SOHO Solar Extreme Ultraviolet Full Disk Flux Observations Since SOHO SEM Commissioning in December 1995

Authors: D. L. Judge
Reference: 1st TIGER Sympsium, Freiburg Germany, June 18/19, 1998 Measurements of the solar EUV/UV radiation

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.

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Last Update: April 2, 1999, James Weygand