Probability distribution function of the electric field at t = 277 s that is induced from the central part of the electron beam where E> 0.22 mV/m in the unperturbed case. The top panel has Δn/n = 0. The remaining panels increase Δn/n from Δn/n = 10^-5 to Δn/n = 10^-1.5 in the bottom panel, as indicated on the right-hand side. The black vertical line indicates the thermal level of the electric field from Langmuir waves.