  Auroral kilometric radiation (AKR) is the paradigm of intense radio emission
from planetary magnetospheres. Being close to the electron gyro frequency
and/or its lower harmonics, its observation indicates the non-thermal state of
the source plasma. Emission is produced when the plasma enters a state of
energetic excitation which results in deformation of the electron distribution
function. Under certain conditions this leads to "quasi-coherent" emission. It
is believed that the weakly-relativistic electron-cyclotron-maser instability
is responsible for this kind of radiation. Since energetically radio radiation
normally is not of {primary} importance in the large-scale magnetospheric
phenomena, AKR as such has, for the purposes of large-scale magnetospheric
physics, become considered a marginal problem. Here this notion is questioned.
AKR while applying to the auroral region mainly during magnetospherically
disturbed times {carries just a fraction of the total substorm energy. It is,
however, of diagnostic power in the physics of the upper auroral ionosphere and
Space Weather research}. As a fundamental physical problem of generation of
radiation in non-thermal plasmas {it remains not resolved yet. Many questions
have been left open even when dealing only with the electron-cyclotron-maser.
These can advantageously be studied in the magnetosphere proper both by
observation and theory, the only continuously accessible place in space. The
most important are listet here with hint on how they should be attacked. Its
value is to be sought in the role it should play in application to the other
magnetized planets, extra-solar planets, and to strongly magnetized
astronomical objects} as an important {tool to diagnose the matter state
responsible for radiation in the radio frequency range beyond thermal, shock or
synchrotron radiation.
