Temperature and heating-induced temperature differences were measured along a chain of thermistors. SIMBA 2021T86 (a.k.a. AWI_0904) is an autonomous instrument that was installed on landfast sea ice in the Antarctic Atka Bay during the expedition Polarstern PS124 (COSMUS) in 2021. The thermistor chain was 5 m long and included 241 sensors with a regular spacing of 2 cm. The resulting time series describes the evolution of temperature and temperature differences after two heating cycles of 30 and 120 s as a function of location, depth and time between 2021-03-17 17:45:00 and 2021-10-14 16:55:00. Sample intervals are commonly between 1 and 24 hours, but most frequently hit intervals of 6 hours for temperature and 24 hours for temperature differences. In addition to temperature and geographic position, barometric pressure, air temperature measured approximately 1m over the ice level, tilt and compass were measured. The data set has been processed and contains quality flags for different kinds for erroneous data. Flag values are the sum of individual error codes. The value of 0 refers to no error. Quality flag, position: The geographic position is flagged +1 if the drift velocity, as derived from the GPS longitude and latitude, exceeds a threshold of 10 deg latitude or 50 deg longitude per time step; +2 if the position exceeds extreme values, such as longitude > 360 deg; +4 if the position is exactly 0.0. This instrument was deployed as part of the project Multidisciplinary Ice-based Distributed Observatory (MIDO)Sea Ice Physics @ AWI (AWI_SeaIce).