Evolution of Mirror Axion Solitons

Abstract: We study an axion soliton, which weakly interacts with background matter and magnetic fields. A mirror-symmetric soliton, for which the magnetic flow is due to secondary magnetic helicity invariant, is described by the Iroshnikov-Kreichnan spectrum. For a large scale magnetic field dynamo is not observed. In a mirror axionic soliton, a phase transition, which produces a magnetic helical flow, is possible. Using this transition, the soliton becomes mirror-asymmetric. When the mirror symmetry is broken, the axion soliton allows the magnetic energy, which is the result of the transformation of the axionic energy. In the main result, for an initial stage of the process, we calculate a scale for which the generation of large scale magnetic fields is the most intense. By making numerical simulations, we received that lower lateral harmonics of the magnetic field have greater amplitudes compared to higher ones. A simplest statistical ensemble, which is defined by the projection of all harmonics onto principal harmonics is constructed. We put forward an assumption that it was the indication to some instability in axionic MHD. Now, we can provide a possible explanation of this feature. 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 the result, the axion soliton acquires the magnetic energy and becomes helical.