Rocket observation of VLF hiss in aurora

Abstract

Three rocket payloads with VLF receivers have been launched during auroral disturbances to study VLF hiss in the ionosphere. One payload was launched during relatively quiet conditions and penetrated a quiet auroral arc. Another payload was launched during the break up phase of the aurora at the start of a magnetic substorm. The third payload was launched in a situation that was similar to an after break up auroral glow situation. From the 7.35 kHz hiss observations on these three payloads it is concluded that in some circumstances auroral hiss is generated in the auroral arcs at altitudes of 105–110 km. The observations also show that auroral hiss often is present in the ionosphere even if it is not observed on the ground. The observed variation in intensity of auroral hiss is interpreted as being caused by absorption as the hiss propagates away from a generation region at 105–110 km altitude or propagates down from greater altitudes. This interpretation together with the occasional spin modulation of the observed signals indicates that the wave normal direction for auroral hiss often is within a few degrees of the resonance cone (89.7°).