Comparison of Langmuir wave frequency f_pk with the electron plasma frequency f_pe, SC calculated from the spacecraft potential: (a) Scatterplot of f_pk/f_pe, SC versus n_e, SC, where each point corresponds to a single TDS snapshot where Langmuir waves are observed; (b) Number density n_e, pk calculated from Langmuir wave frequency versus n_e, SC. (c) n_e, pk − n_e, SC versus n_e, SC; (d) f_pk/f_pe, SC versus θ_Bx. The different colored symbols correspond to the different time intervals indicated in the legend in panel a.