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Ann. Geophys., 16, 1212-1225, 1998
www.ann-geophys.net/16/1212/1998/
© European Geosciences Union 1998
Geophysical phenomena during an ionospheric modification experiment at Tromsø, Norway
N. F. Blagoveshchenskaya1, V. A. Kornienko1, A. V. Petlenko1, A. Brekke2, and M. T. Rietveld3
1Arctic and Antarctic Research Institute (AARI), 199397, St. Petersburg, Russia
2Auroral Observatory, University of Tromsø, N-9037, Tromsø, and The University Courses of Svalbard, Svalbard, Norway
3EISCAT, N-9027 Ramfjørdbotn, Norway
Abstract. We present an analysis of phenomena observed
by HF distance-diagnostic tools located in St. Petersburg combined with
multi-instrument observation at Tromsø in the HF modified ionosphere during a
magnetospheric substorm. The observed phenomena that occurred during the Tromsø
heating experiment in the nightside auroral Es region of the
ionosphere depend on the phase of substorm. The heating excited small-scale
field-aligned irregularities in the E region responsible for field-aligned
scattering of diagnostic HF waves. The equipment used in the experiment was
sensitive to electron density irregularities with wavelengths 12–15 m across the
geomagnetic field lines. Analysis of the Doppler measurement data shows the
appearance of quasiperiodic variations with a Doppler frequency shift, fd
and periods about 100–120 s during the heating cycle coinciding in time with the
first substorm activation and initiation of the upward field-aligned currents. A
relationship between wave variations in fd and magnetic
pulsations in the Y-component of the geomagnetic field at Tromsø was
detected. The analysis of the magnetic field variations from the IMAGE
magnetometer stations shows that ULF waves occurred, not only at Tromsø, but in
the adjacent area bounded by geographical latitudes from 70.5° to 68° and
longitudes from 16° to 27°. It is suggested that the ULF observed can result
from superposition of the natural and heater-induced ULF waves. During the
substorm expansion a strong stimulated electromagnetic emission (SEE) at the
third harmonic of the downshifted maximum frequency was found. It is believed
that SEE is accompanied by excitation of the VLF waves penetrating into
magnetosphere and stimulating the precipitation of the energetic electrons
(10–40 keV) of about 1-min duration. This is due to a cyclotron resonant
interaction of natural precipitating electrons (1–10 keV) with heater-induced
whistler waves in the magnetosphere. It is reasonable to suppose that a new
substorm activation, exactly above Tromsø, was closely connected with the
heater-induced precipitation of energetic electrons.
Key words. Ionosphere (active experiments; ionosphere
· magnetosphere interactions). Radio science (nonlinear phenomena).
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