It is shown that, in the case of isotropic distribution function, plasma conditions (including pressure) are invariant along the flux tube and can be estimated using measurements by low-orbit satellites. Criteria for selecting the tubes with isotropic characteristics are discussed, and the possibility of directly estimating the current density in the plasma sheet, in flux tubes near the electron isotropic boundary (EIB), is shown. The calculation of these conditions using NOAA-10 satellite data yielded reasonable values of current density, and magnetosphere pressure and showed a pressure growth in the IEB tube by an order of magnitude (from 0.3 to 2.5 nPa), when the magnetosphere tail current increased (at decreasing the EIB latitude from 67° to 61°), and the AE activity index increased from <100 to >500 nT.