and
3 Auroral Radio
Emissions
and
3 Auroral Radio
Emissions
A lower ionospheric electron cyclotron maser has recently
been suggested as a possible source of natural radio
emissions that have been observed with ground-based
instruments in the auroral zone.
According to this theory, the observed emissions are
interpreted as X-mode electromagnetic waves at two and/or
three times the ionospheric electron cyclotron frequency
(
).
In the present study, we carry out a more thorough analysis
of the cyclotron maser instability using a more complete
growth rate expression and more realistic energetic electron
distribution functions.
Growth rates of various electromagnetic modes and harmonics
are calculated.
It is shown that growth rates for 2 and 3
X-modes can exceed collisional ionospheric damping rates,
confirming the original estimations.
However, the present analysis also reveals that O-mode
waves at the fundamental cyclotron frequency, ,
should also be excited at the source.
The propagation of excited electromagnetic waves from the
source region to the ground inside a model horizontal
density cavity structure is also examined by means of ray
tracing technique.
It is found that the O-mode wave is unable to reach the
ground, which explains why the emissions at are
not detected on the ground.
AGU Index Terms: 2471 Plasma waves and instabilities; 2407 Auroral ionosphere; 2487 Wave propagation; 2439 Ionospheric irregularities
Keywords/Free Terms: Auroral roar, ionsopheric density cavity, cyclotron maser instability.
JGR-Space 96JA02664
Vol. 101
, No. A12
, p. 27,015