Image: A NASA Solar Dynamics Observatory image of the multimillion degree plasma of the sun, plus lines depicting the paths of the magnetic field originating from the sun’s interior. Credit: NASA/SDO
Every 11 years or so, the sun does something quite profound — its magnetic field completely swaps polarity. This event occurs at the peak of the solar cycle, heralding the mid-point and the most active phase of Solar Cycle 24.
“It looks like we’re no more than 3 to 4 months away from a complete field reversal,” said solar physicist Todd Hoeksema of Stanford University in a NASA news release. “This change will have ripple effects throughout the solar system.”
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Solar astronomers have been keeping a close eye on the magnetic conditions in the lowest regions of the sun’s atmosphere, measuring its magnetic field strength and direction. “The sun’s polar magnetic fields weaken, go to zero, and then emerge again with the opposite polarity. This is a regular part of the solar cycle,” said solar physicist Phil Scherrer, also of Stanford University.
Hoeksema and Scherrer work at Stanford’s Wilcox Solar Observatory, one of the few observatories on the planet that is capable of acquiring solar magnetograms. Wilcox has been monitoring the sun’s polarity since 1976, seeing in three “grand reversals” from three solar cycles. This will be its fourth and excitement is mounting, especially as we’re only a few months away from complete reversal.
The solar cycle ebbs and flows over an approximate 11 year period. From “solar minimum” to “solar maximum,” our nearest star’s internal magnetic field gets wound up by the sun’s differential rotation. Differential rotation means that the sun rotates faster at the equator than it does at the poles, dragging the magnetic field — like an elastic band — that is embedded in the superheated plasma. As the sun approaches solar max (as it is now) the magnetic field is at its most stressed, causing magnetic arcs to be forced from the solar interior and into the lower corona.
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It is during this period that space weather is at its most ferocious, creating beautiful aurorae at the Earth’s poles caused by an intensified solar wind blasting energetic particles into the Earth’s magnetosphere. Also, this is a period of intensified flare and CME activity, potentially damaging satellites and interfering with communications on the ground.
A visible marker of the progression of the solar cycle is the appearance of sunspots — dark blemishes in the sun’s photosphere, marking the location of active regions and potential sites of magnetic eruptions.
So, as we experience solar maximum, the sun’s interior reaches a tipping point in its magnetic polarity, signified by a magnetic field weakening. When the field does switch polarity, it’s not just a local event. The sun’s magnetic field projects from the sun and sweeps throughout the sun’s environment — the heliosphere. As the field flips inside the sun, so does the interplanetary magnetic field, causing the magnetic field and associated electric “current sheet” to ripple and warp.
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Although the underlying reasons for the solar cycle are yet to be understood, Hoeksema and Scherrer know what’s going to happen next.
“The sun’s north pole has already changed sign, while the south pole is racing to catch up,” says Scherrer. “Soon, however, both poles will be reversed, and the second half of Solar Max will be underway.”