 |
| Titel |
Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude |
| VerfasserIn |
E. Malnes, N. Bjørnå, T. L. Hansen |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
0992-7689
|
| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 14, no. 12 ; Nr. 14, no. 12, S.1328-1342 |
| Datensatznummer |
250012499
|
| Publikation (Nr.) |
 copernicus.org/angeo-14-1328-1996.pdf |
|
|
|
|
|
| Zusammenfassung |
| We have observed a number of strong echoes
with the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5°
and 78.6° with the geomagnetic field and at about 100-km altitude north in the
auroral zone. The echoes are short-lived and occur in single 2- or 10-s data
dumps. They are offset by 125–130 kHz with respect to the transmitted frequency.
In most cases the offset compares well with the frequency of gyro lines in the
incoherent-scatter spectrum, as given by the standard linear dispersion
relation. But sometimes the measured offsets deviate significantly from the
model calculations, and the interpretation in terms of gyro lines becomes
questionable. The discrepancy could possibly be explained by local deviations in
the magnetic field from the model (IGRF 1987), which are generated by incoming
particle beams. A more serious problem with the gyro-line theory is how the line
can be excited at altitudes where the collisional damping is substantial. The
high intensity and short lifetime of the signal point to a fast-growing plasma
instability as the likely excitation mechanism, if the gyro-line interpretation
is correct. The cause of the instability could be the same particle beams as
those causing the disturbances in the magnetic field. Alternatively, the
observations may be interpreted as meteor head echoes. The large Doppler shifts,
the short lifetimes and the altitudes of the signals support this explanation.
The main difficulty is that the distribution of measured offsets appears to be
different in magnetically active conditions and in less active conditions. Also,
the occurrence of echoes does not seem to follow the expected changes in meteor
density. More observations in different conditions are needed to decide between
the two interpretations. As it is, we are inclined to believe in the meteor head
echo theory, the objections to the gyro-line theory being more fundamental. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|