Recently, Ph.D. Candidate ZHANG Xiaojuan and Prof. DENG Linhua from Yunnan Observatories of the Chinese Academy of Sciences (CAS) investigated the long-term variation of the rotation in the transition region using Lyα irradiance from 1947 February 14 to 2023 February 20. This work was published in The Astrophysical Journal Letters.

Solar transition region, located between the chromosphere and corona, plays an important role in the formation of solar wind and coronal heating mechanism. Solar and space physicists have realized that solar transition region is not a statically layered structure, but a dynamic region with very uneven magnetic field and plasma structure. As a long-term unsolved frontier subject of solar and space physics fields, the acceleration mechanism of solar wind and its source region have important scientific significance.

Rotation is crucial in the generation and evolution of solar magnetic field, and also plays an important role in the study of solar-like stars. Generally speaking, solar rotation can be studied through three ways: tracking magnetic features, spectrum measurements and flux modulation method. Additionally, the internal rotation of the Sun is usually diagnosed by helioseismology. However, the previous studies of temporal variation of the solar rotation focused on the photosphere, chromosphere, and corona. Work on the rotation of the solar transition region has been relatively infrequent. Based on Lyα spectral line (originating from the low transition region) irradiance data from 1947 to 2022, ZHANG Xiaojuan et al. carefully studied the rotation period, secular trend and its relationship with solar cycle.

It is found that the rotation period in the low transition region varies from 19.25 to 31.25 days, with an average of 25.45 days. The large-scale variation range of the rotation period length is similar in the photosphere and corona, for instance, the period range of the photospheric rotation is from 21.0 to 30.3 days, and the coronal rotation length is from 19.0 to 29.5 days. Further studies indicate that the global rotation of the low transition region exhibits a continuous downward trend (with a sustained increase in rotation rate). Two possible reasons could lead to this secular feature: the weakening of the solar global magnetic field and the decrease in the heliospheric environment pressure.

Contact:
DENG Linhua
Yunnan Observatories, CAS
Email: lhdeng@ynao.ac.cn