Sky & Telescope - April 2025 - 72

BEGINNER'S SPACE by Monica Young
What Is
the Solar
Cycle?
OUR SUN IS NOT a constant
star. It changes on a roughly
11-year cycle, waxing and
waning in activity that includes
sunspots, flares, and eruptions of
energetic subatomic particles.
Much of this activity remained
invisible until the Space Age - we
couldn't know that the Sun regularly
emits X-ray flares and ejects material
until we had the space-based detectors
for those photons and particles. (English
astronomer Richard Carrington
famously recorded a visible flare in
1859, however.) But long before
satellites offered us access to the Sun
in parts of the spectrum we can't
see with our eyes, observers found
another way to monitor solar cycles as
they unfolded.
For more than 400 years, astronomers
have kept watch on solar activity
by means of sunspots. Within these
regions, strong magnetic fields inhibit
the movement of gas in the Sun's photosphere,
or surface layer. Magnetically
trapped gas cools by more than 1,000
degrees and appears dark in contrast
with the surrounding hotter gas.
German amateur astronomer
Heinrich Schwabe started observing
sunspots nearly daily in 1825. After
more than a decade, he realized that
the sunspot count exhibited a roughly
10-year period of variation - close to
the modern average of 11 years.
Sunspots wax and wane in number
because they are a marker of magnetic
activity. A quiet Sun is an orderly Sun,
its magnetic field arranged similar to a
bar magnet (that is, with a north and
72 APRIL 2025 * SKY & TELESCOPE
south pole). But the Sun isn't a solid
body, and its equator rotates faster
than its poles. This differential rotation
interacts with interior flows of plasma
that create the magnetic field, tangling
the field lines. And where field lines
tangle, sunspots crop up.
Sunspots are only the most obvious
indicator of the solar cycle. As spot
counts peak, the Sun's limb is more
likely to sport magenta arcs of gas
known as prominences. This phenomenon
becomes visible during total solar
eclipses or at any time when observing
the Sun through hydrogen-alpha
filters. Likewise around the peak of
the solar cycle, sudden flares of highenergy
radiation become more likely.
There may also be huge blasts of material
from the Sun's outer atmosphere,
usually from above sunspots, known
as coronal mass ejections. Sometimes,
these streams of energetic solar particles
head straight for Earth - bringing
on the geomagnetic storms that
both threaten satellites and delight
aurora chasers.
A CYCLE Photos of the lower solar corona
taken between 1996 and 2006 show the evolution
of the Sun's magnetically driven activity
during Solar Cycle 23.
These activities associated with
sunspots release the energy of the tangled
magnetic fields, weakening them.
Eventually, the Sun's magnetic poles
flip and, as an orderly field reasserts
itself, the sunspot count decreases
again - for a while.
Right now, we're in the maximum
phase of the current solar cycle, as
noted by the National Oceanic and
Atmospheric Administration last fall.
The peak may continue for several
more months. The activity we've seen
thus far - including two strong aurora
shows in May and October of 2024
- indicates that this cycle has shown
unexpected strength compared to the
exceedingly weak cycle that preceded
it (S&T: Nov. 2013, p. 10). Once the
current solar fervor fades, we'll have
to wait until 2035 or so to see its like
again. So enjoy it while it lasts! ¢
NASA / GODDARD SPACE FLIGHT CENTER
Sky & Telescope - April 2025

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