First Detection of Low-frequency Striae in Interplanetary Type III Radio Bursts

We report the first detection of type III solar radio burst striae in the 30–80 kHz range, observed by the Cluster-4 spacecraft during an exceptionally quiet solar period. These low-frequency fine structures, which drift slowly in frequency and exhibit narrow bandwidths, provide a novel diagnostic of plasma processes in the inner heliosphere. The detected striae, interpreted as fundamental plasma emission, exhibit a frequency drift rate of 0.328 Hz s⁻¹ and a bandwidth of 1.3 kHz. By combining high-resolution radio observations with well-calibrated in situ electron velocity distribution function data from the Wind spacecraft, we characterized the plasma properties of the burst source region near 0.32 au. Our analysis estimates relative density fluctuations, at the effective turbulence scale length, as approximately 3.4% (inferred from striae bandwidths), 0.62% (from intensity fluctuations), and 3.5% (from a heliocentric distance-based empirical model). These findings offer critical insights into small-scale density inhomogeneities and turbulence that affect electron beam propagation. This study underscores the potential of combining well-calibrated in situ electron data with radio burst measurements to probe the physical conditions of the solar wind and to refine our understanding of solar radio bursts across a broad frequency range.