Quasi-thermal Noise in the Heliosphere, Including the Electron, Proton, and Shot Noise

Abstract The total noise spectrum in the frequency band near the plasma frequency f p plays an important role in plasma and radio wave detections in the solar wind. The noise comes from the electron and proton quasi-thermal noise (QTN) and the shot noise, and their radial distributions in the heliosphere are investigated based on plasma measurements from several spacecraft. Power spectral profiles of the generic total noise are calculated at different heliocentric radial distances, and the separate contributions from the three noises with different antenna lengths are then given. Our results show that one could perform QTN diagnostics of plasma density and temperature within a wider range of heliocentric distance if a longer antenna were to be used. The influence of model parameters—especially the drift velocity of the solar wind—on the QTN spectrum is further discussed. In general, a larger density and higher temperature causes a more significant proton QTN, while the drift velocity directly leads to the Doppler shift effect on the noise spectrum. A new approximation is proposed to estimate the profile of the electron QTN with considerable drift velocity, but it should be treated cautiously in practical situations, because of the rough simplification and inevitable overestimation of the shift. This work may provide a reference for future deep-space explorations in the whole heliosphere and also for the design of antenna systems.