arXiv subjects

Kyogo Tokoro

Publications and source records attributed to Kyogo Tokoro.

3 recordsLinked to original sources

arXiv2026

Investigation of Solar Wind Speed Characteristics Using IPS Observations and the PFSS+SCS Model

Understanding the relationship between solar wind speed and global coronal magnetic field is essential for space-weather forecasting and provides key diagnostics of the underlying acceleration mechanisms. Most observational studies to date have relied on near-ecliptic measurements dominated by slow wind, and the full range of wind speed, including fast wind, is insufficiently explored. Interplanetary scintillation (IPS) observations offer global coverage of wind speed; however, previous IPS-based studies have relied solely on the potential field source surface (PFSS) model, which does not adequately reproduce key features of the heliospheric magnetic field, potentially leading to the poor connectivity between the solar wind and its coronal source regions. Here we perform a comprehensive statistical analysis of the wind speed using IPS observations combined with PFSS and the Schatten current sheet (SCS) model. We find that the parameter f_SS/B_sun bifurcates the solar wind into two distinct groups: one showing a strong negative correlation and the other showing no correlation. This grouping is better organized by the footpoint magnetic field strength and the distance from coronal hole boundary (DCHB) than by solar magnetic activity, suggesting that the two groups may reflect fundamentally different acceleration mechanisms.

astro-ph.SR↗

arXiv2026

Proposal of a Novel Physical Parameter Characterizing Solar Wind Speed in a Wave-Driven Model

Empirical solar wind speed models play an important role in enabling space weather forecasting with low computational cost. Among these, one model called WS model is based on the asymptotic expansion factor. However, it is known that it fails in the case of pseudostreamers. In this study, as a first step toward constructing a solar wind speed empirical model based on physical parameters, we investigated the effect of the radial profile of flux-tube shape on the solar wind speed using one-dimensional numerical simulations. In the simulations, ad hoc Alfvén waves are injected from the photosphere at $r=R_\odot$ as the energy source, and the MHD equations are solved out to the interplanetary space at $r=70R_\odot$ to reproduce solar wind acceleration. As a result, even when the coronal base magnetic field and the asymptotic expansion factor are fixed, the final solar wind speed varies by approximately 300 km s$^{-1}$ depending on changes in the expansion height or non-monotonic expansion. Additionally, across all simulations performed, a better correlation is found with the quantities that reflect the information about the radial profile of flux-tube shape than the asymptotic expansion factor. Our results suggest that, as a physical characteristic parameter of the solar wind speed, an operation that can account for the expansion factor throughout the corona is necessary.

astro-ph.SR↗

arXiv2025

Empirical Optimization of the Source-Surface Height in the PFSS extrapolation

The potential field source surface (PFSS) method is a widely used magnetic field extrapolation technique in the space weather community. The only free parameter in the PFSS method is the source-surface height ($R_{\rm SS}$), beyond which all field lines are open. Although $R_{\rm SS}$ is known to vary with solar activity, there is no consensus on how to determine it for a given surface magnetic field distribution. In this study, we investigate the nature of $R_{\rm SS}$ using a long-period (2006-2023) data, covering two solar minima and one maximum. We adopt ADAPT-GONG magnetograms and determine $R_{\rm SS}$ by matching the open flux estimated from observations at 1 au with that calculated using the PFSS method. Our analysis reveals that $R_{\rm SS}$ increases slightly after the solar minima and around the solar maximum, and that it can be characterized by both the mean unsigned photospheric magnetic field strength and the dipolarity parameter $f_{\rm dip}$, defined as $f_{\rm dip} = B_{\rm dip}^2/(B_{\rm dip}^2 + B_{\rm quad}^2 + B_{\rm oct}^2)$, with $B_{\rm dip}$, $B_{\rm quad}$, and $B_{\rm oct}$ denoting the magnitudes of dipolar, quadrupolar, and octupolar components of photospheric radial magnetic field, respectively. Our results suggest that $R_{\rm SS}$ does not exhibit a simple monotonic dependence on the solar activity and must be determined by properly considering both surface magnetic field strength and global field structure.

astro-ph.SR↗