Measuring the Suns radial velocity variability due to supergranulation over a magnetic cycle

TL;DR

This study analyzes how the Sun's supergranulation-related radial velocity variability changes over its magnetic cycle, highlighting its impact on exoplanet detection and proposing observational strategies for other stars.

Contribution

It provides the first long-term measurement of supergranulation variability over a solar cycle and suggests optimal observational strategies for stellar studies.

Findings

Supergranulation variability is largest at solar activity minimum.

Supergranulation time-scale varies by an order of magnitude over the cycle.

At least 23 nights of observation are needed to characterize supergranulation in other stars.

Abstract

In recent years supergranulation has emerged as one of the biggest challenges for the detection of Earth-twins in radial velocity planet searches. We used eight years of Sun-as-a-star radial velocity observations from HARPS-N to measure the quiet-Sun's granulation and supergranulation properties of most of its 11-year activity cycle, after correcting for the effects of magnetically active regions using two independent methods. In both cases, we observe a clear, order of magnitude variation in the time-scale of the supergranulation component, which is largest at activity minimum and is strongly anti-correlated with the relative Sunspot number. We also explored a range of observational strategies which could be employed to characterise supergranulation in stars other than the Sun, showing that a comparatively long observing campaign of at least 23 nights is required, but that up to 10 stars can be monitored simultaneously in the process. We conclude by discussing plausible explanations for the "supergranulation" cycle.