Statistical and case studies, as well as data-mining reconstructions
suggest that the magnetotail current in the substorm growth phase has a
multiscale structure with a thin ion-scale current sheet embedded into a
much thicker sheet. This multiscale structure may be critically
important for the tail stability and onset conditions for magnetospheric
substorms. The observed thin current sheets are found to be too long to
be explained by the models with isotropic plasmas. At the same time,
plasma observations reveal only weak field-aligned anisotropy of the ion
species, whereas the anisotropic electron contribution is insufficient
to explain the force balance discrepancy. Here we elaborate a
selfconsistent equilibrium theory of multiscale current sheets, which
differs from conventional isotropic models by weak ion anisotropy
outside the sheet and agyrotropy caused by quasi-adiabatic ion orbits
inside the sheet. It is shown that, in spite of weak anisotropy, the
current density perturbation may be quite strong and localized on the
scale of the figure-of-eight ion orbits. The magnetic field, current and
plasma density in the limit of weak field-aligned ion anisotropy and
strong current sheet embedding, when the ion scale thin current sheet is
nested in a much thicker Harris-like current sheet, are investigated and
presented in an analytical form making it possible to describe the
multiscale equilibrium in sharply stretched 2-D magnetic field
configurations and to use it in kinetic simulations and stability
analysis.