This column from The Tablet first ran in 2018.
The pull of a magnet on a refrigerator is strong enough to hold up a child’s drawing; but move it just a fraction of an inch away and suddenly the tug between the fridge and the magnet is almost nothing. Planets have magnetic fields, too; Earth’s magnetic field points compass needles and gives us the auroras, or “northern lights”. But the strengths of planetary fields, too, drop quickly as you move away from the planet. Earth’s magnetic field doesn’t even reach to its own Moon.
The Sun, however, has a magnetic field whose effect can be felt even at the Earth, and indeed out beyond the orbits of the outer planets. Magnetic storms on the Sun can affect our auroras, and occasionally interfere with cell phone and radio transmissions. How is that possible? It’s all connected to a phenomenon called the “solar wind”.
The Sun is made mostly of hydrogen gas; but when the hydrogen atoms get especially hot, they break into their constituent protons and electrons. This ionized gas, or “plasma”, makes up the corona of the Sun — the part you see glowing around the Sun during a solar eclipse.
Some coronal electrons and protons are so hot that they can move fast enough to escape the Sun’s gravity. As they move farther away from the Sun, the pull of the Sun’s gravity gets weaker, and so these particles are free to move out ever faster. The result is a constant flow of plasma blowing off the Sun: the solar wind.
But electrically charged particles are tied to magnetic fields. A strong magnetic field can move a plasma around; magnets in an old fashioned television tube would steer a stream of electrons back and forth across the TV screen, painting a picture there 25 or 30 times a second. If a strong magnetic field can direct a thin plasma, what happens to a weak field in a strong plasma? The plasma wins. As the solar wind flows away from the Sun it drags the Sun’s magnetic field along with it, filling the space between the planets with both plasma and magnetic field.
This solar wind model was first proposed by Eugene Parker in the 1950’s, to explain why the ionized tails of comets always appear to be pushed in the direction aimed away from the Sun. Magnetic field detectors on many spacecraft since then have vindicated Parker’s original model.
I remember as a kid reading several science fiction stories inspired by the idea of a solar wind, with spaceships using giant “solar sails” like futuristic windjammers to travel among the planets. (Ten years later I’d be writing my PhD thesis on space plasmas under a professor who had studied with Parker.) Indeed, several real space probes have used the power of the solar wind to propel and steer themselves.
Parker’s ideas, mind you, were developed before any spacecraft. He pictured the Sun’s magnetic field as a set of tangled lines, embedded like spaghetti in a thick ragu sauce. He knew the model was over-simplified; but it was good enough to let him picture what was going on… much as simple images as “father” or “king” can nonetheless begin to give us an idea of God.
The one place that spacecraft haven’t yet reached is Sun’s corona itself, where the solar wind begins. That’s why, on August 12 , NASA launched the Parker Solar Probe to orbit just 6 million kilometers above the Sun’s surface. Witnessing the launch in Florida was the then 91-year-old Eugene Parker.