Unraveling the mysteries of the Solar Corona
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Our Sun, being the primary source of energy for our solar system has a significant influence on our lives, and has always instilled a curiosity in humankind.. Though we have a fairly good understanding of the origin of energy and other various aspects of the Sun, several potentially life-changing phenomena still remain a mystery. Being the nearest star, understanding the Sun also allows us to learn about other distant stars better
Some of these mysteries are related to the hot outer atmosphere of the Sun, known as ‘corona’, which emits profusely in ultra-violet and X-ray wavelengths of the electromagnetic spectrum. We know that the corona consists of ionized gas at temperatures exceeding one million Kelvin, which is much higher than photospheric temperature of 6000 K, the visible surface temperature of the Sun. However, this observation is against the natural expectation that the temperatures should reduce as we go away from the source of energy, and this is known as the ‘coronal heating problem’. From observations, such as the presence of even hotter corona, called active regions above the Sunspots (dark patches seen in visible images of the Sun) where the magnetic fields are known to be stronger, it is suggested that the magnetic fields have an important role in the coronal heating. While there are different theories regarding the actual mechanism, one of these relies on the occurrence of a large number of small solar flares called ‘nanoflares’. Another puzzling observation about the corona is that certain elements are found to have abundances three to four times higher in active regions than in the photosphere. This happens for elements which are easier to ionize, or require lesser energy to ionize. In more technical terms, these elements have their First Ionization Potential (FIP) lower than 10 eV, and hence this phenomenon is generally termed as ‘FIP bias’. The exact reason behind the FIP bias and its origin remains an open question.
A team of scientists from the Physical Research Laboratory (PRL), Ahmedabad, used observations of the Sun in soft X-rays with Solar X-ray Monitor (XSM) onboard ISRO's Chandrayaan-2 mission during the deepest solar minimum of the past hundred years to learn exciting details about the solar corona. For the first time, absolute abundances of elemental Mg, Al, Si in the quiet solar corona are derived. The team discovered and characterized around 100 “sub-A class” microflares in the quiet corona providing new insight into coronal heating puzzle.
The XSM, designed and developed by PRL with support from various ISRO centres, provides measurement of soft X-ray (1-15 keV) spectrum of the Sun. The XSM also supports the quantitative measurements of elemental abundances of the lunar surface using the companion payload CLASS (Chandrayaan-2 Large Area Soft X-ray Spectrometer) developed by URSC, Bangalore, which measures the X-ray fluorescence spectrum from the lunar surface. At present, XSM is the only instrument that provides soft X-ray spectral measurements of the Sun, i.e., measures the intensity of X-ray in different energies over the 1 to 15 keV. More importantly, XSM provides such measurements with very good energy resolution at every second, the highest cadence for any instrument so far.
XSM started observations of the Sun in September 2019, during the period of solar minimum when typically there were very few Sunspots and active regions on the Sun. The solar minimum of 2019-2020 was even more peculiar as the Sun was extremely quiet, and its activity was at the lowest level over the past century. This provided a unique opportunity for XSM to observe the quiet corona without active regions for long periods. The solar X-ray flux as observed by the XSM during this period is shown in the figure. Intervals highlighted with blue color correspond to a period of 76 days when no active regions were present on the solar disk, and XSM was observing the quiet corona.
A remarkable and surprising observation is the detection of a large number (98) of extremely small flares in the quiet corona (see Figure below). These flares are so small that their intensity is well below the standard scale to classify solar flares (i.e. A, B, C, M, and X class flares, where each class is 10 times more intense than previous), and hence these are termed as “sub-A class microflares”. Using the X-ray spectra of these microflares obtained with the XSM and contemporary images in Extreme Ultra-violet obtained with the Atmospheric Imaging Assembly (AIA) of NASA's Solar Dynamics Observatory (SDO), the energy content of these flares could be estimated. This was the first observation and statistical study of such a large sample of microflares in the quiet Sun, supporting the hypothesis of the presence of even smaller scale flares everywhere on the solar corona that could be responsible for the coronal heating.
The X-ray emission over these 76 days, excluding the durations of the microflares, is unusually constant. This is the lowest intensity of X-ray emission observed from the Sun since space-borne observations began. Analysis of the XSM spectra of the quiet Sun, excluding the microflares, provided the measurement of abundances of various elements. The abundances of the low FIP elements Mg, Al, and Si were estimated and found to be lower than the abundances seen in active region corona but higher than that in the photosphere. This is the first report of measurement of abundances as well as reduced FIP bias in the quiet Sun. Our observations of FIP bias in the quiet Sun provides significant inputs towards understanding the FIP bias and suggests that it arises due to the presence of Alfvén waves in the closed magnetic loops.
These outstanding science results on the solar corona and heliophysics obtained during a unique solar extremely quiet period using a sensitive instrument XSM aboard Chandrayaan-2 observations are published in two companion papers in the May issue of the Astrophysical Journal Letters.
Both the Chandrayaan-2 orbiter and the XSM instrument are performing extremely well, and expected to provide many more exciting and new results.
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Figure : Panels (a) and (b) show the X-ray flux in the 1–15 keV energy range with a time cadence of 120 seconds, as measured by the XSM during two observing seasons. Different background shades represent activity levels on the Sun, with orange representing periods when active regions are present; pink representing periods of enhanced activity visible in both the XSM light curve as well as EUV/X-ray images but not classified as Active Regions; and blue representing periods selected for the present study when no major activity was observed on the Sun. The micro flares detected during the quiet periods are marked with red points, representing their peaks; and red vertical bars, representing their time.
Publications:
"Observations of the Quiet Sun During the Deepest Solar Minimum of the Past Century with Chandrayaan-2 XSM: Elemental Abundances in the Quiescent Corona”
Vadawale, Santosh V.; Mondal, Biswajit; Mithun, N. P. S.; Sarkar, Aveek; Janardhan, P.; Joshi, Bhuwan; Bhardwaj, Anil; Shanmugam, M.; Patel, Arpit R.; Adalja, Hitesh Kumar L.; Goyal, Shiv Kumar; Ladiya, Tinkal; Tiwari, Neeraj Kumar; Singh, Nishant; Kumar, Sushil,
Astrophysical Journal Letters, Vol. 912, Id. L12 (7pp), 2021, https://doi.org/10.3847/2041-8213/abf35d
"Observations of the Quiet Sun During the Deepest Solar Minimum of the Past Century with Chandrayaan-2 XSM: Sub-A Class Microflares Outside Active Regions”
Vadawale, Santosh V.; Mithun, N. P. S.; Mondal, Biswajit; Sarkar, Aveek; Janardhan, P.; Joshi, Bhuwan; Bhardwaj, Anil; Shanmugam, M.; Patel, Arpit R.; Adalja, Hitesh Kumar L.; Goyal, Shiv Kumar; Ladiya, Tinkal; Tiwari, Neeraj Kumar; Singh, Nishant; Kumar, Sushil, Astrophysical Journal Letters, Vol. 912, Id. L13(11pp), 2021, https://doi.org/10.3847/2041-8213/abf0b0
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