The team led by Professor Chen Pengfei from School of Astronomy & Space Science (SASS), Nanjing University, disclosed the fine structures and formation of solar filaments, reported their paper published in Nature Astronomy on May 25, 2020 (https://www.nature.com/articles/s41550-020-1094-3).
Figure 1. Comparison of the solar filaments in the solar atmosphere with the earth in size
While the temperature on the solar surface is only 6000K, that of the solar corona over it can reach several million Kelvin. Afloat in the solar corona like clouds above the sun are cold materials, which are only 7000K in temperature but 100 times denser than their coronal background.
Since the corona and cold materials look like earrings put on the sun, these clusters are named solar filament or ri’er (meaning sun earrings in Chinese) as shown Figure 1.
The solar filament is not only an observation object obsessed by astronomy enthusiasts, but a focus of research interest to astronomers.
Typically, it has a width several times and length dozens of times those of the earth.
The solar filament is also called dark thread because it appears as a long dark structure, as shown in Figure 2, when it suspended in the edge of the sun turns to the surface.
Figure 2. The solar filament on the solar surface appears as a long dark structure so that it is called dark thread (observed through Nanjing University’s ONSET)
It is demonstrated by the high-resolution observations that the solar filaments (or dark threads) consist of many fibril-like threads (see Figure 2). More interestingly, these threads are moving all the time with leftward and rightward velocities appearing alternatively.
After decades of research, it is now almost clear how the solar filaments come into being: the 6000K cold materials from the solar surface are heated to several million Kelvin and evaporate into the corona, and then thermal disequilibrium leads them to drastically cool down to 7000K, thus prompting the solar filaments.
However, there are still some unanswered questions: (1) why do the solar filaments have fine structures like threads? (2) why do the filament threads flow in alternating directions? (3) Do the threads localize the footpoints of the magnetic field lines?
To answer these questions, the team hypothesized that the random turbulent heating at the solar surface unequally evaporates materials from the solar surface to the corona, causing the formation of the threads which therefore always sway to the left or the right in the random turbulence.
On this basis, the team did the simulations on the computers at the High Performance Computing Centre of Nanjing University to reproduce the fine structures and counter-streamings of the threads in the solar corona, as shown in Figure 3.
The relation between the filament threads and the local magnetic field lines has been argued worldwide, and the research shows that the filament threads are basically aligned with the local magnetic field lines.
More importantly, it suggests that there are always counter-streamings in the hyperthemal materials between numerous cold threads.
Figure 3. The result of simulation on the computers of the fine structure of the solar filament (the background is the solar surface and the long dark structure is the solar filament)
Zhou Yuhao, the first author of the paper, obtained his doctorate from SASS at Nanjing University in 2019. His advisor Chen Pengfei is the corresponding author.
This research was supported by the Chinese Natural Science Foundations (NSFC 11533005 and 11961131002) and Jiangsu 333 Project (BRA2017359). Zhou Yuhao was supported by the Program A for Outstanding Ph.D. Candidates at Nanjing University. The simulations were conducted at the High Performance Computing Centre of Nanjing University.
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