Investigating WIND quasi-thermal noise spectra between the ion and the electron plasma frequency

When immersed in plasma, electric field antennas measure fluctuations due to the thermal motion of ambient particles. This phenomena is called quasi-thermal noise (QTN). Analysis of QTN spectra in the solar wind yields accurate measurements of local electron density and core temperature, as well as somewhat less accurate measurements of wind speed and suprathermal electron properties. The thermal noise receiver (TNR) on board the WIND spacecraft provides high cadence spectra (4.5 sec in normal mode) with clear signatures of QTN. However, at frequencies between the ion and electron plasma frequencies, the measured TNR spectra at this high-time resolution often deviates from the theoretical spectra calculated with plasma parameters directly measured by the WIND/3DP instrument. We report result of a careful study of two possible sources of the deviation, namely, the spacecraft spin which modifies the proton noise, and coherent electrostatic waves. We discuss implications for applying the QTN technique for plasma diagnosis.