How Coronal Loops Form? (Planetary Science)

Astronomers found first direct observational evidence for the formation of coronal loops through magnetic reconnection

You may have seen several pictures of coronal loops on google. They are bright, curving structures that appear as arcs above the Sun’s surface. Hot plasma causes these loops to glow. Based on their temperature, they are actually classified into 3 groups: cool, warm and hot loops. Cool loops have temperature of about 0.1–1 MK and could be observed in ultraviolet (UV) spectral lines or narrow-band images. Warm loops consist of plasma at a temperature of around 1–2 MK, and they are well observed by extreme ultraviolet (EUV) imagers and spectrographs. While, hot loops have temperature more than 2 MK, which are typically observed in some spectral lines with a high formation temperature and filters with a high temperature response, often at the wavelengths of soft X-ray, EUV and UV. Both hot and warm loops may be called coronal loops.

Since coronal loops are building blocks of solar active regions (ARs), it is important to understand how they are formed. However, despite intensive investigations on the plasma properties of quiescent coronal loops, their formation process has rarely been studied and thus the formation mechanism is not well understood.

But now, a team of international astronomers for the first time, reported on the direct observational evidence for the formation of coronal loops through magnetic reconnection as new magnetic fluxes rise into the upper atmosphere. Their study recently appeared in Arxiv.

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Figure 1. Images of the HMI LOS magnetic field (a), NVST Hα line core (b), AIA 171 Å (c) and AIA 211 Å (d) taken at 06:53 UT. The white dotted line marks the approximate location of the newly formed overlying loops. The red dotted line and blue arrows in (b) indicate some dark threads that are located close to the top part of these loops. The red arrows indicate a pair of bright ribbons that correspond to the footpoints of the newly formed small loops underneath the plasma sheet. The red and blue contours in (c) represent positive and negative magnetic fluxes with the levels of ± 300 G, respectively. The black lines in (c) outline the geometry of the reconnection region. The cyan box in (c) marks the field-of-view of the 171 Å image sequence in (e1)–(e4), which shows the formation process of quiescent coronal loops during the second episode. The arrows in (e2) – (e4) mark several newly formed coronal loops. An animation of this figure is available, showing the formation process of coronal loops. It includes the images of the HMI LOS magnetic field, NVST Hα line core, AIA 171 Å and AIA 211 Å, and covers 82 minutes starting at 05:49 UT with a cadence of 12 s. © Hou et al.

They identified this evidence, with the help of EUV observations of Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) and Hα line core observations of NVST.

Astronomers observed that, the magnetic reconnection occurs within plasma sheet at the interface of two-approaching loop like structures. They also clearly observed, converging motions of opposite-polarity magnetic fluxes and the subsequent flux cancellation, from photospheric magnetograms.

“The reconnection results in the formation of overlying loops with typical coronal temperatures and low-lying small loops/fibrils above and below the plasma sheet, respectively. In the meantime, the transverse magnetic field in the photosphere is enhanced. After reconnection, the transverse field becomes weaker and the fibrils disappear, indicating the submergence of the low-lying loops.”

In addition, they have also revealed the presence of numerous bright plasma blobs in the plasma sheet with the help of EUV observations. These blobs have an average width of 1.37 Mm, and they appear intermittently in the plasma sheet and move upward with projected velocities of ∼114 km s¯1. Through a DEM analysis, they found the temperature, emission measure and density of the blobs to be about 3 MK, 2.0×1028 cm¯5 and 1.2×1010 cm¯3, respectively.

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Figure 2. Velocities and sizes of plasma blobs in the plasma sheet. (a) An AIA 171 Å image taken at 06:53:45 UT. (b) AIA 171 Å intensity along slice I–J shown in (a). The black and green curves indicate the original intensity profile and the Gaussian fit. (c) and (d) Distributions of the projected velocity and size for the identified plasma blobs. © Hou et al.

Finally, they have performed a power spectral analysis for these blobs, and found a spectral index that is distinctly different from the expected one in a turbulent reconnection scenario. It has also been found that, plasma flows with speeds of 20 to 50 km s¯1 towards the footpoints of the newly formed coronal loops.

Reference: Zhenyong Hou, Hui Tian, Hechao Chen, Xiaoshuai Zhu, Zhenghua Huang, Xianyong Bai, Jiansen He, Yongliang Song, Lidong Xia, “Formation of solar quiescent coronal loops through magnetic reconnection in an emerging active region”, Arxiv, pp. 1-15, 2021.

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