Interstellar medium modulation of nonlinear kinetic Alfvén morphology in structured Galactic environments

¹ School of Computing and Artificial Intelligence, Southwest Jiaotong University, Chengdu 610031, PR China
² School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, PR China
³ Department of Physics, Guru Nanak Dev University, Amritsar 143005, India

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 January 2026
Accepted: 9 June 2026

Abstract

Context. The ionized interstellar medium (ISM) is a structured, magnetized plasma in which large-scale variations in density, magnetic field strength, temperature, and plasma beta can regulate the local existence and morphology of nonlinear waves such as kinetic Alfvén (KA) solitons.

Aims. The aim of this work is to determine how the plasma conditions of a structured Galactic ISM control where nonlinear KA solitons can exist and how their amplitude and width change across different Galactic environments.

Methods. We constructed a multicomponent Galactic ISM model for the diffuse warm ionized medium together with embedded H II regions, stellar-wind bubbles (SWBs), and supernova remnants (SNRs). The reductive perturbation method was then used to derive the Korteweg–de Vries (KdV) equation and its local coefficients, allowing the admissibility, amplitude, and width of KA solitons to be evaluated under realistic plasma conditions, including suprathermality, plasma beta, temperature, and density gradients.

Results. We identify distinct exclusion zones for KA solitons in high-β H II regions and hot SWB and SNR interiors, as well as ultra-low-β regions near central pulsar wind nebulae. The compressed shells of SWBs and SNRs emerge as preferential zones where the KA ordering can remain valid. Within the present leading-order KdV treatment, the soliton width shows the strongest environmental sensitivity, broadening significantly near admissibility boundaries, whereas the amplitude varies more weakly.

Conclusions. This work provides Galactic maps of where nonlinear KA solitons are locally admissible in a structured ISM, together with the corresponding changes in their width and amplitude. These results establish a physically grounded framework for future observational tests for small-scale ISM structures associated with KA solitons.