Sky and Telescope - January 2018 - 20

Solar Physics

By Monica Bobra

1
Start with a dipolar magnetic
ﬁeld. (This is a simpliﬁcation.)

2
The Sun rotates faster at the
equator than the poles. This differential rotation stretches out
the ﬁeld along the equator.
(v is for velocity)

3
Solar rotation also twists the ﬁeld
into loops. When these loops
poke up through the surface, we
see them as sunspots. Sunspots
usually appear in pairs, each with
a north and south pole. (Note: The
spot pairs are at an angle, and
the higher footpoint has an opposite polarity as the hemisphere
it's in. Important!)

4
Most of the magnetic ﬁeld cancels out quickly: Burbling plasma
motions move sunspots around,
and their magnetic ﬁeld lines
snap into new conﬁgurations. But
these cancellations leave behind
bits of magnetism. Think of them
as sunspot debris.

5
Now another motion becomes
important: the meridional ﬂow.
It slowly transports stuff toward
the poles. It's also stronger farther
from the equator. It drags the bits
of opposite-polarity magnetism to
the poles, where over time (about
11 years) they chip away at the
Sun's polar ﬁeld.

JA N UA RY 2 018 * SK Y & TELESCOPE

their magnetic ﬁelds using common processes that somehow
create cyclic behavior.
But the way this cyclic magnetic ﬁeld manifests itself from
star to star is far from clear. Subsequent researchers discovered that stars in the Mount Wilson data fall into two rough
classes: young, swiftly rotating stars with strong magnetic
activity and cycles that last for 300 to 500 rotations, and
older, more sluggish stars with less magnetic activity and
cycles that last less than 100 stellar rotations.
One of the big problems with this picture is that it's based
on a small amount of data. But this picture also caught
astronomers' attention because it casts the Sun as the odd
man out, sitting squarely in between both classes and producing a new cycle every 160 rotations. Some researchers decided
to ﬁgure out why.

Kepler Weighs In
Soon after Kepler launched from Cape Canaveral, Florida,
on a clear night in March 2009, the mission made headlines
around the world for its discovery of planets orbiting stars in
the Milky Way. But Kepler discovered more than that. It also
found hundreds of stars with starspot cycles.
A few months ago, Travis Metcalfe (Space Science Institute) and Jennifer van Saders (Carnegie Observatories)
studied Kepler observations of F-, G-, and K-type stars to
understand how the Sun might evolve over time. In the process, they unearthed other stars that didn't fall into the two
groups from the Mount Wilson data.
Perhaps, Metcalfe and van Saders reasoned, the Mount
Wilson data don't describe two static classes of cycles, but
rather two bookends of a star's evolutionary track. In this
scenario, a star starts its life in the swift group, hits a critical
turning point (which depends on a star's rotation rate and
the depth of its convective bubbles), and experiences a weakening of its magnetic ﬁeld, which transitions it to the other,
sluggish group. Eventually, the ﬁeld stops cycling altogether.
In fact, maybe our Sun sits between these two classes because
its magnetic ﬁeld has already started to decay.
If so, this transition may take another 800 million to 2.4
billion years, according to Metcalfe and van Saders. During
this period, the solar cycle will slow down. As it does, our
familiar Sun - with dark, concentrated spots that appear and
disappear over time - will change its look completely: Thanks
to its steady ﬁeld, it will wear a permanent coat of magnetic
ﬂecks like speckles on an egg. Eventually, the Sun, no longer
an anomaly, will sit comfortably with its elderly companions.
It's an elegant theory, but not without controversy. Some
worry we're seeing a relationship that, with more observations,
we'll realize isn't there. Others think Kepler's sample may be
biased, because Kepler can only observe cycles on stars with
huge spots. Cycles on stars with tiny spots would go unnoticed by Kepler's detectors. Furthermore, the original Kepler
mission's data only span four years - which means the space
telescope couldn't detect cycles as long as the Sun's.
Luckily, solar observations go back much longer than that.

LE AH TISCIONE / S&T, SOURCE: M ONICA BOBR A

HOW THE SOLAR CYCLE WORKS

Sky and Telescope - January 2018

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