We present a technique for deriving the temperature anisotropy of solar wind protons observed by the Parker Solar Probe mission in the near-Sun solar wind. The variation in the temperature of solar wind protons in the radial direction measured by the SWEAP Solar Probe Cup is compared with variation in the orientation of the local magnetic field measured by the FIELDS fluxgate magnetometer, and the components of the proton temperature parallel and perpendicular to the magnetic field are extracted. This procedure is applied to both moments of the proton velocity distribution function (VDF) and to the results of a non-linear fit of proton core and proton beam Maxwellian components of the VDF, and the results are compared and optimum timescales for data selection and trends in the uncertainty in the method are identified. We find that the moment-based proton temperature anisotropy is more

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Various solar wind ion species move with different speeds and theoretical considerations as well as limited observations in a region close to the Sun show that heavy solar wind ions tend to flow faster than protons, at least in less-aged fast solar wind streams. The solar wind flow carries the frozen-in interplanetary magnetic field (IMF) and this situation evokes three related questions: (i) what is the proper solar wind speed, (ii) is this speed equal to the speed of the dominant component, whatever that may be, and (iii) what is the speed of the magnetic field? We show that simple theoretical considerations based on the MHD approximation as well as on the dynamics of charged particles in electric and magnetic fields suggest that the IMF velocity of motion (de Hoffmann-Teller (HT) velocity) would be deliberated as the velocity appropriate for solar wind studies. Our analysis based on the Wind, Helios, ACE, and SOHO observations of differential streaming of solar wind populations shows that their energy is conserved in the HT frame. On the other hand, the noise and temporal resolution of the data do not allow us to decide whether the total momentum is also conserved in this frame.

We study the polarization properties of the velocity fluctuations in solar wind turbulence using high-resolution data from the Spektr-R spacecraft. The ratio of perpendicular to parallel velocity fluctuations in the inertial range is smaller than the equivalent ratio for magnetic fluctuations, but gradually increases throughout this range. In the kinetic range, there is alarge decrease in the ratio, similar to the magnetic fluctuations. We compare the measurements to numerical solutions for acombination of kinetic Alfvén waves and slow waves, finding that both the slow increase and sharp decrease in the ratio are consistent with amajority population of Alfvén waves and minority population of slow waves in critical balance. Furthermore, the beta-dependence of this scale-dependent ratio can be successfully captured in the model when incorporating abeta-dependent Alfvén to slow wave ratio similar to that observed in the solar wind.

Relative properties of solar wind protons and α particles are often used as indicators of a source region on the solar surface, and analysis of their evolution along the solar wind path tests our understanding of physics of multicomponent magnetized plasma. The paper deals with the comprehensive analysis of the difference between proton and α particle bulk velocities at 1 au with a special emphasis on interplanetary coronal mass ejections (ICMEs). A comparison of about 20 years of Wind observations at 1 au with Helios measurements closer to the Sun (0.3–0.7 au) generally confirms the present knowledge that (1) the differential speed between both species increases with the proton speed; (2) the differential speed is lower than the local Alfvén speed; (3) α particles are faster than protons near the Sun, and this difference decreases with the increasing distance. However, we found a much larger portion of observations with protons faster than α particles in Wind than in Helios data and attributed this effect to a preferential acceleration of the protons in the solar wind. A distinct population characterized by a very small differential velocity and nearly equal proton and α particle temperatures that is frequently observed around the maximum of solar activity was attributed to ICMEs. Since this population does not exhibit any evolution with increasing collisional age, we suggest that, by contrast to the solar wind from other sources, ICMEs are born in an equilibrium state and gradually lose this equilibrium due to interactions with the ambient solar wind.