Solar Wind Heating Near the Sun: A Radial Evolution Approach

TL;DR

This study uses Parker Solar Probe SPAN-I observations from Encounters 1–24 to characterize the radial evolution of near-Sun solar wind plasma and magnetic-field properties in both sub- and super-Alfvénic regimes. By applying rigorous field-of-view corrections and selecting fully observed VDFs, the authors derive radial profiles for $|B|$, $N$, $V$, $T$, $T_{ elax parallel}$, $T_{ elax perp}$, $T_{ elax perp}/T_{ elax parallel}$, $eta$, $M_A$, and $|oldsymbol{ abla B}|/B$, revealing distinct sub- and super-Alfvénic trends. They find that $T_{ elax perp}$ decreases monotonically with distance while $T_{ elax parallel}$ increases just beyond the Alfvén surface, interpreted as proton-beam signatures, and that magnetic fluctuations show enhanced perpendicular power near the Sun, providing free energy for beam formation and heating via wave–particle interactions. These results underscore regime-dependent heating mechanisms near the Sun and highlight the role of near-Sun fluctuations in energizing solar wind particles, with implications for kinetic models of solar wind acceleration.

Abstract

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.

Figures (8)

• Figure 1: Example of FOV calculation for the time indicated by the white vertical solid, dashed and dotted lines in Figure \ref{['fig:fov_energy']}. Panels (a) to (c) show an example of 0.99 (99%), 0.89 (89%) and 0.62 (62%) FOV coverage. The lower three panels (d)-(f) show the associated VDF slices in the $v_x$-$v_y$ ($\phi$) plane. The 1D Gaussian fits are also displayed for three cases. The effect of the occultation is evident in the VDFs that are cut off in their lower part.
• Figure 2: Energy Flux and FOV observations from PSP Encounter 19. The upper and lower panels display the variation of energy flux with respect to the $\theta$ and $\phi$ angles, respectively. The color bar represents the energy flux, while the right y-axis (in black) indicates the FOV. The white dashed horizontal line marks the 0.85 (85%) FOV in both panels. The vertical solid, dashed and dotted white line corresponds to the FOV calculation illustrated in Figure \ref{['fig:fov_ex']}.
• Figure 3: The variation of SPAN-I FOV for Encounters 1–24. Panels (a)–(d) display the radial distance from the Sun ($R_s$), the FOV for $\theta$, the FOV for $\phi$, and the combined FOV ($\theta \times \phi$), respectively. The green line in the lower three panels shows the FOV=0.85. The color bar represent the number of observations.
• Figure 4: The variation of SPAN-I FOV during Encounters 1 - 24 with radial distance. The FOV for $\theta$, the FOV for $\phi$, and the combined FOV ($\theta \times \phi$) are shown. The black dashed horizontal line shows the FOV=0.85. The median values are shown with black curve. The 10% and 90% levels are shown by black dashed lines. The lime line based on the right y-axis shows that percentage of data having FOV$>$0.85. The dashed vertical line shows $R_s$=30.
• Figure 5: Radial evolution of (a) magnetic field strength ($|B|$, nT), (b) proton density ($N$, cm$^{-3}$), (c) proton bulk speed ($|V|$, km s$^{-1}$), (d) total temperature ($T$, eV), (e) parallel temperature ($T_\parallel$, eV), (f) perpendicular temperature ($T_\perp$, eV), (g) temperature anisotropy ($T_\perp/T_\parallel$), (h) plasma beta ($\beta$), and (i) Alfvén Mach number ($M_A$). The black solid line and the black dashed lines represent the logarithmic median and 10-90 percentile levels, respectively. The black dashed vertical line marks the mean Alfvén critical surface at $16R_s$ where $M_A = 1$. The blue (10–16 $R_s$) and pink shaded region (17–30 $R_s$) indicates the radial range used for the power-law fitting. The blue and red lines shows the power-law fits for sub- and super-Alfvénic region respectively.