Researchers from Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute under the Department of Science and Technology, along with collaborators from Physical Research Laboratory, Indian Institute of Technology Delhi, Indian Institute of Space Science and Technology, and NASA's Goddard Space Flight Center, have made a significant discovery about plasma flow in the Sun's upper atmosphere. The study analyzed 27 years of radio observations collected by the Nobeyama Radioheliograph in Japan, providing the first clear evidence that poleward meridional flow exists nearly 3,000 kilometres above the Sun's visible surface in the upper chromosphere region.
The research team developed a novel image-correlation technique comparing thousands of full-disk radio images of the Sun taken one day apart, measuring tiny shifts in brightness patterns over nearly three decades. They found that plasma in the upper chromosphere moves toward the poles at speeds of about 5 to 15 metres per second, similar to speeds measured in deeper layers of the Sun. The flow changes over the course of the solar cycle, with northern and southern hemispheres sometimes behaving differently depending on which hemisphere is more magnetically active.
One of the most significant findings emerged when radio observations were compared with long-term maps of the Sun's magnetic field. Bright features seen in radio images were found to move poleward in close step with the transport of magnetic fields, suggesting structures observed high in the Sun's atmosphere remain connected to magnetic fields rooted much deeper inside the Sun. This provides strong observational evidence for the 'magnetic tree' hypothesis which proposes that magnetic structures extending high above the Sun's surface are connected to deeper layers similarly to how tree branches remain connected to trunk and roots.
The findings, published in The Astrophysical Journal, open a new window for exploring the Sun's internal dynamics through radio astronomy and provide an important new tool for studying the solar dynamo—the process responsible for generating the Sun's magnetic field and powering its 11-year activity cycle. Understanding how plasma and magnetic fields move through the Sun is essential as these processes drive solar activity and influence solar storms and other space-weather events that affect satellites, communication systems, navigation networks and power grids on Earth.