Observations of Spatial Collapse of Langmuir Waves in Solar Type III Bursts

Thejappa Golla, Univ Maryland

High-time-resolution in situ wave observations show that Langmuir waves associated with solar type III radio bursts often occur as coherent localized one-dimensional magnetic-field-aligned wave packets with short durations of a few milliseconds and peak intensities well above the strong turbulence thresholds. We report the observations of one of such wave packets obtained by the time domain sampler of the STEREO WAVES experiment, which is unique in the sense that it is the most intense wave packet ever detected in association with a solar type III radio burst, with peak intensity of $E_t \sim 215$ mVm$^{−1}$, which is equivalent to the normalized peak energy density of $\frac{W_L}{n_eT_e} \sim 2.3\times 10^{-2}$. We show that this wave packet provides evidence for (1) oscillating two-stream instability (OTSI), (2) a collapsing soliton formed as a result of OTSI, and (3) a density cavity created by its ponderomotive force. We also show that the peak intensity and spatial width of this wave packet also satisfy the threshold condition for it to be the collapsing Langmuir wave packet formed as a result of nucleation processes even when $\delta n_b > \delta n_p$, where $\delta n_b$ and $\delta n_p$ are the levels of background and ponderomotive-force-induced density fluctuations, respectively. Thus, these observations provide unambiguous evidence for the spatial collapse of Langmuir waves in the source region of a type III radio burst, and the observed spectral evidence for OTSI and the ponderomotive-force-induced density cavity strongly suggest that the OTSI is mostly likely responsible for the collapse of the observed wave packet. We also report that the FFT spectrum of this wave packet contains peaks at harmonics of the electron plasma frequency, $f_{pe}$ up to n = 5, whose peak intensities fall off with increasing frequency. We discuss the implication of these observations for theories of solar radio bursts.