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 Å 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 Alfvén 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.