## The solar wind and magnetosphere

The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cc and magnetic field intensity around 2–5 nT. The interplanetary magnetic field (IMF) may also be much stronger.

Southern vertical component of the Interplanetary Magnetic Field (IMF Bz < 0) is usually regarded as a predictor of the magnetospheric electromagnetic state disturbance. If IMF Bz < -5 nT for 3 hours consecutively, this component indicates the occurrence of a magnetic storm and its development.