The Fraunhofer spectrum appears to show subtle variability at the 0.1% level indicative of non-constant temperature and possibly mechanical wave flux in the solar atmosphere. Persistent or secular changes in these physical conditions on time scales of years has terrestrial climate implications. Spectrum line monitoring requires control of spectrometer scattered light, zero point, resolution, and linearity to high accuracy (< 0.05%). Inevitably system components such as filters, detectors, gratings, etc. will break or deteriorate and have to be replaced. The general problem of maintaining a time invariant instrument is discussed.
W. C. Livingston,
"Long Time-Base Monitoring Of The Solar Spectrum-An Instrumental Challenge", Proc. SPIE 0172, Instrumentation in Astronomy III, (3 May 1979); doi: 10.1117/12.957076; https://doi.org/10.1117/12.957076