  Upper atmospheres of Hot Jupiters are subject to extreme radiation conditions
that can result in rapid atmospheric escape. The composition and structure of
the upper atmospheres of these planets are affected by the high-energy spectrum
of the host star. This emission depends on stellar type and age, which are thus
important factors in understanding the behaviour of exoplanetary atmospheres.
In this study, we focus on Extrasolar Giant Planets (EPGs) orbiting K and M
dwarf stars. XUV spectra for three different stars - epsilon Eridani, AD Leonis
and AU Microscopii - are constructed using a coronal model. Neutral density and
temperature profiles in the upper atmosphere of hypothetical EGPs orbiting
these stars are then obtained from a fluid model, incorporating atmospheric
chemistry and taking atmospheric escape into account. We find that a simple
scaling based solely on the host star's X-ray emission gives large errors in
mass loss rates from planetary atmospheres and so we have derived a new method
to scale the EUV regions of the solar spectrum based upon stellar X-ray
emission. This new method produces an outcome in terms of the planet's neutral
upper atmosphere very similar to that obtained using a detailed coronal model
of the host star. Our results indicate that in planets subjected to radiation
from active stars, the transition from Jeans escape to a regime of hydrodynamic
escape at the top of the atmosphere occurs at larger orbital distances than for
planets around low activity stars (such as the Sun).
