The Solar Wind

The solar wind is the continuous flow of charged particles, ions, electrons and neutrons that comes from the Sun in every direction.  The high temperature of the Sun's corona creates a high pressure which causes the particles to be pushed out.  This is why the solar wind can also be thought of as the continuing expansion of the corona.

How does gravity fit into this?  Without gravity, all of the particles that make up the Sun would just fly out into space.  Gravity keeps the particles in while the pressure from the Sun tries to push them out.  But because the pressure is greater than the gravity, we have the solar wind.  It's sort of like a "leak" in gravity's force.

As the wind moves out from the Sun, it accelerates and becomes less dense.  This is because it covers more volume and cools as the particles hit each other less often.  This process is called adiabatic expansion because no energy is gained or lost.

There are two types of solar wind, the fast and the slow.   The difference in speed depends on where on the Sun the wind comes from:

Fast--Open field lines extend from coronal holes.  The solar wind is not restricted by the field lines, however, and easily expands.  So the fast solar wind originates from coronal holes and extends toward the poles of the Sun.

Slow--The slow solar wind is associated with closed magnetic field lines, which causes its speed to be less than that of the fast solar wind.  It generates from near the solar equator.
 

Another subject of recent interest in our research are pick-up ions.  Pick-up ions come about when neutral particles become ionized and are then swept into the heliosphere by the Sun's magnetic field that is carried into space by the solar wind.  There are three ways in which these neutrals become ionized:

Photoionization can take place in which photons from the Sun bombard the neutrals and tranfer their energy to them, causing the neutral to emit an electron and become charged

A charge exchange can take place, where a charged particle and a neutral particle exchange an electron, causing the neutral to become charged

Electron impact ionization occurs when an electron has enough energy to collide with a neutral to knock off an electron from it, making it a charged particle

The interstellar medium  is the main source of the neutrals that pick-up ions come from.  They can also be knocked off of interplanetary dust grains (and are then called inner source pick-up ions).  The diagram below shows how pick-up ions make their way into the solar wind from the interstellar medium on a magnetic field line.