Observations of the magnetic fluctuation enhancement in the earth's foreshock region

Upstream waves have been postulated to be a major source of energy for the dayside magnetic pulsations within the magnetosphere. Thus, it is of interest to determine over what frequency range in the ion foreshock the power of fluctuations in the solar wind is enhanced. The magnetic field data from pairs of spacecraft, when they stay on either side of the ion foreshock boundary, were examined. It was found that the power of magnetic fluctuations is enhanced only at periods less than about two minutes, not at longer periods. Thus the upstream waves may contribute to Pc 3 and Pc 4 pulsations in the dayside magnetosphere, but they cannot be directly responsible for the longer-period waves.