It must measure the supergranules. This refers to the physical pattern casing the exterior of the quiet Sun with an archetypal horizontal scale of approximately 30,000 km and a lifetime of around 1.8 d. Its most noticeable or visible signature is as a fluctuating velocity field of 360 m st-1 rms whose components are mostly horizontal. This was exposed more than fifty years ago, conversely clearing up why and how it originates still represents one of the core challenges of modern solar physics.
