Radial Evolution of the Near-Sun Solar Wind: Parker Solar Probe Observations

A statistical study of the radial evolution of the solar wind within 0.3 au is shown in this Letter based on Parker Solar Probe observations. We show the radial distribution of the main solar wind parameters, including the solar wind speed V sw, magnetic field divide B divide , the number density of electrons N e , protons N p , and alpha particles N alpha , and the temperature of protons T p and alpha particles T alpha . The power-law fitting results of these parameters in the near-Sun solar wind are compared with previous radial models. We also show the radial distribution of the angle between the magnetic field B , solar wind V sw, and radial vector R . In the solar wind within 0.3 au, theta BVsw , and theta BR mainly concentrate around 135 degrees, and theta RVsw almost concentrates in the region less than 20 degrees. Furthermore, we also present the radial distribution of the relative values between the solar wind parameters, including the electric neutrality estimation ((2 x N alpha + N p )/N e similar or equal to 1 within 0.2 au), relative number density ratio (N alpha /N p similar or equal to 0.02 in slow solar wind and N alpha /N p similar or equal to 0.04 in faster solar wind), relative temperature ratio (T alpha /T p decreases with the increase of heliocentric distance and its decay rate is larger in faster solar wind), and differential speed (both V alpha - V p and (V alpha - V p )/V A are larger in the faster solar wind and decrease as heliocentric distance increases).