Sky and Telescope - January 2018 - 21

Cycle number

300

Sunspot number

Every day
Missing 1-10 days
Missing 11-20 days
Missing >20 days

200

100

1750

1800

1850

1900

1950

2000

Year
p HOW SUNSPOT NUMBERS CHANGE WITH TIME Scientists use monthly averages of the International Sunspot Number (shown), itself a statistical combination of many observations, to track the solar cycle. (The number of missing days from 1750 to 1818 might be an overestimate.) Dotted
lines mark the approximate start of each cycle. As clear from the chart, the solar cycle's strength and duration vary on multiple time scales.

G R A PH: G REGG DINDER M A N / S&T, SOURCE: DAVID H. H ATH AWAY / LIVING R E VIE WS IN SOL AR PHYSICS 2015;
SUNSPOTS: THO M AS BERG ER (LMSA L) A ND M ATS LÖFDA HL (ISP) / SWEDISH 1-ME TER SOL A R TELESCOPE

Superﬁcial Sun
The interior of the Sun pulsates as rhythmically as a human
heart. Magnetic ﬂuids swish around the solar interior at
thousands of different frequencies. Pressure changes inside
the Sun create these reverberations, just like pressure changes
in the air create sound.
Rachel Howe (University of Birmingham, UK) and colleagues listened to these heartbeats to study the structure of
the solar interior. In particular, they wanted to know why
this last sunspot cycle was the weakest one in a hundred
years (S&T: Nov. 2013, p. 10). Their stethoscope of choice was
a series of ground-based telescopes called the Birmingham
Solar-Oscillations Network.
Using 31 years of observations, they found that the structure of the Sun's surface layer, where sunspots form, changed
markedly in 1994. Since then, far more tiny, weak sunspots
have dotted the solar surface than before. And these spots
live in the shallowest layer of the Sun, unanchored to deeper
layers as in previous cycles. Perhaps, Howe's team speculated
in 2017, the Sun's surface magnetic ﬁeld is thinning and its
magnetic activity is weakening, its cycle slowing down.
We still don't know whether this change is permanent or
temporary. The Sun has deviated from its regular behavior
before: The most famous example is a 70-year period in the
17th and 18th centuries called the Maunder Minimum, during which the Sun shed all but a few of its sunspots, only to
resume its cycle again. But our star has done the same thing
during the Spörer (1450-1540) and Dalton (1790-1830)
minima. It might simply be doing it again.

In a few months, NASA will launch the Transiting Exoplanet Survey Satellite, which will observe nearly the entire
night sky. Kepler, by contrast, only looked at a single patch
of the Milky Way during its primary mission, and it can only
observe along the ecliptic in its revitalized form (see page 22).
With many more nearby stars to observe, we'll undoubtedly
learn more about spot cycles. These data will help support,
or quench, today's controversial theories. In the process we'll
learn a little more about the star closest to home.
¢ MONICA BOBRA is a solar physicist at Stanford University.

Enduring Mystery
Whether we're in a momentary lull or an everlasting decline,
spot cycles can teach us about a lot more than the fate of
our Sun. We can also learn about the fate of other planets.
For example, scientists pore through data from Kepler and
other, ground-based observatories, to search for exoplanets
that might be able to support life. But determining a planet's
habitability also depends on its host star. If a star plastered
with spots releases giant ﬂares, hundreds of times larger than
the ones we see on the Sun, a planet in a habitable zone might
not be so habitable after all.

p SUNSPOT CLOSE UP Twisted, concentrated field lines poking out
of the solar surface keep surrounding convective plasma (bright orange)
from flowing into their locations, creating local cool spots that we see as
sunspots. The widespread, bumpy cells are called granules; the feathery
edges are the sunspot's penumbra, where magnetism leaks out to the
rest of the surface.
sk yandtelescope.com * JA N UA RY 2 018

http://www.skyandtelescope.com/

Sky and Telescope - January 2018

Table of Contents for the Digital Edition of Sky and Telescope - January 2018