Many violent phenomena continuously keep occurring on surface of the Sun and its atmosphere known as the corona. This solar activity follows an eleven-year cycle, which means, it goes through its 'solar maxima' and 'solar minima' once every eleven years. While the cumulative emission of solar X-rays emitted over a year varies with the solar cycle, these are often punctuated with extremely large x-ray intensity variations over very short periods, few minutes to hours. Such episodes are known as solar flares.

Chandrayaan-2 orbiter utilizes X-rays emitted by the Sun in a clever way to study elements on the lunar surface. Solar X-rays excite atoms of constituent elements on the lunar surface. These atoms when de-excited emit their characteristic X-rays (a fingerprint of each atom). By detecting these characteristic X-rays, it becomes possible to identify various major elements of the lunar surface. However, in order to determine their concentration, it is essential to have simultaneous knowledge of the incident solar X-ray spectrum.

The Chandrayaan-2 orbiter carries two instruments, Chandrayaan 2 Large Area Soft X-ray Spectrometer (CLASS) and Solar X-ray Monitor (XSM), to measure the lunar elemental composition using this technique. Here, the CLASS payload detects the characteristic lines from the lunar surface and the XSM payload simultaneously measures the solar X-ray spectrum.

Currently, the solar cycle is heading towards minima and the Sun has been extremely quiet for past few months. On 30th September 2019 00:00 UTC - 1st October 2019 23:59 UTC, a series of small flares were observed by XSM.

The figure shows the solar X-ray flux as measured by XSM (in blue) during this period, and for comparison, the flux measured by X-ray sensor on the Geostationary Operational Environmental Satellite (GOES-15) is also shown (in orange), which is considered the standard for solar X-ray intensity measurement.It shows that XSM is able to detect the intensity variations of the Sun much beyond the sensitivity limit of GOES. The gaps seen in GOES light curve around 09:00 UTC are due to instrumental artifacts. The GOES data was obtained from the National Center for Environmental Information of National Oceanic and Atmospheric Administration, USA.

Apart from the better sensitivity, XSM also measures the spectrum of solar X-ray in the energy range of 1 - 15 keV with highest energy resolution so far for any broadband solar X-ray spectrometer over intervals as short as 1 second.

Although this solar flare observed at present may not enable the study of the lunar surface composition due to the large angle between Sun, lunar surface and Chandrayaan-2 (close to 90 deg in this case against a desirable low value, close to zero), such XSM observations provide very useful data to understand various processes on the Sun.