A geomagnetic storm rapidly changes the horizontal magnetic field at Earth's surface. By Faraday's law that time-varying field drives a quasi-DC geoelectric field in the ground:
E ≈ k · (dB/dt) [simplified surface-impedance model]
Long grounded conductors — transmission lines, with earth as the return path — pick up an EMF equal to that field integrated along their length, offset by how well the line's azimuth lines up with the storm's field direction:
EMF = E · L · cos θ
GIC = EMF / (R_line + 2·R_ground + R_winding) [Ohm's law, lumped circuit]
θ is the angle between the line and the storm's E-field direction — a line running parallel to the field picks up the most current, one running perpendicular picks up almost none. This is why north–south lines are usually hit hardest during storms at mid-to-high latitudes.
The GIC itself is quasi-DC (period of minutes), so it rides on top of the 50/60 Hz AC flux in every transformer core it passes through:
φ(t) = φ_dc(GIC) + φ_ac·sin(ωt)
Once |φ(t)| exceeds the core's saturation flux, the magnetizing current stops being a small sinusoid and spikes sharply on the half-cycle where the DC and AC flux add — half-cycle saturation. The scope in the top-right of the viewport plots exactly that current for the most heavily loaded transformer (shown at a slowed, visualization-friendly frequency). Saturation drives harmonic-rich currents, extra core heating, and reactive-power swings — the real mechanism behind 1989's Hydro-Québec blackout and the risk that keeps grid operators watching space weather.
- dB/dt slider — storm intensity; real severe storms reach several hundred to ~1000+ nT/min at auroral latitudes.
- Azimuth slider — rotates the storm's geoelectric field; watch which line lights up as it sweeps past each line's own orientation.
- Grounding resistance — resistors some utilities add in transformer neutrals specifically to throttle GIC; raising it visibly cuts the current.
- Substorm impulse — a sudden-impulse-style transient spike in dB/dt, as real substorms produce.