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

TL;DR

This study investigates how the inclination angle of the solar rotation axis affects the observable chromospheric activity, revealing that viewing angle significantly influences the detection of solar rotation and activity cycles in observational data.

Contribution

The paper introduces a method to simulate solar images at various inclination angles using Ca II K line observations, quantifying the impact of viewing angle on solar activity detection.

Findings

Solar rotation modulation is detectable up to 70° inclination with dense sampling.

Activity-cycle modulation remains detectable with at least 20 observations per year and 30% cycle amplitude.

Inclination angle significantly affects the visibility of solar rotation and activity signals in observations.

Abstract

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.