  Despite of over thirty years of research of the black hole thermodynamics our
understanding of the possible role played by the inner horizons of
Reissner-Nordstr\"om and Kerr-Newman black holes in black hole thermodynamics
is still somewhat incomplete: There are derivations which imply that the
temperature of the inner horizon is negative and it is not quite clear what
this means. Motivated by this problem we perform a detailed analysis of the
radiation emitted by the inner horizon of the Reissner-Nordstr\"om black hole.
As a result we find that in a maximally extended Reissner-Nordstr\"om spacetime
virtual particle-antiparticle pairs are created at the inner horizon of the
Reissner-Nordstr\"om black hole such that real particles with positive energy
and temperature are emitted towards the singularity from the inner horizon and,
as a consequence, antiparticles with negative energy are radiated away from the
singularity through the inner horizon. We show that these antiparticles will
come out from the white hole horizon in the maximally extended
Reissner-Nordstr\"om spacetime, at least when the hole is near extremality. The
energy spectrum of the antiparticles leads to a positive temperature for the
white hole horizon. In other words, our analysis predicts that in addition to
the radiation effects of black hole horizons, also the white hole horizon
radiates. The black hole radiation is caused by the quantum effects at the
outer horizon, whereas the white hole radiation is caused by the quantum
effects at the inner horizon of the Reissner-Nordstr\"om black hole.
