Magnetic Mirrors
Magnetic mirror structures have been observed in the day
side of the terrestrial magnetosphere, between bow-shock
and magnetopause in the magnetometric data from EQS
spacecraft. These structures have also been observed by previous
missions both in the day side of the magnetosheath
(ISEE 1 and 2, OGO 5) and in the distant magnetotail
(ISEE 3).
We present a self-consistent model of the magnetic mirrors using a perturbative magnetohydrostatic
approach. With the help of this model a number of features have been revealed like geometry, stability and behavior for different
temperature anisotropies (A=T⊥/T||). The basic relations we use in order to derive the model for the mirror structures are the
magnetohydrostatic equilibrium condition and an expression for the anisotropy in the case of bi-Maxwellian distribution. Based on these equations, we have found analytical expressions for the magnetic field
(δB), pressure (δp) and temperature (δT) perturbations. From the investigation of the dependence of the magnetic mirrors
on the unperturbed anisotropy (A0), we have found the well-known behavior (opposite phase variations of the magnetic field
intensity and number density) for A0>1. For A0<1, the behavior is different
but the mirror structures still exist. However, if the anisotropy is in a range of values depending on the plasma parameter
β0⊥, the magnetic mirrors can no longer exist. From the comparison between the current density deduced
from the Ampere law, necessary to sustain the magnetic mirror, and the gradient-curvature drift current density actually being
inside the magnetic mirror, we have been able to determine instability regions in the (A0,β0⊥)-plane.
We present a method for identification of magnetic mirror
structures (MMS) in multi-point spacecraft measurements
and for deriving their full 3D geometry. Multiple minima in
magnetic field data are no longer regarded as separate MMS
but as belonging to the same mirror structure. The approach
is based on fitting a model magnetic field to measured data
from one or many spacecraft. Because of the complexity
of the model magnetic field, fitting the data from only one
spacecraft has proved not to be reliable even if data from the
others spacecraft can be used for checking the results. The
main reason is the instability of the numerical fit and the
non-uniqueness of the solution found. However, performing
the fit on the data from multiple spacecraft allow the use
of information from different regions of the structure eliminating
false results and improving the numerical stability of
the fit.
Contact: Dragos Constantinescu
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