You may have heard people talk about the vacuum of space. In fact a perfect vacuum is very hard to find, even in what we call 'space'. On the other hand, the region between the Sun and the planets is a pretty good vacuum even with the solar wind flowing through it. Although the solar wind is very important, there really is almost nothing to it!
In one cubic metre of 'solar wind space' we would find about one million protons. Sounds like a lot doesn't it? But how much do you think a teaspoonful of water would have to expand so that there were one million water molecules per cubic metre? It would have to expand into a cube with 500 km long sides! That's huge, and just shows how many molecules there are in a teaspoonful of water - or how little material there is in the solar wind.
Click on the ‘Moon’ or ‘Earth’ text to see what happens
The fact is that the solar wind is actually much closer to a true vacuum than our present-day technology can create here on Earth. It's amazing that we know the wind is there at all, and even more surprising that it can have such an influence on the Earth.
What is a vacuum?
A vacuum is a volume of space with absolutely nothing in it: no atoms, no protons, no electrons, nothing. A well-known saying is that 'Nature abhors a vacuum' which simply means that material will happily spread into the empty space of a vacuum if it can.
Remove the seal on the illustration above, and see what happens to the density of the gas.