Low Frequency Waves at the Dayside Auroral Oval

Abstract

This paper is focused on naturally generated waves in the ULF to VLF range observed by ground based receivers near the dayside auroral oval. Generally, this region is characterized by its high level of emission activity over a wide frequency range. The magnetic field is in an interval ~ 3 hours on either side of magnetic noon perturbed by strong micropulsations. Superimposed on these “background pulsations” are intensive pulsation burts associated with enhancements in 6300Å auroral emissions. At extremely low frequencies the dominating emission is a structured hiss band below 1 kHz. Comparisons with satellite recordings show that these waves are generated and propagate on closed field lines. The high latitude cut-off of the emission may give information about the position of the last closed field line. Short, intensive brusts of VLF hiss appear during the midday hours. In the post-noon sector the occurrence of this emission type increases. It is supposed that this is connected to the increased occurrence of localized, short-living discrete auroral structures in this part of the oval.