Temperature and heating-induced temperature were measured along a chain of thermistors. Digital Thermistor Chain DTC09 is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the MOSAiC expedition on 28 January 2020. The thermistor chain was 4.16 m long and included sensors with a regular spacing of 2 cm. The resulting time series describes the evolution of temperature during the heating cycle of 20 s and after the heating cycle during the following 40 s as a function of geographic position (GPS), depth, and time between 27 February 2020 and 1 July 2020 in sample intervals of 6 hours. It also contains manually estimated position of air-snow, snow-ice, and ice-water interfaces. The DTCs was installed in level first-year ice next to the Fort Ridge. Ice mass balance SIMBA 2020T60 was installed at the Fort Ridge: doi:10.1594/PANGAEA.924269. Radiation station 2020R10 was installed in level first-year ice next to the Fort Ridge: doi:10.1594/PANGAEA.949124.

Temperature after the cooling cycle and temperature before the heating cycle both represent in situ temperature. The temperature before the heating cycle may contain errors in measurements and is provided for reference. It is recommended to use temperature after the cooling cycle and temperature difference after the heating cycle for further analysis.