Why does the electrical conductivity of earth's atmosphere increase with altitude?

Explanation

We know that, with an increase in altitude, the
atmospheric pressure decreases. The high energy
particles (i.e., `gamma rays ` and cosmic rays) coming
from outer space and entering our earth's
atmosphere cause ionistation of the atoms of the
gases present there. The ionising power of these
radiations decreases rapidly as they approach
the earth, due to increase in number of collision
with the gas atoms. it is due to this reason that
the electrical conductivity of earth's atmosphere
increases with altitude.