Detection of decayless oscillations in solar transition region loops

Context. Decayless kink oscillations have been frequently observed in coronal loops, serving as a valuable diagnostic tool for the coronal magnetic field. Such oscillations have never before been reported in low-lying loops of the transition region (TR).Aims. The aim of this study is to detect decayless kink oscillations in TR loops for the first time.Methods. We used the SI IV 1400 & Aring; imaging data obtained from the Interface Region Imaging Spectrograph. We applied the Multiscale Gaussian Normalization method to highlight the TR loops, and generated time-distance maps to analyse the oscillation signals.Results. Seven oscillation events detected here exhibit a small but sustained displacement amplitude (0.04-0.10 Mm) for more than three cycles. Their periods range from 3 to 5 min. The phase speed is found to increase with loop length, which is consistent with the decrease in Alfven speed with height. With these newly detected oscillations, we obtain a rough estimate of the magnetic field in the transition region, which is about 5-10 G.Conclusions. Our results further reveal the ubiquity of decayless kink oscillations in the solar atmosphere. These oscillations in TR loops have the potential to be a diagnostic tool for the TR magnetic field.