Abstract
We have examined high time resolution magnetic field data for different separations of ISEE 1 and 2 in the Earth's ion foreshock region to study the coherence length of upstream ULF waves. By examining the correlation coefficients of the low frequency waves as a function of separation distance, we find that the correlation coefficient depends mainly on the separation distance of ISEE 1 and 2 transverse to the solar wind flow. It drops to about 0.5 when the transverse separation is about 1 RE, a distance much larger than the proton thermal gyroradius in the solar wind. Thus the coherence length of the low‐frequency waves is about one Earth radius, which is of the order of the wavelength, and is consistent with that estimated from the bandwidth of the waves.
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