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.Ī remarkable and surprising observation is the detection of a large number (98) of extremely small flares in the quiet corona (se e Figure below). The solar X-ray flux as observed by the XSM during this period is shown in the figure. This provided a unique opportunity for XSM to observe the quiet corona without active regions for long periods. 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. 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. More importantly, XSM provides such measurements with very good energy resolution at every second, the highest cadence for any instrument so far. 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. 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. 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 team discovered and characterized around 100 “sub-A class” microflares in the quiet corona providing new insight into coronal heating puzzle. For the first time, absolute abundances of elemental Mg, Al, Si in the quiet solar corona are derived. The exact reason behind the FIP bias and its origin remains an open question.Ī 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. 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’. This happens for elements which are easier to ionize, or require lesser energy to ionize. 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. 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’. 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. 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’. 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. 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. Being the nearest star, understanding the Sun also allows us to learn about other distant stars better 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. 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.