Space observations of the variability of solar irradiance in the near and far ultraviolet

Abstract

Satellite observations of UV solar irradiance in selected wavelength bands between 1200 and 3000 A have been made continuously by photometers consisting of broad band sensors operated on Nimbus 3 and 4, which were launched in April 1969 and April 1970, respectively. In addition, on Nimbus 4, spectrophotometer measurements of solar irradiance have been made with a dispersive instrument at 12 selected wavelengths from 2550 to 3400 A with a 10‐A band pass. Variations of solar irradiance associated with the solar rotational period have been observed since the launch of Nimbus 3. These variations are apparently associated with two source regions separated by about 180° in solar longitude. The change in irradiance with solar rotation was found to increase with decreasing wavelengths. Different types of observed variations in UV solar irradiance can be classified in accordance with characteristic times; in order of increasing periods, they are (1) flare‐associated enhancements, (2) 27‐day variations due to solar rotation, (3) a possible biennial effect, and (4) long‐term variations associated with the 11‐year solar cycle.

NOAA 11 Solar Backscattered Ultraviolet, model 2 (SBUV/2) instrument solar spectral irradiance measurements in 1989–1994: 1. Observations and long‐term calibration

“…The accumulation of extended data sets suitable for characterizing solar UV variability on different timescales requires orbiting satellites. Early satellite instruments used one or more broadband photodiodes [e.g., Heath, 1969;Prag and Morse, 1970;Heath, 1973] or spectrometers such as the Backscatter Ultraviolet (BUV) instrument [Heath, 1973] to measure solar UV irradiance. These early instruments confirmed the existence of solar UV variability on rotational timescales and provided initial estimates of its magnitude and spectral characteristics.…”

Short Term Periodicities in Ionospheric Total Electron Content.

“…As the ionosphere responds markedly to the 11-year solar variability, if a 27-day spectral variability exists, the ionosphere should respond similarly (considering the short time response of the ionosphere to both these periods). Heath (1973) finds a 27-day UV wavelength dependence variability (in 1969) of about 25%…”

A mechanism for solar ultraviolet flux variability

“…The UV enhancements of Heath (1973Heath ( , 1981 and Hinteregger et al (1977) show a 27 day variability of order 30% near A = 100 nm and 1% near 300 nm. Further, if we take a particular wavelength, La for example, the rotational variation, near 50% in early 1969 decreases to near 10% in 1972 and then increases to about 20% in 1973.…”