|
|
 |
|
|
Ann. Geophys., 23, 55-74, 2005
www.ann-geophys.net/23/55/2005/
© European Geosciences Union 2005
An interferometer experiment to explore the aspect angle dependence of stimulated electromagnetic emission spectra
Isham1, Hagfors2, Khudukon3, Yu. Yurik3, Tereshchenko3, Rietveld4, Belyey5, Grill6, La Hoz5, Brekke5, and Heinselman7
1Interamerican University, Bayamón, Puerto Rico, USA
2Max-Planck Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany
3Polar Geophysical Institute, Murmansk, Russia
4EISCAT Scientific Association, Ramfjordbotn, Norway
5University of Tromsø, Tromsø, Norway
6Lancaster University, Lancaster, UK
7SRI International, Menlo Park, California, USA
Abstract. When the Earth's ionosphere is irradiated by a
radiofrequency (RF) electromagnetic wave of sufficiently high power density
and tuned to match a natural E- or F-region plasma frequency, ionospheric
magnetoionic wave modes may be excited and may generate RF electromagnetic
sideband waves via nonlinear interactions. These secondary emissions, which
may then escape from the ionosphere, have been termed stimulated
electromagnetic emission or SEE. The frequency spectra of this radiation has
been studied extensively, and a number of characteristic spectral features
have been identified and in some cases related to particular plasma
processes. The separation in frequency between the RF pump and the harmonics
of the local electron gyrofrequency is critical in determining the amount of
anomalous absorption suffered by the pump wave and the spectral properties
of the stimulated sidebands. The pump can excite electrostatic waves which
do not propagate away but can in some cases be observed via radio-wave
scattering from the electron density fluctuations associated with them.
These enhanced density fluctuations are created by processes commonly
referred to as upper-hybrid and Langmuir turbulence. Langmuir turbulence has
been the subject of 930-MHz scattering observations with antenna scanning
through several pre-selected angles between the geographic and geomagnetic
zenith directions, and a preference for pointing angles between the Spitze
angle and geomagnetic field-aligned was identified. Other phenomena, such as
the generation of enhanced electron temperatures and artificial aurora, have
more recently been shown to have special behavior at similar angles, near
but apparently not quite at field-aligned. In view of this evidence for
angular structure in several pump-induced effects, in light of the rich
variety of SEE phenomena strongly dependent on the geomagnetic field via the
frequency interval between the pump and the gyrofrequency harmonics, and in
view of the not yet understood but complex relationship between
electrostatic fluctuations and SEE, it is of interest to investigate
experimentally whether a similar angular structure is present in the various
spectral features of the SEE signals and to compare the results with radar
and other observations of RF-pump-induced effects. To this end we describe a
simple two-element radio interferometer designed to search for aspect angle
dependence of SEE features. We present an example of the initial data produced
by this system, and draw preliminary conclusions based on the example data.
Full Article in PDF (5384 KB) |
|
|
