Solar Wind Turbulence Spectra and Correlation Scale from 4 to 50 r_: STEREO and PUNCH Observations

We investigate the radial evolution of solar wind turbulence using remote-sensing brightness observations from the Solar Terrestrial Relations Observatory (STEREO) and the first science-ready images from the Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission. Exploiting the combined wide fields of view of STEREO's COR2 coronagraph (4-9 r_sun) and PUNCH's Wide Field Imager (WFI, 21-50 r_sun), we track the radial evolution of the power spectral density and correlation scale of density fluctuations from 4 to 50 r_sun. Line-of-sight integration systematically steepens the "usual" turbulence scaling laws by a constant offset of about -0.5 across all resolved scales, so that the energy-containing range (nominally k^-1) and the inertial range (nominally k^-5/3) appear as k^-1.5 and k^-2.1, respectively -- consistent with previous synthetic-image simulations. The correlation scale increases with heliocentric distance across the full range of heliodistances probed, in accordance with previous theoretical models and observations at closer distances from the Sun.