Solar Physics Division Meeting 2000, June 19-22
Session 1. Helioseismology, Magnetic Fields, Chromosphere and Transition Region
Display, Chair: C. U. Keller, Monday-Thursday, June 19, 2000, 8:00am-6:00pm, Forum Ballroom

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[1.27] Differences in the Sun's Radiative Output in Cycles 22 and 23

O. R. White (HAO/NCAR), G. de Toma (LASP/CU), G. A. Chapman, S. R. Walton (SFO,CSUN), K. L. Harvey (SPRC), A. M. Cookson, D. G. Preminger (SFO,CSUN)

We study total solar irradiance (TSI) observations during the rising phase of cycle 22 and 23 for the 5--year periods during 1986--1990 and 1996--2000. The increase in TSI in cycle 23 is greater than expected from the observations of the photospheric magnetic flux, and by traditional activity indices, like sunspot number, 10.7\,cm radio flux, MgII and HeI indices, all of which indicate that cycle 23 is a relatively weak cycle. Space observations of TSI from SOHO/VIRGO and UARS/ACRIMII show an increase in TSI of about 1\,W/{\rm m}2 from 1996 to 2000. This is comparable to the increase observed in TSI during the previous cycle, from 1986 to 1990 as observed from Nimbus7/ERB. To resolve the discrepancy between the variability in TSI observed in the two last cycles, we used the San Fernando Observatory (SFO) photometric data at 393.4\,nm and 672.3\,nm to evaluate sunspot and facular contributions to TSI. The SFO image decomposition technique has been tested against NSO/KP magnetograms decomposition for selected days, and they are in good agreement. A 3--parameter fit to Nimbus--7 data for the years 1988--1993 based on SFO data and MgII index gives a correlation coeff.~{\rm r}2\,=\,0.9. Extrapolation of the fit in time largely underestimates the current SOHO/VIRGO TSI measurements. This suggests there is either a problem in the observations or a change in nature of radiative sources on the Sun.

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