  Context. The zebra patterns observed in solar radio emission are very
important for are plasma diagnostics. The most promising model of these
patterns is based on double plasma resonance instability, which generates
upper-hybrid waves, which can be then transformed into the zebra emission.
Aims. We aim to study in detail the double plasma resonance instability of hot
electrons, together with a much denser thermal background plasma. In
particular, we analyse how the growth rate of the instability depends on the
temperature of both the hot plasma and background plasma components. Methods.
We numerically integrated the analysed model equations, using Python and
Wolfram Mathematica. Results. We found that the growth-rate maxima of the
upper-hybrid waves for non-zero temperatures of both the hot and background
plasma are shifted towards lower frequencies comparing to the zero temperature
case. This shift increases with an increase of the harmonic number s of the
electron cyclotron frequency and temperatures of both hot and background plasma
components. We show how this shift changes values of the magnetic eld strength
estimated from observed zebras. We confirmed that for a relatively low hot
electron temperature, the dependence of growth rate vs. both the ratio of the
electron plasma and electron cyclotron frequencies expresse distinct peaks, and
by increasing this temperature these peaks become smoothed. We found that in
some cases, the values of wave number vector components for the upper-hybrid
wave for the maximal growth rate strongly deviate from their analytical
estimations. We confirmed the validity of the assumptions used when deriving
model equations.
