  The Mg II k \& h line intensity ratios can be used to probe the
characteristics of the plasma in the solar atmosphere. In this study, using the
observations recorded by the Interface Region Imaging Spectrometer (IRIS), we
study the variation of the Mg II k \& h intensity ratio for three flares
belonging to X-class, M-class, and C-class, throughout their evolution. We also
study the k-to-h intensity ratio as a function of magnetic flux density
obtained from the line-of-sight magnetograms recorded by the Helioseismic and
Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Our
results reveal that while the intensity ratios are independent of magnetic flux
density, they show significant changes during the evolution of the C-class and
M-class flares. The intensity ratios start to increase at the start of the
flare and peak during the impulsive phase before the flare peak and decrease
rapidly thereafter. The values of the ratios fall even below the pre-flare
level during the peak and decline phases of the flare. These results are
important in light of the heating and cooling of localized plasma and provide
further constraints on the understanding of flare physics.
