Current and Future Solar Irradiance Measurements from SBUV/2 Instruments

 

Authors:Matthew T. DeLand and Richard P. Cebula

Affiliations:Science Systems and Applications, Inc. (SSAI)

 

SBUV and SBUV/2 instruments have made solar irradiance measurements since November 1978.These instruments make daily measurements in both continuous scan mode (170-405 nm, 1.1 nm resolution) and discrete mode, sampling 12 wavelengths about the 280 nm Mg II absorption feature.Proxy indexes derived from these data provide accurate monitoring of solar UV variations, circumventing many of the difficulties involved in maintaining an instrumentís long-term calibration.The most popular index uses the core-to-wing ratio of the Mg II absorption line at 280 nm.For Nimbus-7 SBUV, the Mg II index was derived from continuous scan data.An electronics change on the SBUV/2 instruments lowered the signal-to-noise ratio (SNR) of their continuous scan data, hence discrete mode solar measurements are currently used to create a high quality Mg II index.The SBUV/2 Mg II data are employed as input for space weather model predictions of satellite orbit changes due to atmospheric drag.When two SBUV/2 instruments are in operation simultaneously (as is currently the case with NOAA-16 and -17), the Mg II measurements can be scheduled to provide increased temporal resolution and accuracy for the model predictions.

The SBUV/2 data complement the SORCE measurements, overlapping both the SOLSTICE and SIM wavelength ranges.This provides an opportunity to validate the calibration of these instruments near 300 nm, where SOLSTICE coverage ends and SIM coverage begins.Unlike SIM, the SBUV/2 instruments also resolve the Ca II K and H lines at 393 and 397 nm.The recently launched NOAA-17 SBUV/2 instrument uses a new electronic design that has significantly improved its SNR.We have constructed a daily Ca II core-to-wing index that reveals solar rotational modulation as low as 0.2% peak-to-peak.These Ca II index data can be compared to ground-based measurements from the Big Bear Solar Observatory.

The SBUV/2 instruments on NOAA-16 and NOAA-17 are fully operational and in good health.An additional SBUV/2 instrument is scheduled to be launched on NOAA-N in October 2004, and the final SBUV/2 instrument is scheduled for launch in 2008.Both of these instruments offer expanded onboard memory for discrete mode operations, so that additional solar measurements at selected spectral regions may be possible.SBUV/2 solar measurements should continue well beyond the scheduled end of the SORCE mission in 2007.