Solar EUV irradiances and indices

Abstract

Information about the solar EUV spectrum and its temporal variability is essential for studies of the earth's upper atmosphere. Despite a data base which spans two solar cycles, neither the EUV irradiance variations associated with the eleven year solar activity cycle, nor the EUV spectral irradiance of the inactive (“quiet”) sun have yet been accurately determined. Because ground based solar observations have been made more frequently, and with a long term accuracy superior to that of EUV irradiance measurements, they have assumed an important role as indicators of solar activity in studies of the earth's thermosphere. In this paper, the solar EUV irradiance data base acquired over the past 25 years is briefly described. The extent to which emissions at different EUV wavelengths and ground based solar indices are similar, is examined statistically over time scales of the 27-day solar rotation, and less rigorously over the eleven year solar cycle. It is shown that the detailed temporal structure of the EUV irradiance variations is a complicated function of wavelength, and cannot be exactly reproduced by any of the traditional ground based indices, over either short or long time scales.