Definition
Stand near a fireplace and you feel its heat. The Sun doesn't just send us light and warmth, either. Its outer atmosphere is so hot that the particles in it can't be held back, and they escape in every direction. This stream of particles flowing out from the Sun and sweeping through the whole solar system is called the solar wind.
Particles the Sun Can't Hold On To
When water boils in a kettle, steam leaks out from under the lid. Something similar happens in the Sun's outer atmosphere. That outer layer is so hot that the particles in it break free of the Sun's gravity and escape into space.
What escapes isn't ordinary air. It's electrically charged particles. When electrons get stripped away from atoms, the result is called plasma, and the solar wind is a flow of this plasma. So it's less like a breeze and more like a river of charged particles.
Its speed reaches hundreds of kilometers per second. That's fast enough to cross the distance to Earth in just a few days. The flow continues even when the Sun is quiet, and it gets stronger and faster when the Sun's surface is stirred up.
This flow stretches out beyond the edge of the solar system and carves out a giant bubble of space. The planets orbit inside that bubble, getting hit by the solar wind as they go.
How Does Earth Block It?
Earth has an invisible shield. A magnetic field generated deep inside our planet reaches far out into space. Charged solar wind particles can't punch straight through it, so they slide off to the sides.
As a result, the magnetic field gets squashed on the side facing the wind and stretched into a long tail on the other side. It looks like the way water streams out behind a rock in a creek.
But the shield has gaps. As the solar wind tangles with Earth's magnetic field, the energy that builds up speeds up particles that were trapped inside the field. These particles travel down magnetic field lines that funnel toward the polar regions. When they collide with oxygen and nitrogen high in the sky, they give off light. That light is the aurora. Solar wind particles themselves don't pour straight into the poles.
That's why auroras are easiest to see close to the polar regions. When the Sun is active, they can be seen at lower latitudes than usual.
A Little More Precisely
To be a bit more precise, the solar wind is not like the wind we know. Air molecules aren't being pushed along. Instead, charged particles move together and drag a magnetic field with them. So when the solar wind gets strong, it can shake Earth's magnetic field and the upper atmosphere above us (the ionosphere) all at once.
It's easy to lump the solar wind together with solar explosions. The steady, gentle stream is the solar wind. The event where a huge clump of material is blasted off the Sun's surface has its own separate name. If that clump heads toward Earth, the solar wind gets stronger for a while.
When that happens, satellites can malfunction, and GPS and radio communications can get disrupted. Unwanted currents can also flow through power grids on the ground and even cause blackouts.
That's why many countries keep watch on the Sun and report space weather like a daily forecast.
Common misconceptions
The solar wind is hot air blowing out from the Sun.
It isn't air. It's a stream of charged particles such as electrons and protons. The wind on Earth is moving air, so it behaves very differently.
The solar wind only reaches Earth when a big explosion happens on the Sun.
The flow continues even when the Sun is calm. A big explosion just makes that flow stronger and faster for a while.
Where you meet it
The green auroras that light up the winter sky over northern Europe or Alaska.
A comet's ion tail (gas tail), which points straight away from the Sun no matter which way the comet is moving.
In one sentence
The solar wind is a flow of charged particles that escape from the Sun's hot outer atmosphere and stream through the solar system. It interacts with Earth's magnetic field to create auroras.