Resolving Differences in Absolute Irradiance Measurements Between the SOHO/CELIAS/SEM and the SDO/EVE

Abstract

The Solar EUV Monitor (SEM) onboard SOHO has measured absolute extreme ultraviolet (EUV) and soft X-ray solar irradiance nearly continuously since January 1996. The EUV Variability Experiment (EVE) on SDO, in operation since April of 2010, measures solar irradiance in a wide spectral range that encompasses the band passes (26 – 34 nm and 0.1 – 50 nm) measured by SOHO/SEM. However, throughout the mission overlap, irradiance values from these two instruments have differed by more than the combined stated uncertainties of the measurements. In an effort to identify the sources of these differences and eliminate them, we investigate in this work the effect of reprocessing the SEM data using a more accurate SEM response function (obtained from synchrotron measurements with a SEM sounding-rocket clone instrument taken after SOHO was already in orbit) and time-dependent, measured solar spectral distributions – i.e., solar reference spectra that were unavailable prior to the launch of the SDO. We find that recalculating the SEM data with these improved parameters reduces mean differences with the EVE measurements from about 20 % to less than 5 % in the 26 – 34 nm band, and from about 35 % to about 15 % for irradiances in the 0.1 – 7 nm band extracted from the SEM 0.1 – 50 nm channel.

INTERSTELLAR NEUTRAL HELIUM IN THE HELIOSPHERE FROM IBEX OBSERVATIONS. II. THE WARSAW TEST PARTICLE MODEL (WTPM)

“…Recent studies (Snow et al 2014;Wieman et al 2014) showed that the rate of the dominant ionization process for helium, i.e., photoionization, may be biased by systematic instrumental effects. This topic is still a subject of research, but for now we cannot rule out that the ionization model we use is systematically biased upward or downward.…”

The EUV spectrum of the Sun: SOHO, SEM, and CDS irradiances

“…The SEM data processing algorithm for converting raw detector count rates to calibrated irradiance values is based on the NIST-measured channel spectral response functions, a time-and wavelength-dependent degradation model, and a solar reference spectrum that provides an appropriate spectral weighting of the response function and, in the case of the SEM 1 channels, a correction for sensitivity outside of the nominal 260-340 Å band pass. For further information on the SEM instrument, see references in Wieman et al (2014).…”

“…In Wieman et al (2014) the aforementioned differences between SEM version 3.1 irradiance values and those from SDO EVE MEGS and EVE ESP were traced to inaccuracies in the SEM 1 instrument response function and in the solar reference spectrum used in data processing. Although the response function for the on-orbit SEM was measured prior to the launch of SOHO, a 2007 NIST calibration of the SEM sounding rocket clone instrument (which is nominally identical to the in-flight version) revealed that the SEM 1 sensitivity outside of its nominal 260-340 Å band pass is greater than what was measured pre-launch owing to the grazing incidence reflection off of a portion of the instrument housing that was left off for the original calibration.…”

“…In the Wieman et al (2014) study, it was shown that reprocessing the SEM data for the interval of SOHO and SDO mission overlap using a revised response function based on the 2007 SEM clone calibration and concurrent measured reference spectra from EVE MEGS resulted in a 15% to 25% shift in the SEM irradiance values, which brought them into very good agreement with those from EVE version 2.…”

“…Solar Dynamics Observatory (SDO) Extreme ultraviolet Variability Experiment (EVE, see Woods et al 2012) observations at moderate resolution with the Multiple EUV Grating Spectrographs (MEGS) were available across a wide wavelength range from May 2010 until May 2014, when MEGS-A ceased to operate. Wieman et al (2014) compared the EVE MEGS spectra during 2010-2014 to the SEM observations. The SEM 1 (version 3) irradiances were found to be overestimated by about 20% to 25%, when compared to those estimated from the SDO/EVE MEGS version 3 data.…”