Terrestrial low-frequency bursts: Escape paths of radio waves through the bow shock

From the analysis of 119 low-frequency (LF) burst spectra observed onboard the Wind spacecraft, we propose an interpretation of the frequency–time characteristics including the low frequency cutoff of the LF burst spectra, and we use these characteristics to sound the bow shock structure at large tailward distances from Earth. When observed from within the solar wind, LF bursts appear to be made of two spectral components. The high frequency one is bursty and observed above twice the solar wind plasma frequency fpsw. The low frequency one is diffuse (ITKR) and its spectrum extends from about 2fpsw to a cutoff frequency fc not much higher than fpsw; its onset time δt(f) increases as the frequency f decreases. For each of the 119 events observed from near the Lagrange point L1, the solar wind density variations were measured and the variations of the density jump across the shock calculated from plasma data all along a shock model over more than 2000RE. But, except for a few events, neither the solar wind overdensities nor the shock density barrier can prevent waves with frequencies below fc from reaching the spacecraft. Scattering on plasma density inhomogeneities was then introduced to account for the propagation of the LF burst waves in the magnetosheath, from near Earth to their escape point through the bow shock at a frequency-dependent distance |Xesc(f)| (GSE), and then in the solar wind to the spacecraft. In such media, at frequencies between 2fpsw and fpsw, the bulk speed of the scattered waves decreases rapidly as f decreases, and this accounts for the observed variations of the onset time δt(f). Angular scattering can also account for the observed cutoff at fc if the distance |Xesc(f)| increases exponentially when f/fpsw decreases. As the shock model we used meets that requirement, we consider that this model is valid, which implies that the bow shock still exists beyond 1000RE from the Earth. The observed decrease of the average spectral intensity of the LF burst between about 1.5fpsw and 2fpsw can also be explained by the scattering in the solar wind if we take into account the angular distribution of the rays when they leave the bow shock.