Backmapping of the High- and Low-latitude Solar Wind under Multiple Heliospheric and Coronal Magnetic Field Configurations

TL;DR

The paper addresses how different coronal and heliospheric magnetic-field configurations influence backmapping of solar wind origins. It deploys a systematic combination of PFSS/PFCS/CSSS coronal-field models with Parker and Fisk heliospheric-field models, validating the mappings against Ulysses/Wind in situ data and STEREO-A EUVI observations. The results show that Parker and Fisk HMFs have similar strength and polarity overall but produce longitudinal footpoint shifts at the source surface, while CMF models yield small high/mid-latitude variations but larger divergences for ecliptic/low-lat wind; the wind source locations are further modulated by the PFSS source surface height. The findings highlight model-dependent uncertainties in solar wind-source identification and suggest that PFSS + Parker is adequate for polar studies but low-lat wind requires cautious, multi-model interpretation for accurate space-weather mapping.

Abstract

Solar wind backmapping is a critical technique for analyzing the origin of the solar wind and space weather events by correlating in situ measurements with solar remote-sensing observations. This technique typically traces magnetic field lines using a heliospheric magnetic field (HMF) model coupled with a coronal magnetic field (CMF). However, the impact of different HMF and CMF configurations on backmapping uncentainty-particularly regarding high-latitude solar wind-remains inadequately quantified. This study comprehensively evaluates solar wind backmapping by combining two HMF models (Parker spiral, Fisk-type) with three CMF models (Potential Field Source Surface (PFSS), Potential Field Current Sheet (PFCS), Current Sheet Source Surface (CSSS)). Our analysis primarily uses in situ measurements from Ulysses and remote-sensing data from STEREO-A. Key findings are that: (1) while both Fisk and Parker HMF models show comparable consistency with measured magnetic field strength and polarity, they produce certain longitudinal displacements in their back-mapped footpoints on the source surface (2.5$R_{\odot}$); (2) For CMF models (PFSS, PFCS, CSSS), predicted photospheric footpoints exhibit minor variations for high/mid-latitude solar wind but some divergences for ecliptic/low-latitude wind; (3) All three CMF models link high/mid-latitude wind to active regions or coronal holes, yet associate a fraction of ecliptic/low-latitude wind with quiet-Sun regions; (4) Ecliptic/low-latitude sources show significantly stronger dependence on the PFSS source surface height compared to high-latitude wind. These results demonstrate that simpler models (PFSS + Parker) appear reasonably adequate for polar coronal hole wind studies, while low-latitude/ecliptic solar wind exhibits the heightened sensitivity to model choices.

Figures (13)

• Figure 1: Predicted HMFs by the Parker and Fisk models with in situ measurements obtained from the Ulysses (A, B, C, D) and Wind (E, F, G, H). Panels A and E: the agreement (orange) and disagreement (green) between measured and predicted HMF polarity. Panels B and F: black dots and light green dots denote one-hour-cadence and filtered measured magnetic field strength, respectively; pink dots and lightskyblue dots shows the predicted magnetic field strength by the Parker and Fisk models, respectively. Panels C and G: the observed (circular markers) and predicted values (red dotted line for Parker and blue dotted line for Fisk) of $B_\theta*r_0 / B_r*r$ (where $r_0= 1$ AU and r is the heliocentric distance), representing the meridional magnetic field component $B_\theta$ divided by the radial magnetic field component $B_r$. Panels D and H: the same as Panels C and G but for azimuthal field component ($B_\phi$).
• Figure 2: Backmapping of the solar wind measured by the Ulysses and Wind spacecraft to the source surface of the PFSS model ($2.5 R_{\odot}$) based on the Parker and Fisk HMF model. Panel A: examples of Parker (red) and Fisk (blue) magnetic field lines traced from identical measurement points to $r=2.5 R_{\odot}$. Panel B: magnetic footpoints at $2.5R_{\odot}$ traced from measurements by Ulysses (solid markers) and Wind (hollow markers) using the Parker (circles) and Fisk (triangles) models. Color coding indicates polarity: olive for negative, magenta for positive, and black dotted lines for neutral line. Panel C: latitudinal ($\Delta\theta$) and longitudinal ($\Delta\phi$) deviations between Parker and Fisk magnetic footpoints at $r=2.5 R_{\odot}$, traced from Ulysses measurements. The color encodes the count. The blue and red lines overlaid on the histograms represent the smoothed probability density distributions of the deviation values, derived from Kernel Density Estimation (KDE). Panel D: the same as Panel C but for Wind measurements.
• Figure 3: Backmapping of the solar wind measured by the Ulysses and Wind spacecraft from the source surface ($2.5 R_{\odot}$) to the photosphere based on the Fisk HMF model. Panel A: representative magnetic field lines from the PFSS (pink), PFCS (darkviolet), and CSSS (mediumseagreen) CMF models. Panels B and C: corresponding photospheric footpoints traced from the Ulysses (B) and Wind (C) in situ observations, overlaid on a synoptic magnetogram, respectively. Panel D: A view of the Sun's north pole created by reprojecting the STEREO-A/SECCHI EUVI 195 $\mathring{\mathrm{A}}$ synoptic map onto a spherical coordinate system, with backmapped solar wind source regions color-coded by speed during Ulysses' mid-latitude scan. Panel E: the same as Panel D but for Wind measurements.
• Figure 4: Backmapped source regions of the Wind-measured solar wind superimposed on an EUVI synoptic image.
• Figure 5: Latitudinal ($\Delta\theta$) and longitudinal ($\Delta\phi$) deviations of photospheric footpoints among the PFSS, PFCS, and CSSS models for Ulysses measurements. Representative magnetic field lines traced from identical points at the model outer boundaries to the photosphere are included for contextual comparison.