How does the solar chromospheric activity look like under different inclination angles?

Chromospheric plages are distributed between mid-latitude and the Equator and
never close to the Poles. Therefore, we suspect that the inclination angle of
the solar rotation axis has an impact on the observable chromospheric emission.
We reproduce the solar images from any inclination in order to study the effect
of the inclination axis on the solar variability by using direct observations
of the Sun in the Ca II K line. More than 2700 days of observations since the
beginning of the Ca II K observations with USET, in July 2012, were used in our
analysis. For each observation day, we produce synoptic maps to map the entire
solar surface during a full solar rotation. Then by choosing a given
inclination, we generate solar-disk views, representing the segmented brightest
structures of the chromosphere (plages and enhanced network), as seen under
this inclination. The area fraction are extracted from the masks for each
inclination and we compare the evolution of those time series to quantify the
impact of the inclination angle. We find a variation of the area fraction
between an Equator-on view and a Pole-on view. Our results show an important
impact of the viewing angle on the detection of modulation due to the solar
rotation. With the dense temporal sampling of USET data, the solar rotation is
detectable up to an inclination of about $|i| = 70^{\circ}$ and the solar-cycle
modulation is clearly detected for all inclinations, though with a reduced
amplitude in polar views. When applying a sparse temporal sampling typical for
time series of solar-like stars, the rotational modulation is no longer
detected, whatever the inclination. On the other hand, we find that the
activity-cycle modulation remains detectable as long as the sampling contains
at least 20 observations per year and the cycle amplitude reaches at least 30\%
of the solar-cycle amplitude.