Authors: T. Woods, L. W. Acton, S. Bailey, F. Eparvier, H. Garcia, D. Judge, J. Lean, J. T. Mariska, D.
McMullin, G. Schmidtke, S. C.Solomon, W. K. Tobiska, H. P. Warren, and R. Viereck
Reference: Accepted ISCS 2001 (AGU Monograph),2001.
Abstract:
The solar extreme ultraviolet (EUV) radiation at wavelengths shortward of
120 nm is a primary energy source for planetary atmospheres and is also a
tool for remote sensing of the planets. For such aeronomic studies, accurate
values of the solar EUV irradiance are needed over time periods of minutes t o
decades. There has been a variety of solar EUV irradiance measurements
since the 1960s, but most of the recent observations have been broadband
measurements in the X-ray ultraviolet (XUV) at wavelengths shortward of 35
nm. A summary of the solar EUV irradiance measurements and their variability
during the last decade is presented. One of the most significant new
solar irradiance results is the possibility that the irradiance below 20 nm is as
much as a factor of 4 higher than the reference Atmospheric Explorer E (AE-E) spectra established in the 1970s and
1980s. The primary short-term
irradiance variability is caused by the solar rotation, which has a mean period
of 27 days. The primary long-term variability is related to the solar dynamo
and is known best by the 11-year sunspot cycle. The solar cycle variability as
a function of wavelength can be characterized as 20% to 70% between 120
and 65 nm and as a factor of 1.5 to 10 between 65 and 1 nm. The magnitude
of the 27-day rotational variability is usually no more than one third of the
solar cycle variability. There is not a smooth transition of variability between
wavelengths, but instead the amount of intrinsic solar variability depends
on the source of the radiation in the solar atmosphere. In general, the
coronal emissions vary the most, the transition region emissions vary somewhat
less, and the chromospheric emissions vary even less. The variability of
the total solar EUV irradiance, integrated from 0 to 120 nm, is estimated t o
be 30-40% for a large 27-day rotational period and a factor of about 2 for the
11-year solar cycle during the recent, rather active, solar cycles.