13541­13548.
Elsen, William G., Orbital Anomalies in Goddard Spacecraft for CY 1989, klabs.org/richcontent/Reports/NASA_Reports/gsfc_anomalies/oags89/pdf
Kyle H. L., D. V. Hoyt, J. R. Hickey, R. H. Maschhoff, and B. J. Vallette, Nimbu2­7 Earth radiation budget calibration history, I. The solar channels, Rep. RP­1316, 82pp., NASA Goddard Space Flight Cent., Greenbelt, Md., 1993.
Lee III R. B., M. A. Gibson, R. S. Wilson, and S. Thomas, “Long­term total solar irradiance variability during sunspot cycle 22,” J Geophys Res, 100(A2), 1667­1675, 1995.
Scafetta, N., “Total solar irradiance satellite composites and their phenomenological effect on climate”, Geological Society of America conference session, 2009.