On the Relation between Source Region and In-situ Variability of Low $δB$ Solar Wind Streams

TL;DR

The study investigates how solar source-region variability governs in-situ variability of low-δB solar wind streams using Parker Solar Probe data from Encounter 23. It employs a PFSS model up to $R_{ss}=2.5\,R_0$ combined with ballistic propagation to map footpoints, building an ensemble from 36 magnetograms to estimate uncertainties and extract footpoint field strengths $B_0$ and expansion factors. The authors find that about half of the in-situ low-variability intervals correspond to low-variability footpoints, and regions with small $dB_0/d\lambda_{ss}$ tend to align with low in-situ variability, though a perfect one-to-one mapping is precluded by model assumptions and solar evolution. The results support a solar-origin for many low-variability periods and highlight the need for time-dependent coronal models and composition diagnostics to further disentangle source versus in-situ evolution in solar wind variability.

Abstract

Parker Solar Probe (PSP) observed a high speed stream near the Sun ($\sim 9.8 R_\odot$) in March 2025. As this stream was observed near the Sun, it allowed for an unparalleled opportunity to observe pristine, coronal hole wind with little evolution (expansion, stream interaction, etc) effects impacting its properties. Through an ensemble of magnetic connectivity analysis utilizing Potential Field Source Surface (PFSS) modeling and ballistic propagation, we make estimates of the footpoints, and corresponding source region parameters, associated with the stream. We then look at how the low variability regions (small gradient in the in-situ $B_r$) are related to the variability in the field strength at their source ($B_0$). We find that about half of these periods are associated with low source region variability. Lastly, we examine the low $δB / B$ periods between switchback patches and similarly find that they show statistically less variability in their associated $B_0$ value than their switchback patch counterparts. We believe that these results point to a solar source of these ``low-variability" periods, that warrants further investigation with composition diagnostics and more complex modeling techniques.

Figures (4)

• Figure 1: Estimated footpoints (PFSS + backmapping) colored by the measured in-situ proton radial velocity ($v_R$) plotted on a 193Å SDO/AIA observation. The light gray shading shows the $1 \sigma$ uncertainty in the modeled footpoints associated with the ensemble of potential field extrapolations from different input magnetograms.
• Figure 2: Overview of observations and modeling results for the Encounter 23 fast stream (2025-03-23 00:29:02 to 04:38:35). (a) the proton radial velocity (partial moment) from SWEAP/SPANi (b) the scaled radial magnetic field component (FIELDS/FGM) and (c) the footpoint field strength derived from combining modeling results and remote observations of the photospheric field (see Section \ref{['sec:modeling']}).
• Figure 3: We calculate gradients of the magnetic field with respect to the source surface longitude at both (b) PSP and (c) the footpoints. Given certain selection parameters to determine low-variability regions, we identify all streams that satisfy these at PSP and the footpoints as seen in (b) purple and (c) orange respectively. If the longitudinal spans of these selected regions agree between PSP and the footpoints, we highlight them in green in all three panels. The top panel (a) shows the radial velocity at PSP, with the coinciding green regions projected onto it.
• Figure 4: We manually identify regions where $\delta B / \langle |B| \rangle$ is small ($\leq 0.2$) to study the gradient in the source region magnetic field footpoint strength. (a) The scaled radial magnetic field highlighting our identified regions. (b) Probability density functions comparing changes in $\mathrm{B_0}$ between our identified regions (purple) and the rest of the observations (black).