  A sequence of prompt and delayed radio signals at tens of kilohertz should
reach the Earth (or Jupiter) due to graviton--photon conversion in interstellar
as well as local planetary magnetic fields. These radio fluxes may be a
detectable probe of a huge gravitational burst expected from Supernovae
explosions. The earliest prompt radio signal, coinciding with the neutrino
burst, is due to conversion in the terrestrial (or Jovian) magnetic field and
is below the micro-Jansky (or milli-Jansky) level for a galactic Supernova like
SN1987A. A later radio signal, a ``tail'', due to the same graviton - radio
wave conversion in random interstellar fields will maintain a relic radio
``noise'' for hundreds or thousands of years and might even be still detectable
by a very sensitive network of satellite antennas at the kilohertz band. Exact
solutions are presented here for the graviton-photon conversion in a refractive
medium, as well as their consequences for high energy supernovae and the 2.726
K background radiation.
