Speaker
Description
First Name: Daniel
Last Name: Müller
Affiliation ESA
All Authors: Emily Sandford, Frédéric Auchère, Annelies Mortier, Laura A. Hayes, Daniel Müller
Abstract: SOHO/EIT has been taking images daily in four EUV bandpasses since early 1996. We converted this extensive archive into a set of “Sun-as-a-star” light curves in EIT’s four bandpasses. For this, we summed the flux in each EIT image into one flux value, with an uncertainty accounting for both the background noise in the image and the potential spillover of flux beyond the bounds of the image. We corrected for long-term instrumental systematic trends in the light curves by comparing our 304 Å light curve to the EUV light curve taken by SOHO’s CELIAS/SEM solar wind monitoring experiment. We corrected for SOHO’s viewing angle by fitting a trend to the flux values with respect to SOHO’s heliocentric latitude at the time of each observation. We produced two sets of Sun-as-a-star light curves with different uncertainty characteristics, either of which might be preferred for different types of future analyses. We find that our EUV light curves trace the solar cycle and ∼27-day rotation period much better than comparable optical observations. In particular, we can accurately recover the solar rotation period from our 284 Å light curve for 26 out of 28 calendar years of EIT observations (93% of the time), compared to only 3 out of 29 calendar years (10% of the time) for the VIRGO total solar irradiance time series, which is dominated by optical light. Our EIT light curves, in conjunction with Sun-as-a-star light curves at optical wavelengths, will be valuable to those interested in inferring the EUV/UV character of stars with long optical light curves, but no intensive UV observations, as well as to those interested in long-term records of solar and space weather.