Solar Wind Heating Near the Sun: A Radial Evolution Approach

Characterizing the plasma state in the near-Sun environment is essential to constrain the mechanisms that heat and accelerate the solar wind. In this study, we use Parker Solar Probe (PSP) observations from Encounters 1 through 24 to investigate the radial evolution of solar wind plasma and magnetic field properties in this region. Using intervals with high field-of-view ($>85%$) coverage, we derive the radial profiles of magnetic field strength ($|B|$), proton density ($N$), bulk speed ($V$), total proton temperature ($T$), parallel ($T_\parallel$) and perpendicular ($T_\perp$) temperatures, temperature anisotropy ($T_\perp/T_\parallel$), plasma beta ($β$), Alfvén Mach number ($M_A$), and magnetic field fluctuations ($δB/B$) for sub and super-Alfvénic regions. In super-Alfvénic regions, power-law of $|B|$, $N$, $V$, and $T$ as a function of heliocentric distance are broadly consistent with previous \textit{Helios} results at $>0.3$ AU. The radial evolution of the components of the temperature tensor reveals distinct behavior: $T_\perp$ decreases monotonically with distance, whereas $T_\parallel$ exhibits a non-monotonic trend – decreasing in the sub-Alfvénic region, increasing just beyond the Alfvén surface. We interpret the increase in $T_\parallel$ as a proxy for proton beam occurrence. We further examine the evolution of magnetic field fluctuations, finding decreasing radial/parallel fluctuations but enhanced tangential/normal/perpendicular fluctuations in sunward direction. These fluctuations may provide free energy for beam generation and particle heating via wave-particle interactions.

💡 Research Summary

This paper presents a comprehensive statistical analysis of the radial evolution of solar wind plasma properties in the inner heliosphere, utilizing in-situ measurements from the Parker Solar Probe (PSP) during its first 24 perihelion encounters. The primary objective is to characterize the plasma state near the Sun to constrain the mechanisms responsible for solar wind heating and acceleration.

The study focuses on data from the SPAN-I (Solar Probe Analyzer for Ions) instrument. A critical methodological advancement is the careful selection of data based on the instrument’s field-of-view (FOV). Due to obstruction by PSP’s heat shield, SPAN-I sometimes measures only partial velocity distribution functions (VDFs), leading to biased moments like the temperature tensor. The authors developed a method to calculate the effective FOV coverage for each measurement by fitting one-dimensional Gaussian functions to the azimuthal and elevation angle distributions of the differential energy flux. Only intervals with high FOV coverage (>85%) were retained for analysis, ensuring the reliability of derived parameters, especially the anisotropic temperatures. Magnetic field data from the FIELDS instrument suite were also used, and intervals associated with interplanetary coronal mass ejections (ICMEs) were removed.

The key findings are as follows:

1. Overall Radial Trends: In the super-Alfvénic region (where solar wind speed exceeds the Alfvén speed), the power-law dependences of magnetic field strength (|B|), proton density (N), bulk speed (V), and total proton temperature (T) on heliocentric distance are broadly consistent with earlier Helios results beyond 0.3 AU.
2. Divergent Evolution of Temperature Components: A major discovery is the distinct radial behavior of the components of the proton temperature tensor. The perpendicular temperature (T⊥) decreases monotonically with distance from the Sun. In contrast, the parallel temperature (T∥) exhibits a non-monotonic trend: it decreases within the sub-Alfvénic region, reaches a minimum around the Alfvén surface, and then increases just beyond it in the super-Alfvénic region.
3. Interpretation of T∥ Increase: The authors interpret the observed increase in T∥ beyond the Alfvén surface as a proxy for the occurrence of proton beams. They suggest that processes active near the Alfvén critical surface, potentially driven by wave-particle interactions, can accelerate a subset of protons into a beam-like population. This beam increases the velocity space spread in the parallel direction, manifesting as a rise in the measured T∥.
4. Evolution of Magnetic Fluctuations: Analysis of magnetic field fluctuations (δB/B) reveals an anisotropic evolution. Fluctuations in the radial/parallel direction decrease with distance, while fluctuations in the tangential/normal/perpendicular directions are enhanced in the sunward direction. These fluctuations may serve as a source of free energy for the proposed beam generation and subsequent particle heating.

In conclusion, this study provides the first detailed observational map of how key kinetic properties of the solar wind evolve with distance in the crucial region below 0.3 AU. The divergent behavior of T⊥ and T∥, coupled with the changing nature of magnetic fluctuations, strongly suggests that the Alfvén surface is not merely a velocity boundary but a dynamic region where specific heating and acceleration mechanisms become operational. The non-monotonic trend in T∥ challenges simple adiabatic or isotropic turbulence models and calls for kinetic mechanisms active near the Alfvén critical point. These results offer stringent observational constraints for future theoretical models and simulations aiming to explain coronal heating and solar wind acceleration. The rigorous FOV-filtering technique established here also sets a new standard for deriving accurate higher-order moments from PSP data.