What is the solar wind?
The solar wind is a plasma, a stream of charged particles (ions and electrons) which are continuously escaping from the Sun into the interplanetary medium. The particles can escape from the hold of the Sun because the solar corona consists of a very hot plasma of which the temperature exceeds millions of degrees.
The magnetic field of our planet protects us almost completely against the incoming solar wind particles by deflecting them. But there are weak zones in this natural shield. Near the poles of Earth, the funnel-like openings in the magnetic shield (the polar cusps) allow these particles to enter.
The fourth state of matter
On Earth, matter mainly comes in three forms: solid, liquid or gaseous. But in space, a fourth state prevails: the plasma state.
Plasma is electrically charged (ionized) gas, a mixture of electrons (with negative electrical charge) and positive charged ions. Most of the visible universe (99% of it) is made of plasma. Plasma is very sensitive to electric and magnetic fields. The plasma state takes over where the ionosphere ends at an altitude of about 1000 kilometers above the Earth’s surface and it prevails in the magnetosphere.
In order to study the solar wind, researchers at the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) investigate its plasma temperature, and density, as well the kinds of particles it contains. Speed is also a key parameter. For instance, there exists a slow solar wind (400 km/sec) and a fast solar wind (800 km/sec) which originate from different regions of the Sun. In that respect, the temperature of the solar corona is an important parameter, just like gravitation and magnetic field strength is. To conclude, researchers also investigate the energy that the solar wind carries with it.