Ionospheric electron heating, optical emissions, and striations induced by powerful HF radio waves at high latitudes: Aspect angle dependence

Abstract

[1] In recent years, large electron temperature increases of 300% (3000 K above background) caused by powerful HF-radio wave injection have been observed during nighttime using the EISCAT incoherent scatter radar near Tromsø in northern Norway. In a case study we examine the spatial structure of the modified region. The electron heating is accompanied by ion heating of about 100 degrees and magnetic field-aligned measurements show ion outflows increasing with height up to 300 m s À1 at 582 km. The electron density decreases by up to 20%. When the radar antenna was scanned between three elevations from near field-aligned to vertical, the strongest heating effects were always obtained in the field-aligned position. When the HF-pump beam was scanned between the same three positions, the heating was still almost always strongest in the field-aligned direction. Simultaneous images of the 630 nm O( 1 D) line in the radio-induced aurora showed that the enhancement caused by the HF radio waves also remained localized near the field-aligned position. Coherent HF radar backscatter also appeared strongest when the pump beam was pointed field-aligned. These results are similar to some Langmuir turbulence phenomena which also show a strong preference for excitation by HF rays launched in the field-aligned direction. The correlation of the position of largest temperature enhancement with the position of the radio-induced aurora suggests that a common mechanism, upper-hybrid wave turbulence, is responsible for both effects. Why the strongest heating effects occur for HF rays directed along the magnetic field is still unclear, but self-focusing on field-aligned striations is a candidate mechanism, and possibly ionospheric tilts may be important.

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

“…If the pump HF radio waves propagate sufficiently close to parallel to Earth's magnetic field B, the waves can be trapped, thereby depositing most of the HF energy in the plasma (Gurevich et al, 2001). Observations which confirm this prediction were made simultaneously at EISCAT/HEATING facility at Tromsø (Leyser et al, 2000;Kosch et al, 2002;Rietveld et al, 2003) and HAARP (Pedersen and Gurevich et al, 2002;Pedersen et al, 2003). This strong enhancement of trapped HF waves near parallel to B, leading to the magnetic zenith effect, has since been studied at these and other HF facilities.…”

“…The generation of strong Langmuir turbulence with upper hybrid waves as the primary source of Langmuir waves has been proposed by Mishin et al (2004), who has shown an extended strong-turbulence region. Rietveld et al (2003) have shown that in the course of EISCAT/HEATING experiments, where both the HF pump beam and the UHF radar beam were scanned in elevation between magnetic zenith and vertical directions, the heating effects, such as large electron temperature increases, intense AFAIs, and HF-induced optical emission, all maximized during HF pumping towards MZ. They also showed that large electron temperature increases of 3000 K above the background were accompanied by ion heating of about 100 K, and electron density decreases of up to 20%.…”

“…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.…”

“…Note that ionosondes provide information about the status of the ionosphere above the heating facility as a whole, but they cannot reproduce the peculiarities in the different parts of the heated volume with high temporal resolution. Hence there is a need to compare data from the dynasonde (Rietveld et al, 2008) and the EISCAT UHF radar directed towards magnetic zenith.…”

“…The status of the ionosphere in the course of the experiment was checked by an ionosonde (dynasonde) at Tromsø (Rietveld et al, 2008). Ionograms were taken every two minutes.…”