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Ann. Geophys., 14, 1437-1453, 1996
www.ann-geophys.net/14/1437/1996/
© European Geosciences Union 1996
Artificial periodic irregularities in the auroral ionosphere
M.T. Rietveld1,2, E. Turunen3, H. Matveinen3, N. P. Goncharov4, and P. Pollari5
1Max-Planck-Institut für Aeronomie, Postfach 20, D-37191 Katlenburg Lindau 3, Germany (rietveld@mirage.mpae.gwdg.de)
2also at EISCAT, N-9027 Ramfjordmoen, Norway
3Sodankylä Geophysical Observatory, FIN-99600 Sodankylä , Finland (esa@sgo.fi)
4Radiophysical Research Institute (NIRFI), Nizhny Novgorod, 603600 Russia (gonch@pole.kis.nnov.su)
5Department of Physical Sciences, University of Oulu, FIN-90570 Oulu, Finland (pp@skynet.oulu.fi)
Abstract. Artificial periodic irregularities (API) are
produced in the ionospheric plasma by a powerful standing electromagnetic wave
reflected off the F region. The resulting electron-density irregularities can
scatter other high-frequency waves if the Bragg scattering condition is met.
Such measurements have been performed at mid-latitudes for two decades and have
been developed into a useful ionospheric diagnostic technique. We report here
the first measurements from a high-latitude station, using the EISCAT heating
facility near Tromsø, Norway. Both F-region and lower-altitude ionospheric
echoes have been obtained, but the bulk of the data has been in the E and D
regions with echoes extending down to 52-km altitude. Examples of API are shown,
mainly from the D region, together with simultaneous VHF
incoherent-scatter-radar (ISR) data. Vertical velocities derived from the rate
of phase change during the irregularity decay are shown and compared with
velocities derived from the ISR. Some of the API-derived velocities in the
75–115-km height range appear consistent with vertical neutral winds as shown by
their magnitudes and by evidence of gravity waves, while other data in the
50–70-km range show an unrealistically large bias. For a comparison with ISR
data it has proved difficult to get good quality data sets overlapping in height
and time. The initial comparisons show some agreement, but discrepancies of
several metres per second do not yet allow us to conclude that the two
techniques are measuring the same quantity. The irregularity decay
time-constants between about 53 and 70 km are compared with the results of an
advanced ion-chemistry model, and height profiles of recorded signal power are
compared with model estimates in the same altitude range. The calculated
amplitude shows good agreement with the data in that the maximum occurs at about
the same height as that of the measured amplitude. The calculated time-constant
agrees very well with the data below 60 km but is larger above 60 km by a factor
of up to 2 at 64 km. The comparisons with the model are considered to be a good
basis for more refined comparisons.
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