An electric noise component with density f−1 identified on ISEE 3

Abstract

An electric noise component with an f−1 spectrum is observed with the SBH radioastronomy receivers on ISEE 3 at frequencies lower than the plasma frequency fp. On the Z antenna (electrical length for long waves is 7 m) this component is 5–10 times more intense than the predicted thermal noise level. Its spectral density is proportional to fpf−1(Tc)1/2, where Tc is the core electron temperature and f is the observing frequency. On the S antenna (90 m tip to tip) the new component is much weaker and most probably represents the high‐frequency part of a noise spectrum found by Kellogg (1981) with an antenna of the same length. This author interpreted it as mostly due to electron acoustic waves and Doppler shifted ion acoustic waves, but this interpretation has not been confirmed by more accurate calculations (Couturier et al., 1982). Kellogg's spectrum also shows an f−1 frequency dependence and, if extrapolated assuming the same law as for the Z antenna, approximately fits our S antenna observations. The S antenna f−1 noise is deeply spin modulated with the minimum electric field in the direction of the solar wind flow as seen in the spacecraft frame of reference. The modulation factor decreases with increasing frequency, becomes negligible when the new component intensity becomes negligible as compared with thermal noise, and increases with the solar wind velocity. The f−1 component shows some of the properties which are expected from shot noise (direction of minimum intensity) but its spectral index is −1 while shot noise is supposed to show a spectral index −2.