Total Solar Irradiance Results from the Upper Atmosphere Research Satellite Active Cavity Radiometer Irradiance Monitor II Experiment OCTOBER 1991 THROUGH DECEMBER 1997 Contacts: Dr. Richard C. Willson Address: Center for Climate Systems Research Columbia University 2845 Windfall Ave., Altadena, CA, 91001 Phone: 818-398-9803 Fax: 818-398-6334 E-mail: HYPERLINK mailto:acrim@primenet.com acrim@primenet.com The second Active Cavity Radiometer Irradiance Monitor experiment (ACRIM II) was launched in September 1991 as part of the science payload of the Upper Atmosphere Research Satellite (UARS). The UARS/ACRIM II results are shown in detail in Figure 1. The variations on solar rotational and active region time scales are clearly seen. The large, short-term decreases are caused by the TSI blocking effect of sunspots in magnetically active regions as they rotate through our view from Earth. The peaks of TSI preceding and following these sunspot are caused by the faculae of solar active regions whose larger areal extent causes them to be seen first as the region rotates onto our side of the sun and last as they rotate over the opposite solar limb. The downward trend through the 1991-1996 period is similar in slope and amplitude to that observed by ACRIM I during the declining activity phase of solar cycle 21. (From the peak of solar cycle 21 to its minimum the TSI decreased by about 0.08 %.) The ACRIM II results through 1997 demonstrate a TSI minimum in early 1996, a period with high variability due to solar magnetic activity between early 1996 and early 1997, and increasing TSI beginning in early 1997 leading to the maximum of solar cycle 23. The results of successive Active Cavity Radiometer Irradiance Monitor (ACRIM) experiments have been related with sufficient precision to resolve a multi-decadal, upward trend in total solar irradiance of 0.036 percent per decade between the minima of solar cycles 21 and 22. (Willson, R.C., Science, v.277, pp 1963-65) The tabulated results of the ACRIM II experiment are attached. Four columns of data are included: Epoch 1980 Day Count, Year, Mean Daily TSI (W/M2) and Uncertainty (W/M2). TSI is reported on ACRIM II native scale defined by operation of sensor, the full-time monitoring sensor. Results are reconciled to 1 A.U. and are fully corrected for sensor degradation. Gaps that may exist in the temporal coverage. Total number of days of observation: 2043 Total number of mission days since activation: 2325 Total number of missing days : 207 Number of days without solar observing opportunity: 170 Number of days with inadequate data: 39 Validation Considerations/Procedure The experience of the UARS/ACRIM II science team with the SMM/ACRIM I and shuttle ACRIM experiments demonstrated repeatedly that two identical days of data are never acquired. Validation is performed on 100 % of the raw data produced by UARS/ACRIM II to guarantee results that will be useful for the most demanding applications of the TSI database. 1. Validation of Level 0 data The level 0 data will be scrutinized for consistency with anticipated size and content for each day's data. Departures from normality will trigger an analysis to determine quality, continuity and size of the data available for each day. Changes from normal data flow that require modification of the data processing path will be flagged and the modifications indicated will be identified. 2. Validation of Level 1 data The Level 1A data corrections for shutter thermal offsets, solar pointing errors, A/D converter calibrations and other perturbations of instrument performance or properties will be monitored for changes in instrument sensitivity to these parameters and for out of boundary values. The SI converted data is averaged over the ACRIM data 'page'. The pre and post observation reference phase observations (ref) of cavity heating powers are averaged over the shutter cycle to derive an equivalent projected value for the solar observation phase (obs). These averages will be scrutinized for erroneous results caused by data drop-outs or bit errors. 3. Validation of Level 2 data Correction of the TSI to a stationary point in the Heliocentric frame of reference provides the top level discriminator for quality and correctness of the data. Comparisons with predicted solar cycle and solar activity TSI trends will be made to flag potentially erroneous data. Data are contained in a 4 column x 1709 row array: Column 1: Epoch 1980 day count Column 2: Decimal time of observations (decimal year) Column 3: Total Solar Irradiance at 1 A.U. in Watts/meters-squared Column 4: Uncertainty of Total Solar Irradiance (Watts/meters-squared) References ********** Willson, R.C., Gulkis, S., Janssen, M., Hudson, H.S., Chapman,G.A., (1981), Observations of solar irradiance variability, Science, v.211, p. 700 Willson, R.C., (1982) J. Geoph. Res., v. 86, P. 4319 Willson, R.C., (1984) Measurements of solar total irradiance and its variability, Space Science Reviews, v. 38, pp 203-242 Willson, R.C., Hudson, H.S., Frohlich, C., Brusa, R.W.; (1986) Science, v. 234, p 1114 Willson, R.C., Hudson, H.S., (1988) Nature, v. 332, No. 6167, 810 Willson, R.C., Hudson, H.S., (1991) Nature, v. 351, pp. 42-44 Willson, R.C., (1997) Total solar irradiance trend during solar cycles 21 and 22, Science, V. 277, pp. 1963-1965. For further information contact Dr. Richard C. Willson