Observations of solar spectral irradiance change during cycle 22 from NOAA‐9 Solar Backscattered Ultraviolet Model 2 (SBUV/2)

Abstract

The NOAA‐9 Solar Backscatter Ultraviolet Model 2 (SBUV/2) instrument is one of a series of instruments providing daily solar spectral irradiance measurements in the middle and near ultraviolet since 1978. The SBUV/2 instruments are primarily designed to measure the stratospheric profile and total column amount of ozone, using the directional albedo as the input to the ozone retrieval algorithm. Almost all optical components are common to both radiance and irradiance measurements, whose ratio forms the directional albedo, so that most response changes cancel out. As a result, the SBUV/2 instrument does not require onboard monitoring of time‐dependent sensitivity changes for production of ozone data. We use vicarious comparisons with coincident Shuttle SBUV (SSBUV) solar irradiance measurements during 1989–1996, combined with observed calibration drift during the solar activity minimum in 1985–1986, to determine the long‐term instrument characterization for NOAA‐9 SBUV/2. This approach allows us to derive more accurate solar spectral irradiances for the period from March 1985 to May 1997, spanning two solar cycle minima with a single instrument. The NOAA‐9 irradiance data show an amplitude of approximately 9.3% at 200–205 nm for solar cycle 22. This is consistent with the result of ΔF200–205 = 8.3% for cycle 21 from Nimbus‐7 SBUV and ΔF200–205 = 10% for cycle 23 from UARS SUSIM. NOAA‐9 data at 245–250 nm show a solar cycle amplitude of ∼5.7%. The observed irradiance change at 200–205 nm between the minima of solar cycles 21 and 22 is not consistent with the UV irradiance change expected from the total solar irradiance trend suggested by Willson and Mordvinov [2003]. NOAA‐9 SBUV/2 data can be combined with data from other instruments to create a 25‐year record of solar UV irradiance.