Active neutron stars – SGRs, reveal the high-quality QPOs at the ‘pulsating tail’ phase. We suggest diagnostics of the trapped fireball plasma, the source of high-frequency pulsations, using coronal seismology. The trapped fireball is represented as a set of current-carrying loops - equivalent of electric circuits. Our approach gives the following magnetosphere parameters in SGRs: an electric current of (2−8) × 1019 A, magnetic field of (0.6−2.7) × 1013 G, and electrons density of (1.3−6.0) × 1016 cm−3. We show high-frequency QPOs can be self-excited for a smaller electric current than the maximum current and/or due to the parametric resonance.