EMIC waves in Earth's Magnetosphere

Abstract:

Since the intensification of studies of “hydromagnetic emissions” and “pearls” observed in ground records in the late 1950s and early 1960s, continued investigations of waves in the Pc 1-2 frequency band (0.2-5.0 Hz) have provided increasingly detailed characterizations of both the occurrence and properties of these waves and of the plasma processes in Earth’s magnetosphere that generate them and control their propagation. Their generation is now attributed, in at least most cases, to the electromagnetic ion cyclotron (EMIC) instability of ion distributions with positive temperature anisotropy. Although many early observations focused on regions near the plasmapause and on the instability of ring current ions, comprehensive ground-satellite studies using elliptically orbiting spacecraft and those in geostationary orbit have extended the range of observed occurrence of these waves from below L = 2 to near the magnetospheric boundary, and even into the polar cap and distant plasma mantle. The role of EMIC waves in thermalizing plasma sheet and ring current ions and also in heating warm He⁺ ions is clear, and their possible role in depleting relativistic radiation belt electrons is becoming increasingly circumscribed. Observations of energetic proton precipitation (proton aurora) are also providing new observational means to identify EMIC wave-particle interactions; these observations may supplement ground-based wave observations during disturbed conditions. However, the “pearl” structure of waves observed in ground data remains poorly understood, and most recently, observations of purely compressional and of purely electrostatic Pc1 waves have posed new observational and theoretical challenges.