  Plasma is prevalent throughout the universe. The cosmic plasma serves as not
only a crucial tracer for studying the evolution of the cosmos but also an
ideal laboratory for investigating the properties of plasma in extreme
conditions. As one of the important contributors to the re-ionization of the
universe, the variability in quasar (driven by the supermassive black hole)
radiation presents a convenient opportunity to study the response of gases
ionized by them. Based on extensive statistical analysis using data from the
Sloan Digital Sky Survey (SDSS), it has been demonstrated that the response of
gases to quasar radiation exhibits asymmetry. Specifically, over 70\% of broad
absorption lines (BALs) gas in quasar host galaxies exhibit a negative
response. Through analytical calculations and photoionization simulations of C
IV, we found that the response of gases to radiation is asymmetric for low and
high ionization states. In high ionization states, the response time scale is
shorter, leading to the detection of more negative responses. In actual case,
the observation time interval is mostly greater than 1 day, and hence the
asymmetric effect of the C IV response gives a typical gas density of upper
limit of $\rm 10^7\ cm^{-3}$. Interestingly, this is consistent with the fact
that most of the measured BAL gas densities are below $\rm 10^7\ cm^{-3}$. In
principle, the detection of this asymmetric effect becomes easier with lower
plasma density or shorter observation time intervals.
