Isotropization of the terrestrial low‐frequency radio bursts

Abstract

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 ≃ fp.sw (the interplanetary (IP) medium plasma frequency) to an upper limit fmax which can reach 5×fp.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 Ll most probably because of propagation effects. In the range of frequencies 2fp.sw < f < 5fp.sw the AKR source angular radius measured from Ll is a few degrees, while that of the ITKR source is close to 90°: the radiation from these two sources must propagate through very different regions of the magneto sphere. The isotropization of the radiation from a magnetospheric source at fmax requires the presence of large overdense structures in the IP medium. For each event, such structures were identified and their peak plasma frequency fp.bump measured using ISEE 3 plasma density data acquired in the ecliptic. The frequency fmax is always larger than fp.bump and there is only a weak correlation between these quantities. Thus fp.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 fp.sw will be encountered. The study of the isotropization of LF bursts requires a deeper knowledge of the dense structures of the IP medium.