The Nimbus 7 solar total irradiance: A new algorithm for its derivation

Abstract

The Nimbus 7 satellite has measured the solar total irradiance from November 1978 to July 1991 (153 months). These measurements are important both in solar physics and for climate change. To insure that the Nimbus 7 measurements are capturing the true behavior of the Sun, it is essential that the properties of the radiometer and its changes over time be understood. The calibration of the radiometer can be viewed as a process of removing instrumental influences from the raw measurements, leaving the experimenter with an estimate of solar variability. In this paper the changing radiometer pointing, the zero offsets, the stability of the gain, the temperature sensitivity, and the influences of other platform instruments are all examined and their effects on the measurements considered. Only the question of relative accuracy (not absolute) is examined. The resulting derived solar irradiances are compared to previous analyses of the Nimbus 7 radiometer and to the Solar Maximum Mission (SMM) measurements. Compared to previous analyses, the newly derived values are higher and somewhat less variable than the older values. Removal of the off‐axis pointing errors and a new treatment of the zero offsets are the major reasons for the changes. Compared to the SMM measurements, both agree quite well so long as any solar activity is present. When the Sun becomes quiet, so its irradiance variability is less than the Nimbus 7 radiometer resolution, the comparison to the SMM results breaks down. Between 1980 and 1988 the correlation of the daily values is 0.83, compared to 0.62 using previously published values from both satellites. The monthly means have a correlation of 0.90, indicating that about 80% of the longer‐term variance is in common. In 1980 when both satellites were operating without problems, Nimbus 7 was 0.3155% higher on average. For May 1984 to December 1988, Nimbus 7 was 0.3255% higher. Therefore if the Nimbus 7 satellite continues to operate properly until a period of about 1 year after the launch of UARS with its active cavity radiometer irradiance monitor (ACRIM), it appears that the differences between Nimbus 7, SMM ACRIM, and UARS ACRIM can be measured to within a few hundredths of a percent. A self‐consistent set of solar irradiance measurements from several satellites over nearly two solar cycles appears feasible.

ACRIM total solar irradiance satellite composite validation versus TSI proxy models

“…During this period the only observations available were those of the Nimbus7/ERB (hereafter referred to as ERB) (Hoyt et al 1992) and ERBS/ERBE (hereafter referred to as ERBE) (Lee III et al 1995). These experiments provided TSI observations that met the needs of the Earth Radiation Budget investigations at that time, but were less precise and accurate than the ACRIM experiments that were designed specifically to provide the long term precision and traceability required by climate and solar physics investigations.…”

The role of the Fraunhofer lines in solar brightness variability

“…Regular spaceborne measurements of the total solar irradiance (TSI, which is the spectrally integrated solar radiative flux at one au from the Sun) started in 1978 with the launch of the NIMBUS 7 mission (Hoyt et al 1992). They revealed that the TSI varies on multiple timescales (Willson et al 1981;Fröhlich 2005).…”

Estimating Solar Forcing of Climate Change during the Maunder Minimum

“…Since the Maunder Minimum, solar activity has risen steadily to the current Modern Maximum and, according to the 10 Be proxy data, has now reached levels as high as those last seen in the 12th century. Space-based measurements with sufficient precision to identify real changes in solar radiative have been made near the peak of the Modern Maximum but for barely more than one 11-year solar activity cycle (Willson & Hudson 1991;Hoyt et al 1992).…”

“…With this approach we investigate the effect of various scenarios of depleted bright magnetic elements on the full disk solar emission, estimate the probable reduction of the total solar irradiance during the Maunder Minimum, and use the Goddard Institute for Space Studies (GISS) General Circulation Model (GCM) to calculate the resultant geographical temperature changes that might be expected from this reduction. (Hoyt et al 1992) and (lower panel) the sunspot number.…”