Observation of the topside ionospheric MF/HF radio emission from

Abstract. We present observations of MF/HF ionospheric radio emission observed between 4-7 Re in the terrestrial magnetosphere by the Wind spacecraft. During a perigee pass, and AKR episode, two distinct radio signatures are observed: an intense, sporadic emission near f 1:8 MHz, and a more steady and weaker emission near f 4:4 MHz. These emissions show characteristics similar to ground ob-servations of ionospheric ‘auroral roar’, although to propa-gate to the spacecraft they must have been generated above the ionospheric F-layer peak. We suggest that the emissions are generated on the topside ionosphere where the condition fuh nfce is met. These observations represent a new com-ponent of the natural terrestrial radio spectrum as viewed from space