From SCHUEHLE@sumop1.nascom.nasa.gov Tue Nov 18 16:43:38 2003
Date: Wed, 10 Mar 1999 16:55:14 GMT
From: SCHUEHLE@sumop1.nascom.nasa.gov
Reply-To: sumer-soft@esa.nascom.nasa.gov
To: sumer-soft@esa.nascom.nasa.gov
Cc: SCHUEHLE@sumop1.nascom.nasa.gov
Subject: SUMER-SOFT: SUMER sensitivity change


      The intercalibration_1 data of 1998 have been analysed by Joerg Hollandt
      (PTB), and the results give us for the first time some quantitative
      information about a sensitivity change of SUMER as a result of the
      SOHO loss period in 1998.

      The SUMER part of the Intercal_1 JOP monitors the intensity of the
      lines of He I (584A), Mg X (609A, 624A), Ne VIII (770A), and N V
      (1238A) at quiet Sun locations since the beginning of the SOHO mission.
      These data are well suitable to track the sensitivity of the
      instrument. Up to June 1998 the data prove that there was no
      sensitivity loss of the SUMER instrument during the entire time period
      starting in March 1996. The first intercal_1 measurement campaign after
      the SOHO accident, however, performed on 6 November 1998 indicates for
      the first time a loss of sensitivity of the SUMER instrument. This
      average sensitvity change at the wavelengths mentioned above is about
      20%. Two intercal_1 measurements have been done, one with the
      A-Detector, one with the B-Detector with consistent results.

      Thus it is most likely that the instrument suffered from the SOHO
      accident with the temperature excursions that happened during the 3
      months period in such a way that contaminants, that have accumulated at
      cold surfaces during three years of nominal operations, have been
      released and distributed inside the instrument.
      These contaminants have obviously not disappeared when the door was
      re-opend after the recovery of SOHO.
      
      For the radiometric calibration of the SUMER data after the recovery we
      will have to correct our calibration curves by this 20% factor until we
      might have more detailed results on the sensitivity change over full
      the spectral range. 

      Further Intercal_1 measurements will be made to confirm these results
      which are always subject to large variations due to the solar
      variability.



      Udo Schuehle, GSFC, 10 March 199