Abstract
OBSERVATIONS from the geosynchronous spacecraft ATS6 of magnetic oscillations transverse to the Earth's magnetic field in the presence of a plasma ion distribution with a ‘bump-in-tail’ form are reported here. We argue that the oscillations are Alfven waves, that the non-monotonicity (that is, population inversion) of the energy distribution provides a source of free energy for the oscillations, and that the particle–wave interaction is through bounce resonance1. The data reported here comes from the UCLA fluxgate magnetometer2 and the UCSD particle detector which measures ions and electrons with energies from 10 eV to 80 keV (ref. 3). The magnetometer data are displayed in a field aligned coordinate system in Fig. 1. A wave with an amplitude of about 5 nT and a period of about 100 s is present. The strongest signal is in Bx, the component in the meridian and perpendicular to the background field, B. The magnetic signal seems typical of a class of pulsations with short wavelengths across B seen in the local afternoon on ATS64. These particular observations were made near 1500 LT.
Published Version
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