Space is not a complete vacuume. In fact there are billions of atomic particles that bombard the earth from the sun each day. These particles form the solar wind and travel at an average speed of 470 km/s away from the sun.
These atomic particles (electrons and protons) are charged, giving the the solar wind an associated magnetic field, which interacts with the earth’s own magnetic field. The solar wind causes the earth’s magnetic field to deform compacting it in the front (Sun facing side) while drawing it our toward the back (away from the sun).
Another result of this interaction is that some particles get trapped by the earth’s magnetic field. The electrons and protons spiral along the magnetic field lines bouncing (called mirroring) before penetrating the earth’s atmosphere. The earth’s magnetic field thus acts as a sort of capacitor.
If these particles interact with a VLF wave travelling in the opposite direction, and energy transfer takes place from the wave to the particle (reletivistic transfer: angular frequency of the particle = frequency of the VLF wave.). If sufficient energy is provided to the particle to enable it to penetrate the earth’s atmosphere the particle is said to be put into the “loss cone”, and the particle will strike an atom in the atmosphere, which will in turn cause an energy (light) release which can be seen as Aurora.
Space Storms or Auroral substorms are caused by two main factors, Solar Flares and CME from the sun or when the Earth’s magnetic field collapses. (Discarge of of the capacitor if you like.)
Solar Flares and CME from the Sun
As seen before the Sun emits a steady stream of particles as it burns its nuclear fuel. Every now and again, the Sun emites burts of particles with much more energy (Solar flares) these particles travel much faster than the normal particles and when these particles reach the earths magnetic field they cause some instability. Should the instability be sufficient an auroral storm is caused. More rare than solar flares are CME or Coronal Mass Ejections, where the Sun violently blows a layer an outer layer away. These CME’s cause particles to travel at speeds in excess of 1500 km/s. When the CME particles reach the earths magnetic field this shock wave causes significant disturbances to the field and hence cause magnetic and auroral storms.
Magnetic Field collapse
As mentioned before the earth acts as a capacitor for solar wind particles. These are stored in the tail (see diagram) that forms on the leaward side of the planet. When this tail extends too far it causes a section of the tail to break free, causing the tail to collapse and the magnetic field lines to reconnect. The result of this collapse causes magnetic storms. Magnetic Field collapse can also be intiated by one of the two solar events discussed above.