Radial Spectral Variations of Elsasser Variable z- in the Slow Solar wind from 0.1 au to 0.5 au

The radial variations and nature of Elsasser Variable z- are important questions in the solar wind turbulence. A background spectrum of z- from 0.3 au to 1.0 au was observed by Helios, suggesting that z- results from the structures. However, it is more prevailing for the idea that z- represents the inward Alfven waves. Here, we analyze the radial spectral variations from 0.1 au to 0.5 au in the slow solar wind using the measurements from Parker Solar Probe. We find that the averaged power spectra of z- do not have obvious radial variations from 0.2 au to 1.0 au and are a bit higher from 0.1 au to 0.2 au and there exist no low-frequency breaks. We show the averaged coherence spectra between z+ and z- for the first time and found that the coherence decreases from around 0.3 to around 0.1 as the frequency increases from 0.2 au to 0.5 au. Inside 0.2 au, the parallel coherence is lower than the perpendicular coherence, which decreases from around 0.5 to around 0.3 as the frequency increases. We try to give explanations for these radial spectral variations by presenting detailed analyses for two intervals on the time series, self-correlation functions and power spectra and provide evidence for the idea that z- is a combination of structures and noises. These results help understand the nature and evolution of z- in the solar wind turbulence.