Alfvénicity and Proximity to Parallel-Mode Marginal Stability in the Slow Solar Wind
Abstract
The proton temperature anisotropy in the solar wind is bounded by the thresholds of pressure-anisotropy-driven kinetic instabilities, and the distance at which the plasma settles from these thresholds is thought to be regulated by compressive fluctuations through the fluctuating anisotropy effect. We test whether the level of Alfvénicity is associated with this distance in the slow solar wind. Using five years of Wind/SWE bi-Maxwellian proton measurements (2004--2008), we separate slow-wind intervals into Alfvénic and non-Alfvénic populations on the basis of normalized cross helicity and residual energy. We compare their magnetic compressive fraction and normalized compressive amplitude, and measure their proximity to marginal stability in the plane of parallel proton beta and temperature anisotropy using the maximum growth rate over the scanned parallel wavenumbers, γ∥,max, from a Vlasov dispersion solver. At low matched parallel beta, the Alfvénic slow wind has lower normalized field-strength fluctuation amplitude and reaches the parallel-mode marginal-stability criterion more often than the non-Alfvénic slow wind. This association is consistent with a weaker fluctuating-anisotropy effect in the Alfvénic slow wind, but does not establish a causal relation.
Related
- Effects of Alfvénicity on the compressible energy cascade in magnetohydrodynamic solar-wind turbulence: Evidence from Parker Solar Probe, Solar Orbiter, and WIND observations
- Effects of Alfvénicity on the Compressible Energy Cascade in Magnetohydrodynamic Solar Wind Turbulence: Evidence from Parker Solar Probe, Solar Orbiter, and WIND observations
- Mean-element Parareal: Parallel-in-Time Integration of the Solar System
- Wave-growth-limited Particle Confinement in a Near-parallel Interplanetary Shock Observed by Parker Solar Probe
- Wave-Growth-Limited Particle Confinement in a Near-Parallel Interplanetary Shock Observed by Parker Solar Probe
BibTeX
@misc{chettri2026alfvénicity,
title = {{Alfvénicity and Proximity to Parallel-Mode Marginal Stability in the Slow Solar Wind}},
author = {Mani K Chettri and Rupak Mukherjee and Hemam D. Singh},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.17671},
eprint = {2609.17671},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.17671},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - UNPB
TI - Alfvénicity and Proximity to Parallel-Mode Marginal Stability in the Slow Solar Wind
AU - Mani K Chettri
AU - Rupak Mukherjee
AU - Hemam D. Singh
PY - 2026
DA - 2026/09/15
PB - arXiv
DO - 10.48550/arxiv.2609.17671
UR - https://arxiv.org/abs/2609.17671
AB - The proton temperature anisotropy in the solar wind is bounded by the thresholds of pressure-anisotropy-driven kinetic instabilities, and the distance at which the plasma settles from these thresholds is thought to be regulated by compressive fluctuations through the fluctuating anisotropy effect. We test whether the level of Alfvénicity is associated with this distance in the slow solar wind. Using five years of Wind/SWE bi-Maxwellian proton measurements (2004--2008), we separate slow-wind intervals into Alfvénic and non-Alfvénic populations on the basis of normalized cross helicity and residual energy. We compare their magnetic compressive fraction and normalized compressive amplitude, and measure their proximity to marginal stability in the plane of parallel proton beta and temperature anisotropy using the maximum growth rate over the scanned parallel wavenumbers, γ_∥,max, from a Vlasov dispersion solver. At low matched parallel beta, the Alfvénic slow wind has lower normalized field-strength fluctuation amplitude and reaches the parallel-mode marginal-stability criterion more often than the non-Alfvénic slow wind. This association is consistent with a weaker fluctuating-anisotropy effect in the Alfvénic slow wind, but does not establish a causal relation.
ER -