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Life, Unbounded

Life, Unbounded

Discussion and news about planets, exoplanets, and astrobiology

Don’t Panic, It’s Just a Solar Field Reversal!

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Magnetic field lines from active solar regions are drawn over an ultraviolet light image of the Sun (SDO/NASA, K. Schrijver & A. Title, Lockheed Martin)

 

At any moment the great cocoon of unseen magnetic fields and particle radiation that surrounds the Earth and all other major planets is going to experience a change of polarity.

More specifically, the Sun's polar magnetic field - its magnetic north and south - is going to reverse. This flip happens at solar maximum, the period of most intense sunspots, flares, and mass ejections that comes with extraordinary regularity every 11 years. But it's not very speedy from our perspective, it can take as long as 18 months for the meandering field lines to complete their pas de deux while opposite polarity sunspot fields migrate across the solar photosphere, accumulating north and south. As this happens the global polar field of the Sun declines to zero and then rebuilds in the opposite orientation.

 

The solar magnetic field strength varying with time (tick marks are for 3 year intervals). Every eleven years it drops to zero and then grows again with the poles flipped (Wilcox Solar Observatory).

 

Clear signs of this were seen a few months back, with the Sun's north pole having already changed its sign (you can think of the magnetic field lines as 'innies' and 'outies'), but with the south not yet changed. The time is getting pretty ripe for the reversal to begin its final act.

 

A schematic of the shape of the heliospheric current sheet - out to the orbit of Jupiter (it extends much further) (NASA).

 

From the point of view of the Earth the most noticeable effect will come from the behavior of a remarkable structure known as the 'heliospheric current sheet'. This is an undulating extension of the rotating equatorial solar atmosphere itself, a spread of electrically charged particles some 10,000 kilometers thick and billions of kilometers wide. Its shape is driven by the Sun's rotation, some liken it to a ballerina's skirt. The current that actually flows in any single part of this sheet, induced by the Sun's magnetic field, is tiny, barely a ten-billionth of an amp per square meter - but that adds up over billions of kilometers.

The entire heliosphere, the vast bubble of solar particle radiation and magnetic fields that the Sun inflates around itself out to the interface with the interstellar medium (the place that Voyager 2 has recently gone through), is in a sense anchored to this current sheet.

So it's not too surprising that a magnetic polarity change in the Sun propagates across the entire solar system. Specifically, it induces extra waviness in the current sheet, ripples that the planets - like Earth - cross again and again. These transitions can produce space weather, the clash of charged particles and our own planetary magnetic fields and upper atmosphere. Aurorae are the most visible manifestation, but induced geomagnetic currents can also play havoc with our human enterprises, just as when solar flares or coronal mass ejections wash across us.

However, there's no need to panic. For one thing, the increased rippling or crinkling of the heliospheric charge sheet may actually shield us from more of the cosmic rays that permeate the galaxy. Down on Earth's surface we're pretty well protected from these high energy particles anyway, but our satellites are vulnerable to damage, so a little extra protection might reduce the rate of radiation damage and failure. And in terms of the Sun itself, this maximum has been one of the least active in the past century, further reducing any risk from a little 'ol pole reversal.

None of which should take away from the sheer majesty of our nearest star. Its fearsome beauty is something to behold.

 

Spot the Earth - shown to approximate scale (if we were located just outside the photosphere) against August 2012 coronal mass ejection event, imaged by NASA's Solar Dynamics Observatory (NASA).

 

The views expressed are those of the author and are not necessarily those of Scientific American.

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