Indian researchers have identified a persistent plasma flow extending nearly 3,000 kilometres into the Sun’s upper chromosphere, providing the first direct observational evidence for the magnetic tree model of solar dynamics. Published in The Astrophysical Journal, this discovery establishes a critical link between the Sun’s surface and its upper atmosphere, revealing that these layers are dynamically interconnected rather than isolated.
The research was spearheaded by Srinjana Routh of the Aryabhatta Research Institute of Observational Sciences, an autonomous institute under the Department of Science and Technology, in collaboration with the Physical Research Laboratory, IIT Delhi, the Indian Institute of Space Science and Technology, and NASA’s Goddard Space Flight Center. For decades, solar physicists have tracked the meridional flow, a slow movement of hot plasma from the equator toward the poles that acts as a conveyor belt for magnetic fields.
While this conveyor belt mechanism is known to regulate the 11-year solar cycle, previous observations were restricted to the Sun’s lower atmospheric layers. The new study demonstrates that this poleward flow persists deep into the upper chromosphere, where magnetic fields exert significantly more influence over plasma behavior. This finding effectively overturns the long-held assumption that such flows were confined to the solar surface.
The breakthrough relied on an analysis of 27 years of radio observations collected by the Nobeyama Radioheliograph in Japan. Rather than tracking individual sunspots, the team utilized an innovative image-correlation technique to compare thousands of full-disk radio images. This method allowed the researchers to detect minute shifts in brightness patterns, mapping the large-scale plasma movement across the upper atmosphere with a precision that exceeds previous manual tracking methods.
The image-correlation process involved comparing radio maps taken exactly one day apart to isolate subtle, long-term displacement trends. By filtering out transient solar noise, the team successfully isolated the meridional velocity components that define the flow. This rigorous computational approach enabled the team to resolve plasma motions that were previously obscured by the high-frequency variability of the solar atmosphere.
The data revealed plasma traveling toward the poles at speeds between 5 and 15 metres per second, velocities remarkably consistent with those observed deep within the solar interior. The study also noted that the flow varies throughout the solar cycle, with the northern and southern hemispheres exhibiting distinct behaviors based on their respective magnetic activity levels. This variability provides a new metric for understanding how the Sun’s internal dynamics fluctuate over time.
The magnetic tree hypothesis posits that magnetic structures extending high above the solar surface remain tethered to fields rooted deep within the Sun, much like branches connected to a trunk. By aligning decades of radio observations with long-term magnetic field maps, the researchers confirmed that bright radio features migrate toward the poles in synchronization with magnetic field transport. This correspondence confirms that the upper atmosphere retains the signature of processes occurring deep within the solar interior.
Understanding these internal plasma movements is vital for refining space weather prediction models, as these processes drive solar flares and coronal mass ejections. These events can cause significant disruption to satellite operations, global communication networks, aviation systems, and electricity transmission grids on Earth. The newly identified flow provides a powerful observational tool for scientists to better understand the solar dynamo, the mechanism responsible for generating the Sun’s magnetic field.
The discovery underscores the increasing role of Indian institutions in international space research and radio astronomy. By proving that the upper atmosphere mirrors large-scale flows occurring beneath the surface, the study offers a new perspective on how different layers of the Sun communicate. Future research will likely focus on utilizing these findings to improve the accuracy of long-term solar activity forecasts and mitigate the risks posed by severe space weather events to modern technological infrastructure.
