Isotropization of the terrestrial low-frequency radio bursts

We analyze ISEE 3 radio data acquired in the solar wind. Using a new techni que we measure the spectrum of the isotropic component (''tail'' or isotrop ic terrestrial kilometric radiation (ITKR)) of low-frequency (LF) bursts wh ich extends from similar or equal to f(p.sw) (the interplanetary (IP) mediu m plasma frequency) to an upper limit f(max) which can reach 5x f(p.sw). Th e relative intensities of the auroral kilometric radiation (AKR) burst, oft en associated with LF bursts, and of the ITKR are highly variable when obse rved from the Lagrange point L1 most probably because of propagation effect s. In the range of frequencies 2f(p.sw) < f < 5f(p.sw) the AKR source angul ar radius measured from L1 is a few degrees, while that of the ITKR source is close to 90 degrees: the radiation from these two sources must propagate through very different regions of the magnetosphere. The isotropization of the radiation from a magnetospheric source at f(max) requires the presence of large overdense structures in the IP medium. For each event, such struc tures were identified and their peak plasma frequency f(p.bump) measured us ing ISEE 3 plasma density data acquired in the ecliptic. The frequency f(ma x) is always larger than f(p.bump) and there is only a weak correlation bet ween these quantities. Thus f(p.bump) cannot be taken as a characteristic o f the three-dimensional IP structures needed to isotropize the radiation. T hese structures should be rough and capable of sending some radiation towar ds the Sun to reach IP regions where higher values of f(p.sw) will be encou ntered. The study of the isotropization of LF bursts requires a deeper know ledge of the dense structures of the IP medium.