Radial increase of the compensated interplanetary magnetic field magnitude as a result of the growth of spherically polarized Alfvén waves
Abstract
Measurements from the NASA Parker Solar Probe (PSP) mission demonstrate that inside ∼20 solar radii (Rs) the magnitude of the interplanetary magnetic field does not fall off simply as 1/R2, with avg|B| R2 15--20\% below its value at ∼60 Rs. Here we use PSP magnetic field measurements over $10.6−−56.8\,\Rs$ to show that the outward rise of avg|B| R2 is not due to the global structure of the heliospheric magnetic field or proximity to the heliospheric current sheet but is the direct result of the growth of large-amplitude, spherically polarized Alfvén waves (so-called `switchbacks'). Over the same distance range, avgBR R2 stays constant, so that the sector-signed radial flux is conserved. The wave amplitude ε= B⊥,rms/avgB∥ grows from $0.26$ to $0.57$ as R1/2cosα, and avg|B|/avgBR rises by 20%. A simple model parametrized by the broadening of the distribution of deflection angles reproduces the effect to high accuracy. These results demonstrate that as the relative wave amplitudes grow with increasing solar distance, a dynamo-like process operates to enhance the field magnitude by up to 20%. By enhancing magnetic pressure this has implications for the large scale solar wind structure in the low-beta inner heliosphere, as well as partially resolving the long-standing discrepancy between remote measurements of the solar magnetic field and those made in situ by earlier missions farther from the Sun.
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BibTeX
@misc{bale2026radial,
title = {{Radial increase of the compensated interplanetary magnetic field magnitude as a result of the growth of spherically polarized Alfvén waves}},
author = {Stuart D. Bale and Timothy S. Horbury and Lorenzo Matteini},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.15741},
eprint = {2610.15741},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.15741},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Radial increase of the compensated interplanetary magnetic field magnitude as a result of the growth of spherically polarized Alfvén waves
AU - Stuart D. Bale
AU - Timothy S. Horbury
AU - Lorenzo Matteini
PY - 2026
DA - 2026/10/11
PB - arXiv
DO - 10.48550/arxiv.2610.15741
UR - https://arxiv.org/abs/2610.15741
AB - Measurements from the NASA Parker Solar Probe (PSP) mission demonstrate that inside ∼20 solar radii (Rs) the magnitude of the interplanetary magnetic field does not fall off simply as 1/R^2, with avg|B| R^2 15--20\% below its value at ∼60 Rs. Here we use PSP magnetic field measurements over $10.6−−56.8\,\Rs$ to show that the outward rise of avg|B| R^2 is not due to the global structure of the heliospheric magnetic field or proximity to the heliospheric current sheet but is the direct result of the growth of large-amplitude, spherically polarized Alfvén waves (so-called `switchbacks'). Over the same distance range, avgB_R R^2 stays constant, so that the sector-signed radial flux is conserved. The wave amplitude ε= B_⊥,rms/avgB_∥ grows from $0.26$ to $0.57$ as R^1/2cosα, and avg|B|/avgB_R rises by 20%. A simple model parametrized by the broadening of the distribution of deflection angles reproduces the effect to high accuracy. These results demonstrate that as the relative wave amplitudes grow with increasing solar distance, a dynamo-like process operates to enhance the field magnitude by up to 20%. By enhancing magnetic pressure this has implications for the large scale solar wind structure in the low-beta inner heliosphere, as well as partially resolving the long-standing discrepancy between remote measurements of the solar magnetic field and those made in situ by earlier missions farther from the Sun.
ER -