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

Abstract

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−1 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(1D) 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.