Abstract:
Long-term periodicities in the solar irradiance are often observed with periods proportional
to the solar rotational period of 27 days. These periods are linked either to some internal
mechanism in the Sun or said to be higher harmonics of the rotation without further discussion of their origin. In this article, the origin of the peaks in periodicities seen in the solar
extreme ultraviolet (EUV) and ultraviolet (UV) irradiance around the 7, 9, and 14 days periods is discussed. Maps of the active regions and coronal holes are produced from six images
per day using the Spatial Possibilistic Clustering Algorithm (SPoCA), a segmentation algorithm. Spectral irradiance at coronal, transition-region/chromospheric, and photospheric
levels are extracted for each feature as well as for the full disk by applying the maps to
full-disk images (at 19.3, 30.4, and 170 nm sampling in the corona/hot flare plasma, the
chromosphere/transition region, and the photosphere, respectively) from the Atmospheric
Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) from January
2011 to December 2018. The peaks in periodicities at 7, 9, and 14 days as well as the solar
rotation around 27 days can be seen in almost all of the solar irradiance time series. The
segmentation also provided time series of the active regions and coronal holes visible area
(i.e. in the area observed in the AIA images, not corrected for the line-of-sight effect with
respect to the solar surface), which also show similar peaks in periodicities, indicating that
the periodicities are due to the change in area of the features on the solar disk rather than
to their absolute irradiance. A simple model was created to reproduce the power spectral
density of the area covered by active regions also showing the same peaks in periodicities.
Segmentation of solar images allows us to determine that the peaks in periodicities seen
in solar EUV/UV irradiance from a few days to a month are due to the change in area of
the solar features, in particular, active regions, as they are the main contributors to the total
full-disk irradiance variability. The higher harmonics of the solar rotation are caused by the
clipping of the area signal as the regions rotate behind the solar limb