Quasi-thermal noise (QTN) spectroscopy is an efficient tool for measuring in situ macroscopic plasma properties in space, using a passive wave receiver at the ports of an electric antenna [1]. The Radio Frequency Spectrometer (RFS) is a dual channel digital spectrometer, designed for both remote sensing of radio waves and in situ measurement of electrostatic fluctuations using signals from the V1-V4 electric field antennas [2]. Usually, the two RFS channels record differences between V1-V2 and V3-V4 antennas (dipole mode). It allows us to retrieve plasma properties independently by two sets of antennas. The plasma line is automatically identified in a frequency range determined by the density model based on spacecraft distance from the Sun. The frequency range is manually adjusted for intervals when the plasma line occurs at lower or higher frequencies than predicted. We assume that the plasma line is well identified if detected at the same frequency by the V1-V2 and V3-V4 dipoles simultaneously. We assume that the plasma frequency is equal to the geometric mean of the plasma line and the preceding frequency channel. In other words, the plasma frequency corresponds to the steepest positive slope below the plasma line. The provided error bars are calculated from the frequency resolution of the RFS instrument.
[1] Meyer-Vernet, N., Issautier, K., and Moncuquet, M. (2017). Quasi-thermal noise spectroscopy: The art and the practice. Journal of Geophysical Research: Space Physics, 122, 7925-7945. https://doi.org/10.1002/2017ja024449
[2] Bale, S. D., Goetz, K., Harvey, P. R., Turin, P., Bonnell, J. W., Dudok de Wit, T., et al. (2016). The FIELDS instrument suite for Solar probe plus. Measuring the coronal plasma and magnetic field, plasma waves and turbulence, and radio signatures of Solar transients. Space Science Reviews, 204(1-4), 49-82. https://doi.org/10.1007/s11214-016-0244-5