Spectroradiometry for Solar Physics in Space
Abstract
Realistic physical and chemical descriptions of the Sun require observations that have been made with spectroradiometrically calibrated telescopes and spectrometers, i.e., with instruments that have a known spectral responsivity. Such calibrations assure that a measured spectral radiance or irradiance is determined on a scale that is defined by the radiometric standards realised and used in laboratories. For ground-based observations of the Sun in the visible or near-infrared spectral regions, comparisons with laboratory standards of radiance or irradiance are relatively straightforward. However, measurements at shorter or longer wavelengths, or measurements of the total solar irradiance with a radiometric accuracy to within one part in 1000 which is indispensable for climatology today, require observations outside the atmosphere. For these the spectral responsivity of the instrumentation must be known. However, calibrating telescope-spectrometer combinations for the wide wavelength range of space observations is a complex and problematic task, particularly for extended space missions. Satellite telescope-spectrometer combinations can be calibrated before launch in the laboratory by use of appropriate primary or secondary source or detector standards. We review such standards and their use in the context of the SOHO instrument calibrations and we note limitations in accuracy and coverage of parameter space. Environmental influences, such as contamination on the ground and the influence of radiation in space, may, however, cause the spectral responsivity of satellite instruments to change between laboratory calibration and initial operation in space and during the subsequent long period of orbital operations. In-orbit monitoring and validation of the responsivity of a satellite instrument is, therefore, necessary. This has been achieved for SOHO by intercomparisons of the responses of the various instruments when a common source is viewed, by observations of stars and by under-flights. In the past, solar physics has often broken new ground and introduced and refined astronomical techniques. The efforts to calibrate solar observations as they are reported in this book should, therefore, be of interest for astronomy as a whole.
- Publication:
-
ISSI Scientific Reports Series
- Pub Date:
- 2002
- Bibcode:
- 2002ISSIR...2...21S