The Sun constantly emits a stream of charged particles — the solar wind. During coronal mass ejections (CMEs), huge plasma bubbles erupt outward; when they reach Earth, the planet's magnetosphere deflects most particles, but strong events cause geomagnetic storms.
Kp ~ log(n_particles * v_wind / B_shield)
F_lorentz = q(v × B) — deflects charged particles
- Solar activity — scales the baseline rate of particles leaving the Sun's surface.
- Solar wind speed — how fast particles travel from Sun to Earth.
- Trigger CME — launches a coronal mass ejection burst toward Earth's magnetosphere.
Earth's magnetosphere (the pale arc) deflects most incoming particles around the shield, which is also what produces the aurora at the poles.