Numerical simulation of electron magnetohydrodynamics with Landau-quantized electrons in magnetar crusts

Abstract

In magnetar crusts, magnetic fields are sufficiently strong to confine electrons into a small to moderate number of quantized Landau levels. This can have a dramatic effect on the crust's thermodynamic properties, generating field-dependent de Haas--van Alphen oscillations. We previously argued that the large-amplitude oscillations of the magnetic susceptibility could enhance the Ohmic dissipation of the magnetic field by continuously generating small-scale, rapidly dissipating field features. This could be important to magnetar field evolution and contribute to their observed higher temperatures. To study this, we performed quasi-3D numerical simulations of electron MHD in a representative volume of neutron star crust matter, for the first time including the magnetization and magnetic susceptibility resulting from Landau quantization. We find that the potential enhancement in the Ohmic dissipation rate due to this effect can be a factor $\sim 3$ for temperatures of the order of $10^8$ K and $\sim 4.5$ for temperatures of the order of $5\times10^7$ K, depending on the magnetic field configuration. The nonlinear Hall term is crucial to this amplification: without it the magnetic field decay is only enhanced by a factor $\lesssim 2$ even at $5\times10^7$ K. These effects generate a high wavenumber plateau in the magnetic energy spectrum associated with the small-scale de Haas--van Alphen oscillations. Our results suggest that this mechanism could help explain the magnetar heating problem, though due to the effect's temperature-dependence, full magneto-thermal evolution simulations in a realistic stellar model are needed to judge whether it is viable explanation.