Observations of E region irregularities generated at auroral latitudes by a high‐power radio wave

Abstract

In September 1983 a series of ionospheric modification experiments was performed in Scandinavia using the high‐frequency (HF) heating facility located near Tromsø, Norway. The experiments were designed to study the production of geomagnetic field‐aligned irregularities in the auroral E region by a powerful HF radio wave. In this initial report, observations of 3.2‐m irregularities made with a mobile 46.9‐MHz field radar are presented. When ionospheric conditions are correct, irregularities having peak cross sections of about 104 m² are excited in the E region over Tromsø. The present results are consistent with theoretical studies which indicate that it is easier to generate short‐scale field‐aligned irregularities at locations where the geomagnetic dip angle is large. When the HF‐induced radar echo at 46.9 MHz is strong, the e‐folding growth times of the echo power are typically between 50 and 150 ms. However, when the E region echo over Tromsø is weak, the growth period can range from seconds to tens of seconds. The irregularities generated by the HF wave often exhibit dynamic structure both in time and in space. In general, the observations provide experimental support for instability theories that employ interactions near the upper hybrid resonance to drive the artificial irregularities. The results also indicate that naturally occurring irregularities may at times play a key role in the excitation of HF‐induced irregularities in the auroral E region. Additional processes are probably required to explain the slower irregularity growth rates often observed when radar echoes are relatively weak and occasionally observed even for relatively strong signals.

Theoretical model for long time stimulated electromagnetic emission generation in ionospheric radio modification experiments

“…In the work of Leyser et al [1989] it was reported that several of the prominent spectral features are absent when the applied frequency is near a harmonic of the electron gyrofrequency. After that, emphasis has been put on the possible role of smallscale magnetic field-aligned density striations, which are known to be generated in these experiments [Fialer, 1974;Minkoff et al, 1974;Djuth et al, 1985]. This overall view is strongly supported by later experimental findings, as will be further discussed in the next Gurevich et al, 1995].…”

Ionospheric HF pump wave triggering of local auroral activation

“…This led to the generation of AFAIs, which scattered the 9-and 12-MHz signals used for Doppler diagnostics. It is believed [Djuth et al, 1985;Noble et al, 1987] that a thermal resonance instability at the upper hybrid (UH) level is the strongest candidate for the excitation of AFAIs in the E region.…”

Phenomena induced by powerful HF pumping towards magnetic zenith with a frequency near the F-region critical frequency and the third electron gyro harmonic frequency

“…Djuth et al (1985) found from VHF radar observations during Tromsø HF heating experiments, that the generation of striations in the auroral E-region was possible when the heater frequency, f H , exceeded the critical frequency foEs by 0.2 MHz. The same result was obtained by Blagoveshchenskaya et al (1999Blagoveshchenskaya et al ( , 2006a when strong heaterinduced striations were observed in the auroral E-region with bi-static HF radio scatter.…”

“…The excitation of artificial small-scale field-aligned irregularities or striations (Minkoff et al, 1974;Djuth et al, 1985;Blagoveshchenskaya et al, 1999Blagoveshchenskaya et al, , 2006b), HF-induced electron heating (Rietveld et al, 2003) and airglow (Brandstrom et al, 1999;Pedersen and Carlson, 2001;Kosch et al, 2002), anomalous absorption (Robinson, 1989) and other phenomena occur at the upper hybrid resonance altitude in HF heating experiments when O-mode polarized HF pump waves reach the ionospheric reflection height. An O-mode HF pump wave couples through striations into electrostatic (upper hybrid) waves at the upper-hybrid resonance altitude, which is below the reflection height of the heating wave.…”