Solar radiation over and under sea ice was measured by radiation station 2020R12, an autonomous platform, installed on drifting Second-Year-Ice (SYI) in the Arctic Ocean during MOSAiC (Leg 3) 2019/20. The resulting time series describes radiation measurements as a function of place and time between 24 April 2020 and 07 August 2020 in sample intervals of 3 hours. The radiation measurements have been performed with spectral radiometers. All data are given in full spectral resolution interpolated to 1.0 nm, and integrated over the entire wavelength range (broadband, total: 320 to 950 nm). Two sensors, solar irradiance and upward reflected solar irradiance, were mounted on a on a platform about 1 m above the sea ice surface. The third sensor was mounted 0.5 m underneath the sea ice measuring the downward transmitted irradiance. Along with the radiation measurements, this autonomous platform consisted of a lightchain, which measured counts of red, green and blue light at 49 positions at hourly intervals. In addition, the evolution of snow height is measured at hourly intervals. All times are given in UTC.