Abstract:
We study an axion soliton that weakly interacts with background matter and magnetic fields. A mirror-symmetric soliton whose magnetic flow is due to secondary magnetic helicity invariant is described by the Iroshnikov–Kraichnan spectrum. For a large-scale magnetic field, a dynamo is not observed. In a mirror axionic soliton, a phase transition producing a magnetic helical flow is possible. Using this transition, the soliton becomes mirror-asymmetric. When the mirror symmetry is broken, the axion soliton acquires magnetic energy, which is the result of the transformation of the axionic energy. Our main result for the initial stage of the process is calculating a scale for which the generation of large-scale magnetic fields is the most intense. Numerical simulations show that lower lateral harmonics of the magnetic field have smaller amplitudes compared to higher ones. We study the simplest statistical ensemble defined by the projection of all harmonics onto principal ones. We conjecture that a certain instability in axionic MHD is observed. We propose a possible explanation for this phenomenon. When the mirror symmetry of the axion soliton is broken, the $\gamma$-term in the axionic mean-field equation, which is related to the axion spatial inhomogeneity, interacts with principal harmonics. As a result, the axion soliton acquires magnetic energy and becomes helical.
Fulltext:
PDF file (467 kB)
First page:
PDF file