It is estimated that at any given moment in time there are about one million spicules in the solar atmosphere. Scientists have known about solar spicules generation since 1877 when they were first discovered by Italian astronomer Father Angelo Secchi. Despite much research, their origin has remained unknown.
The Goode Solar Telescope at the Big Bear Solar Observatory, in San Bernardino County, California, U.S. was named after American theoretical physicist Philip R. Goode and is operated by the New Jersey Institute of Technology. It is the world's largest-aperture and highest-resolution solar telescope. /Photo via Tanmoy Samanta
Samanta and his colleagues used the Goode Solar Telescope (GS at the Big Bear Solar Observatory (BBSO) in California, U.S., to observe the emerging spicules and the nearby magnetic fields at ultra-high spatial resolution.
"Solar spicules, while they rise up to the height of 4,000-5,000 kilometers, their width is quite narrow at around 100 kilometers. As a result, it was extremely difficult to detect them with earlier telescopes. We were very fortunate to use the 1.6-meter telescope (Goode), currently the world's largest-aperture and highest-resolution solar telescope, through which we made clear observations of the spicules in ultra-high resolution," Samanta said.
"We discovered a very sensitive magnetic field measurement at the foot point of these spicules and that provided us the evidence of how they originate," he added.
The team observed spicules emerging as an outcome of interaction between magnetic fields of opposite polarity, through a process that solar physicist call magnetic flux cancellation. The results further offered evidence that the dynamic interaction of magnetic fields, likely due to magnetic reconnection, generated spicules, which then transferred energy and heat to the corona.
Corona is the hot outer atmosphere of the Sun, which extends outward for several million kilometers beyond the solar surface. Although the temperature in the core of the Sun is as high as 15 million degrees Celsius, it drops to a mere 5,700 degrees at the visible solar surface, or photosphere. Above the photosphere, however, the temperature starts to increase again with height, reaching one million degrees or more in the corona.
Layered image sequence (from bottom to top) showing the Sun's photospheric magnetic field as well as emission from the photosphere, chromosphere and corona. (Visuals Credit: T. Samanta et al. 2019, Science; Data courtesy of BBSO/GST and NASA/SDO)
"To give an analogy, you expect to get colder as you move away from a hot oven, not hotter. But such a temperature increase with distance from source is just what happens in the corona," Samanta explained.
Since their discovery 142 years ago, spicules have been suspected to serve as a conduit for mass and energy to flow from the lower atmosphere to the corona. The latest study appears to corroborate the speculations.
"While we have good observational evidence that these small-scale jets could be a solution to the coronal heating problem, we still need modern simulation or advance theoretical calculations to completely understand the energetic point of view of these events," Samanta said.
Samanta recently concluded his post-doctoral research at Peking University under the supervision of Hui, an associate professor at the university's School of Earth and Space Sciences. Samanta and Hui are jointly credited for conceiving the study on solar spicules and writing the manuscript.
Dr Tanmoy Samanta (L) and Professor Hui Tian of Peking University jointly conceived the study on the origin of solar spicules and led a team of 12 scientists for the research. /Photo via Tanmoy Samanta
Other members of the team included Wenda Cao, Vasyl Yurchyshyn, and Kwangsu Ahn (Big Bear Solar Observatory, New Jersey Institute of Technology, U.S.), Hardi Peter (Max Planck Institute for Solar System Research, Germany), Alphonse Sterling (NASA Marshall Space Flight Center, U.S.), Robertus Erdelyi (University of Sheffield, UK and Eotvos University, Hungary), Song Feng (Kunming University of Science and Technology, China), Dominik Utz (University of Graz, Austria), Dipankar Banerjee (Indian Institute of Astrophysics, India), and Yajie Chen (Peking University, China).
Attributing the success of the study to his alma mater, Samanta remarked: "Peking University has a really excellent working condition and environment and they have meaningful collaboration with international institutions. The university helped a lot."
Samanta is now set to pursue advanced research at George Mason University in the U.S. where he is expected to collaborate with scientists from John Hopkins University and NASA. "I'll continue working in the same field of research and try to find more evidence by performing more statistical analysis to confirm our results. In the future I'll be using even larger telescopes and try to find the origin of solar spicules," he remarked, excited about his new prospects.