Temperature and heating-induced temperature were measured along a chain of thermistors. DTC 26 is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the 2nd leg of the MOSAiC expedition in February 2020. The thermistor chain was 7.34 m long and included 368 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 14 February 2020 and 24 July 2020 in sample intervals of 6 hours. The investigated first-year ice ridge known as Alli's ridge was formed on 7-9.01.2020 between second-year ice, 0.9-1.0 m thick first-year ice and lead ice mainly from lead ice of 0.2-0.4 m thick blocks. The buoy was installed 20 m from the ridge crest towards the first-year level ice. Ice mass balance SIMBA 2020T61 was installed at the ridge crest: https://doi.org/10.1594/PANGAEA.926580.This dataset represents the temperature of sea ice 40 s after the 20 s cycle of internal heating or in-situ temperature. The file contains time (1), latitude (2), longitude (3), and temperature measurements in [°C] from 368 sensors (4-371) starting from the top with a 2 cm vertical resolution.