The space shuttle as a platform for observations of ground‐based transmitter signals and whistlers

Abstract

Recent experimental and theoretical studies indicate that coherent VLF waves, such as lightning‐generated whistlers and signals from ground‐based VLF transmitters, can often produce significant pitch angle scattering in energetic particles in the magnetosphere. However, the relative importance of these waves in controlling the lifetimes of energetic particles is only partially understood due to limited knowledge of the global distribution of the coherent waves throughout the magnetosphere. The present paper presents a preliminary global study of VLF transmitter signals and low‐latitude whistlers received at 245 km altitude on the space shuttle. The observations were made in a 5‐day period during the STS 3 mission of the space shuttle in March 1982. The threshold sensitivity of the wave receiver when mounted in the shuttle bay was 0.3 pT ± 10 dB (set by the shuttle electromagnetic interference), which was sufficient to detect the whistler mode signals in large regions of the ionosphere. We find that the direct signals from a 10‐kW transmitter located at 28°S magnetic latitude were received in a roughly circular region with a diameter of 6000 km centered around the transmitter. The signals propagating through the magnetosphere from a 500‐kW magnetically conjugate transmitter at 40°N magnetic latitude were received inside a region extending 5000 km in longitude and 2000 km in latitude. Finally, the direct signals from a 1 MW‐transmitter at 31°S magnetic latitude were received in a region extending 22,000 km in longitude, while the latitudinal extent (5000 km) was limited by the shuttle orbit and the day/night terminator. Except for one case, signals were received only during nighttime. Extremely small dispersion whistlers were detected on L‐shells between 1.04 and 1.11, suggesting that the lack of ducted whistlers on magnetic field lines in this range is not due to transmission loss across the D region boundary or to high ionospheric absorption loss, but most likely to a lack of ducts.