How often does the sun's magnetic field flip?
Category: Space Published: August 7, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
The sun's magnetic field flips every eleven years. The sun is a giant sphere of hydrogen and helium gas that is compressed by gravity to the point of sustaining nuclear fusion, which releases vast amounts of energy that heats the sun to high temperatures. The sun's very high temperatures rip electrons off of many of the hydrogen atoms, leaving them electrically charged (ionized). The natural, steady rotation of the sun means that all these charged hydrogen atoms trace out, on average, giant circles around the axis of rotation. Anyone who has ever played with an electromagnet can tell you that when you have electric charges traveling in loops, you get a strong magnetic field. As each charged hydrogen atom inside the sun loops as part of the sun's rotation, it creates a circular loop of current, which forms a magnetic field with the North magnetic pole pointing up out of the loop and the South magnetic pole pointing down out of the loop. Because all the charged atoms in the sun, on average, are looping in the same direction around the same rotation axis, all of their magnetic fields combine constructively into one giant dipole magnetic field. To a good approximation, the sun acts like a giant bar magnet with the North pole on one end, the South pole on the other end, and both magnetic poles aligned with the axis of rotation. (This is very similar to how the earth has an overall magnetic field due to its rotation. )
This picture is a good starting point but it is not the whole story. Unlike the earth, the sun is not a solid. The sun is a giant ball of bubbling, shifting, swirling gases. As a result, different parts of the sun can rotate at different speeds. In fact, parts of the sun near the equator rotate much faster than parts of the sun near the poles. Because rotation is what causes the magnetic field, the different rotation speeds cause the magnetic field to twist and knot. For the purpose of illustration, think of a magnetic field as a collection of magnetic field lines that act much likes ropes attached to the North and South poles. As the parts of the sun near the equator rotate faster and get ahead of the rest of the sun, they drag the magnetic field lines ahead, causing them to twist and knot.
Magnetic field lines tend to guide charged particles. The hydrogen ions of the sun are charged, so they are forced to travel somewhat along the magnetic field lines. A knot in the magnetic field lines on the surface of the sun therefore leads to a pocket of hot gas getting trapped on the surface which is unable to boil into space in the usual way. We see this region of trapped gas as a dark spot on the sun's surface and call it a "sunspot". Eventually, the knot in the magnetic field either relaxes and the sunspot disappears, or the knot becomes so twisted that the magnetic field lines snap. When the field lines snap, they can no longer keep the hot gas trapped. At this point, the hot gas has been building up pressure, so the snapped field lines causes a violent release of the hot gas. The glowing gas erupts into space, sending a strong fireball of light and matter out into the solar system. This fireball is known as a "solar flare".
Sunspots and solar flares are therefore caused by a knot in the sun's magnetic field. The more knots there are, the more sunspots and solar flares there are. Now, remember that the knotting is caused by the equator spinning too fast. As time progresses, the equator gets more and more ahead, the magnetic field lines get more and more knotted, and there are therefore more sun spots and solar flares. This process can not go on forever (the magnetic field cannot become infinitely knotted). Every eleven years the sun's magnetic field gets so knotted that it all snaps at once. This snap is different from a single break that takes place on a sun spot. The magnetic field lines break all over the place. This colossal snapping every eleven years causes the magnetic field of the sun to flip so that the North Pole switches places with the South pole. This colossal snapping also causes the magnetic field lines to reset back to an unknotted state, leading to few sun spots and solar flares. The sun has been charging ahead on this 11-year cycle of being unknotted, somewhat knotted, very knotted, snapped, unknotted, somewhat knotted, etc. for millions of years. The number of sunspots per month, the number of solar flares, the amount of knotting, and even the brightness of the sun all follow this 11 year cycle.
Solar flares have little direct effect on life on earth's surface, thanks to the protective cover of earth's atmosphere and magnetic field. The sun's magnetic field has been flipping every eleven years for millions of years without any harm to life on earth. But solar flares can do harm to people and equipment above earth's atmosphere. Solar flares present a radiation hazard to astronauts while in space and can damage spacecraft and satellites. Additionally, the collision of a solar flare with earth's magnetic field can cause geomagnetic storms that are strong enough to knock out electric power grids on earth.