Isotropization of the terrestrial low-frequency radio bursts

We analyze ISEE 3 radio data acquired in the solar wind. Using a new technique we measure the spectrum of the isotropic component (''tail'' or isotropic terrestrial kilometric radiation (ITKR)) of low-frequency (LF) bursts which extends from similar or equal to f(p.sw) (the interplanetary (IP) medium plasma frequency) to an upper limit f(max) which can reach 5x f(p.sw). The relative intensities of the auroral kilometric radiation (AKR) burst, often associated with LF bursts, and of the ITKR are highly variable when observed from the Lagrange point L1 most probably because of propagation effects. In the range of frequencies 2f(p.sw) < f < 5f(p.sw) the AKR source angular 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 structures were identified and their peak plasma frequency f(p.bump) measured using ISEE 3 plasma density data acquired in the ecliptic. The frequency f(max) is always larger than f(p.bump) and there is only a weak correlation between these quantities. Thus f(p.bump) cannot be taken as a characteristic of the three-dimensional IP structures needed to isotropize the radiation. These structures should be rough and capable of sending some radiation towards the Sun to reach IP regions where higher values of f(p.sw) will be encountered. The study of the isotropization of LF bursts requires a deeper knowledge of the dense structures of the IP medium.