LOW-FREQUENCY WAVES IN MAGNETOSPHERE

in the magnetosphere are found low‐frequency (100–100,000 Hz) electromagnetic waves generated externally by lightning and ground‐based transmitters and internally by energetic charged particles. Dispersion and anisotropy characterize the medium, and both ducted and nonducted paths of propagation are found. Lightning‐generated ducted ‘whistlers,’ observed on the ground, give information on the distribution of electrons and the latitudinal drift motions of ducts in the magnetosphere. Nonducted whistlers observed in satellites give information on electron and ion concentrations, the earth's magnetic field, and wave damping by energetic particles. Noise generated in the magnetosphere (or ionosphere) includes steady noise, or hiss, and the structured forms known as discrete emissions. in the auroral zone, hiss is seen in close association with the aurora and over a frequency range of at least 4 kHz to several hundred kHz. It has been attributed tentatively to incoherent Cerenkov radiation from the incoming auroral particles. At middle latitudes, noise from the magnetosphere is mostly of the discrete type with associated hiss and appears to be limited to frequencies below the minimum gyrofrequency on the path of propagation. A related type of noise, called ‘polar chorus,’ appears at high latitudes below about 1500 Hz. Discrete emissions may occur spontaneously or be triggered by whistlers or man‐made signals from the ground. The upper cutoff frequency is controlled by the minimum gyrofrequency on the path of propagation, indicating that the region of generation lies in the equatorial plane. Discrete emissions have been attributed to cyclotron resonance between whistler‐mode waves and electrons. Emissions offer new possibilities for studying wave‐particle interactions in the earth's magnetosphere and in other plasmas, both natural and man‐made. Copyright © 1969 by the American Geophysical Union.