Profiles of the daytime atmospheric turbulence above Big Bear solar observatory

Context. Space weather has become acutely critical for today's global communication networks. To understand its driving forces we need to observe the Sun with high angular-resolution, and within large fields-of-view, i.e. with multi-conjugate adaptive optics correction. Aims. The design of a multi-conjugate adaptive optical system requires the knowledge of the altitude distribution of atmospheric turbulence. We have therefore measured daytime turbulence profiles above the New Solar Telescope (NST), on Big Bear Lake. Methods. To this purpose, a wide-field wavefront sensor was installed behind the NST. The variation of the wavefront distortions with angular direction allows the reconstruction of the distribution of turbulence. Results. The turbulence is found to have three origins: 1. a ground layer (<500 m) that contains 55-65% of the turbulence, 2. a boundary layer between 1-7 km comprises 30-40% of the turbulent energy, 3. and the remaining ~5% are generated in the tropopause, which is above 12 km in summer and between 8 and 12 km in winter. Conclusions. A multi-conjugate adaptive optical system should thus aim at correcting the ground turbulence, the center of the boundary layer at roughly 3 km altitude and, eventually, the upper boundary layer around 6 km altitude.