We report a detailed study of an isolated substorm onset with the FAST spacecraft crossing the equatorward = most auroral arc just when/where it starts to break up. Comprehensive ground (optical, riometer and magnetometer) data combined with the space borne field and particle high resolution observations allowed to infer the following properties of the breakup onset region in this well-documented event: (1) The arc flux tube stays in the region of considerable plasma pressure gradient wherethe pressure values are close to ~1--2 nPa. The arc was located (2) just 0.4¿ poleward of the proton isotropic (b2i) boundary (which roughly gives ~40 nT estimate for the equatorial magnetic field) and (3) close to the peak of the diffuse electron precipitation. (4) Tsyganenko 96 model for these particular solar wind conditions (which correspond well to both the observed b2i boundary and estimated plasma pressure) maps the arc to the equatorial distance of ~8RE.All these facts in mutual agreement evidence that the auroral breakup in this particular case was launched in the near-Earth domain of the magnetotail at r ~8RE.