Authors: D. L. Judge
Reference: 1st TIGER Sympsium, Freiburg Germany, June 18/19, 1998
Measurements of the solar EUV/UV radiation
Abstract:
The Solar and Heliospheric Observatory (SOHO) Solar Extreme Ultraviolet
Monitor (SEM) has measured the solar flux output in the EUV continuously since
December of 1995. The full disk zero order and ( first order 304 A solar flux
has been measured with a transmission grating spectrometer whose wavelength
range was limited by the transmission characteristics of aluminum thin film
filters and the response of the silicon diode detectors. The central image
flux is almost exclusively limited to the wavelength range shortward of 500 A,
and includes solar radiation well into the x-ray region. The 304 A ( 40 A
band is dominated by the 304 A line and represents about 50% of the radiation
in the band. Sounding rocket underflights have provided absolute calibration
data obtained using a nominally identical SOHO SEM instrument calibrated at
NIST both before and after the sounding rocket flights. The stability of the
zeroth order channel has been truly remarkable with a decrease in responsivity
of that channel of ( 2% over the fourteen month period between the two rocket
underflights to date. The first order channels, however, have shown
approximately a 16% decrease in responsivity. Based on our modeling results
the most rational explanation of the decreased response of the SEM
instrumentation is hydrocarbon deposition on the aluminum filters since the
wavelength dependent absorption of hydrocarbons can account for the observed
differences.We will present both the zeroth order and first order 304 A
solar flux data adjusted for instrument sensitivity recalibration using
sounding rocket underflight data.