A Needle in a Haystack: The Leptocline.

Studies of solar (and stellar) dynamos face a problem of utter complexity, i.e., the interaction of a turbulent plasma in the convection zone, associated with latitudinal rotation together with magnetic field acting in a highly stratified medium, covering wide ranges of spatial and temporal scales.Recent theoretical models for the solar dynamo in complex 3-D simulations highlighted changes of the Sun's stratification, mainly in the upper zones, pointing the role of the leptocline, a shallow and sharp shear layer in the top ~8-10 Mm.Here, we give here a brief history of the circumstances that led to the discovery of this layer, characterized by a strong radial rotational gradient at mid latitudes and self-organized meridional flows.We give an overview of the physical solar parameters that originate in this layer: opacity, superadiabicity and turbulent pressure changes; the hydrogen and helium ionization processes; sharp decrease in the sound speed; probably an oscillation phase of the seismic radius associated with a nonmonotonic expansion with depth; probably temporal changes in photospheric zonal and sectorial modes and their associated gravitational moments.Likely also the initial place of the solar wind escape.In addition, the leptocline may play a key role in the formation of the magnetic butterfly diagram.Such results should be the starting point of systematic further investigations of structure and dynamics in this layer, leading to a better understanding of the solar cycle.