Results of retrieval investigating the nightside atmosphere of TOI-2109b. Corner plot: posterior distributions and correlations between the atmospheric temperature and uncertainty scaling parameters; chemical, velocity, and broadening parameters are set to fixed values. The dashed vertical lines in the posterior distributions denote the median and 1σ intervals (16th, 50th and 84^th percentiles) for the bounded parameters. For parameters with upper limits, we report the 2σ intervals (95th percentiles). We note that in addition to the free parameters of the retrieval, the present figure also includes log p₂, which is calculated as the sum of the free parameters log p₁ and dp. Right top panel: examples of the T -p profiles sampled by the MCMC analysis. Right bottom panel: upper temperature limits of the nightside atmosphere as a function of atmospheric pressure.