Geophysical phenomena during an ionospheric modification experiment at Tromsø, Norway

Abstract

Abstract. We present an analysis of phenomena observed by HF distance-diagnostic tools located in St. Petersburg combined with multi-instrument observation at Tromsù in the HF modi®ed ionosphere during a magnetospheric substorm. The observed phenomena that occurred during the Tromsù heating experiment in the nightside auroral E s region of the ionosphere depend on the phase of substorm. The heating excited smallscale ®eld-aligned irregularities in the E region responsible for ®eld-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 ®eld lines. Analysis of the Doppler measurement data shows the appearance of quasiperiodic variations with a Doppler frequency shift, f d and periods about 100±120 s during the heating cycle coinciding in time with the ®rst substorm activation and initiation of the upward ®eld-aligned currents. A relationship between wave variations in f d and magnetic pulsations in the Y-component of the geomagnetic ®eld at Tromsù was detected. The analysis of the magnetic ®eld variations from the IMAGE magnetometer stations shows that ULF waves occurred, not only at Tromsù, 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 Tromsù, was closely connected with the heater-induced precipitation of energetic electrons.

Doppler shift simulation of scattered HF signals during the Tromsø HF pumping experiment on 16 February 1996

“…It is interesting to note that similar negative burst-like Doppler tracks were observed from bistatic scatter measurements when turning on the SURA heating facility (see Fig. 6 of Blagoveshchenskaya et al, 1998b).…”

“…There are some grounds to believe that the motion of the heater-induced small-scale irregularities in the auroral E-region is not always consistent with the background plasma drift. Supporting evidence includes a similar Tromsø HF pumping experiment carried out on 17 February 1996 in the nightside auroral ionosphere (Blagoveshchenskaya et al, 1998a). During this experiment on the London -Tromsø -St. Petersburg path, the pump wave reflected from a sporadic E s layer and well-defined heater-induced Doppler tracks were observed.…”

“…Modification experiments in which the nightside auroral ionosphere is irradiated by powerful HF radio waves have revealed a multitude of nonlinear phenomena caused by the interaction of the HF pump wave and ionospheric plasma (Djuth et al, 1985;Noble et al, 1987;Blagoveshchenskaya et al, 1998aBlagoveshchenskaya et al, , 1999Blagoveshchenskaya et al, , 2001Brandström et al, 1999). One…”

Probing of medium-scale traveling ionospheric disturbances using HF-induced scatter targets

“…We report experimental results related to probing medium-scale traveling ionospheric disturbances using HF-induced scatter targets at mid-and high latitudes. With bi-static HF Doppler scatter, the AFAIs are being studied at middle (Blagoveshchenskaya and Troshichev, 1996;Yampolski et al, 1997;Blagoveshchenskaya et al, 1998a) and high latitudes (Blagoveshchenskaya et al, 1998b(Blagoveshchenskaya et al, , 1999(Blagoveshchenskaya et al, , 2006Rietveld et al, 2003). The proposed observational technique was used for the first time to identify the medium-scale TIDs.…”

SPEAR: Early results from a very high latitude ionospheric heating facility

“…It is well known that O-mode heating is capable of generating artificial field aligned irregularities (AFAI) using high power HF radio waves (e.g. Hedberg et al, 1983;Djuth et al, 1985;Blagoveshchenskaya and Troshichev, 1996;Yampolski et al, 1997;Blagoveshchenskaya et al, 1998;Eglitis et al, 1998;Robinson, 1989;Rietveld et al, 2003). These irregularities are detectable by both incoherent and coherent scatter radars (e.g.…”