ELF hiss associated with plasma density enhancements in the outer magnetosphere

The low-frequency, narrow hiss bands associated with plasma density enhancements in the magnetosphere near and beyond L = 4 have been studied with the search coil magnetometer and ion mass spectrometer data from Ogo 5. ELF hiss is found to accompany most of the detached plasmas in the outer magnetosphere and to be sharply limited by the steep ion density gradient at their boundary. The observations indicate that the hiss originates in and is ducted by the plasma density enhancements. In a particularly favorable case after an interval of low AE and Dst indices a series of enhanced plasma density peaks were observed between L = 4 and L = 6 in the afternoon local time sector. In this case it was possible to develop an empirical relation between hiss amplitude, plasma density, and L by using in situ simultaneous measurements.