Sky and Telescope - January 2018 - 18

SOLAR PHYSICS

by Monica Bobra

Feeble sunspot cycles and other stars' behavior have prompted
speculation on whether something's up with our Sun.

The Cyclic Field
The Sun's cycle of activity originates in its magnetic ﬁeld.
Unlike the dipole bar magnets stuck to many refrigerators,

the solar ﬁeld looks like a giant hairball. It pokes out of the
surface all over the place, creating sunspots. Each sunspot is
a region of concentrated magnetic ﬁeld. This ﬁeld acts like a
dam, preventing hot gases from ﬂowing in. The location of
these little magnetic dams, and the number of them, give us
clues about the solar interior.
Solar physicists don't fully understand how the Sun generates its hairball magnetic ﬁeld. They know that this ﬁeld
originates somewhere inside the Sun, due to the complex
combination of various plasma motions in different regions.
From there, the ﬁeld moves outward, roiling around convective bubbles and forming braids, ropes, loops, and kinks.
Finally, this twisted ﬁeld pokes out through the solar surface,
where we see it as a sunspot. For reasons that remain a mystery, sunspots appear closer and closer to the solar equator as
the Sun moves through its 11-year cycle.
It is obvious, however, that the solar magnetic ﬁeld governs this cycle. To study this process, scientists develop mathematical models of the solar magnetic ﬁeld that reproduce
the Sun's behavior. But we can also study the Sun by looking
at other stars.

Oddball Sun
The Mount Wilson Observatory sits atop a hill that overlooks
the sprawling city of Los Angeles. Pine trees surround the
dome, which, on a clear day, affords vistas of Catalina Island
nestled in the shimmering waters of the Paciﬁc. It was from
this serene setting in 1966 that astronomer Olin Wilson used
the 100-inch reﬂector to begin the ﬁrst long-term study of
spot cycles on other stars.
Wilson found spot cycles on all sorts of stars. Stars similar
to the Sun (G-type) sported cycles, and so did ones slightly
hotter (F-type) and cooler (K-type) than the Sun. On all
these stars, the stellar magnetic ﬁeld created spots and powered eruptions. From these data, it's clear that stars generate
t SUNSPOT BANDS Sunspots appear in two bands that sandwich
the Sun's equator. At the beginning of the solar cycle they're high in
the hemisphere (roughly 30° latitude), but as the cycle progresses the
bands move closer to the equator.
u SOLAR BALL OF STRING The Sun's magnetic field is a complex,
intertwined system that continuously rearranges itself. Shown is a magnetic field model overlaid on a picture from September 4, 2017, both
from NASA's Solar Dynamics Observatory.

JA N UA RY 2 018 * SK Y & TELESCOPE

N ASA / HMI / SDO; FACING PAG E: N ASA / SDO / AIA / LMSA L

bout 290 million years ago, a volcano erupted in what
is now eastern Germany. The blast lifted trees straight
out of the ground and coated them with liquid rock.
Beneath this debris, an entire forest fossilized. Last year, scientists studied tree rings from these ancient trees - but not to
learn about Earth. They wanted to learn about the Sun.
To the naked eye, the Sun looks like a uniform whitish
sphere. But the solar surface is often mottled with dark spots,
like the peel of a ripe banana. These sunspots emerge, live for
a few hours or days (or longer), and then decay. Occasionally, 150 or more spots dot the solar surface. During those
times, we observe many eruptions of high-energy radiation and, sometimes, superheated material, which can blast
through space and hit the planets. At other times, hardly any
spots show up at all, and the Sun stays fairly quiet. The Sun
smoothly cycles between these two states, ramping the number of sunspots up and down every 11 years.
By studying fossilized tree rings, Ludwig Luthardt and
Ronny Rössler (Chemnitz Museum of Natural History, Germany) made a striking discovery: The Sun has been going on
like this for at least 300 million years. It's not that long on
astronomical time scales - our Sun is, after all, 4.6 billion
years old - but it means that some predictable mechanism
operates inside the Sun, churning out cycle after cycle for
millions of years.
But recent observations of the Sun and other Sun-like
stars suggest that the solar cycle might eventually taper off
and disappear. This controversial and speculative theory
requires more observations, and calculations, before we can
conﬁrm it. But we can't rule it out, either. Is the Sun on the
brink of a permanent change?

Sky and Telescope - January 2018

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