A New Method to Identify Propagation and Evolution of Electromagnetic Plasma Waves Based on Parker Solar Probe Measurements

The solar wind in the inner heliosphere has been observed by Parker Solar Probe (PSP) to exhibit abundant wave activities. However, their origin and evolution remain a mystery. Specifically, it remains unknown whether it is an emitted signal from the solar atmosphere or an eigenmode growing locally in the heliosphere due to plasma instability. To address and resolve this controversy, we develop a new dedicated method.

In this method, we have the following three steps. (1) We first obtain the calibrated electric field by fitting the equivalent parameters of the antenna. (2) Based on the information of electric and magnetic fields, we establish a set of judgment criteria for diagnosing and identifying different wave modes with different propagation directions based on the wave energy flux density, wave magnetic helicity, and electric field polarization. (3) We propose the formulas for calculating the dispersion relationship and the growth/decay rate spectrum, which lays a foundation for detecting and diagnosing the dispersion propagation and dynamic evolution of space plasma wave modes.

Then, we apply this method to the PSP measurements during its perihelion encounter. Based on the multi-parameter dynamic spectra obtained from the calibrated electric and magnetic fields analysis, we find that the turbulence of the primary solar wind often contains quasi-parallel fast-magnetosonic/whistler waves propagating outward. We further use this method to measure the dispersion relationship and evolution state of the detected fast-magnetosonic/whistler waves. The results show that the fast-magnetosonic/whistler waves have a propagation speed comparable to the solar wind speed in the inner heliosphere (~200 km / s), which leads to the failure of the Taylor freezing hypothesis. Moreover, the growth/decay rate spectrum obtained with the new method clearly shows that the fast-magnetosonic/whistler waves encountered by the PSP have a noticeable growth rate spectrum and are in the process of rapid growth. This set of analysis methods makes full use of and gives full play to the measurements from PSP's FIELDS and SWEAP suites, providing a new research paradigm for studying the inner heliospheric space plasma fluctuations, and finding its application prospects in future space plasma exploration missions.