Two-dimensional Reconstruction of a Time-dependent Mirror Structure from Double-polytropic MHD Simulation

A new reconstruction method incorporated with pressure anisotropy parameter, alpha(B), has recently been developed for magnetohydrostatic equilibria and successfully applied to recovering a two-dimensional (2-D) magnetic field map of mirror structures observed in the Earth's magnetosheath. Here, alpha(B)=mu(0)(p(parallel to)-p(perpendicular to))/B-2 is assumed to be a function of magnetic field strength, B, alone. The fundamental reconstruction theory assumes that the magnetic field and plasma configurations are time-independent and 2-D, which may not be fulfilled in the real applications to satellite observations. When the 2-D structure is time-dependent, the intrinsic field-line invariant F-z=(1-alpha)B-z is violated so that the quantity Fz is not constant for the same field line. This paper aims to examine the performance of the alpha(B) reconstruction of a time-dependent mirror structure, using data from a 2-D, double-polytropic Magnetohydrodynamics (MHD) simulation. With a single-branched fitting function for the field-line invariant, results show that the geometry of time-dependent mirror structure can be reasonably reconstructed, including the distribution maps of gyrotropic pressures p(parallel to) and p(perpendicular to). As expected, the assumption of alpha(B) is well satisfied for the mirror structure. Additionally, another two reconstruction methods are also tested, namely, the Grad-Shafranov reconstruction and the alpha(A) reconstruction. The former is considered isotropic pressure, while the latter assumes that alpha is function of vector potential A alone. As expected, these two reconstruction methods fail to recover the geometry of the mirror structure. We suggest that use of a single-branched fitting function is more appropriate for reconstruction of a time-dependent, wave-like structure, regardless of which magnetohydrostatic reconstruction method is applied.