Sky and Telescope - June 2017 - 9

SUN

Another Maunder Minimum?

The blank Sun in early 2017

SCIENTISTS HAVE reconstructed the
Sun's conditions during a 17th-century
lull in activity with the hope of understanding what our star might look like
if history repeats itself.
Around 1645, the Sun became nearly
spotless. During the 70-year period that
followed, known as the Maunder Minimum, sunspots appeared only rarely
on the surface of the Sun - though our

STELLAR

star did continue its 11-year cycle of
activity during this time.
This spotless period doesn't just have
historical signiﬁcance: After a peak in
magnetic activity that produced a multitude of sunspots during the 1950s and
'60s, the Sun again appears to be headed
toward a minimum. The current 11-year
cycle, known as Cycle 24, peaked in
2013, but the peak was the weakest
recorded in 100 years. Early 2017 has
had two spotless stretches, each nearly
two weeks long. Many experts anticipate that Cycle 25 will be weaker still. If
the trend continues - and it is speculative to predict activity based on an individual cycle - we could be looking at a
largely blank Sun in the next few years.
In Nature's January 31st Scientiﬁc
Reports, Mathew Owens (University of
Reading, UK) and colleagues look back
to the Maunder Minimum in order to
anticipate solar and auroral activity (or
the lack thereof) in our near future.
Based on modern data, the researchers replayed the past 30 years of activity
on a computational model of the Sun's
magnetic ﬁeld, comparing the number
Geminga

of sunspots to other properties, such
as the speed of the solar wind. The
team then took the model back in time,
retracing the Maunder Minimum. With
only the records of sparse sunspots as
input, the model postdicted what the
Sun and the solar wind would have
looked like, reconstructing the space
weather of 400 years ago. The result:
The solar magnetic ﬁeld and solar wind
near Earth would have been half as
strong during the minimum as they are
in the modern era.
The postdiction makes sense, given
the lack of auroras reported during that
period. But it's risky to extrapolate a
model so far back in time, cautions Tom
Schad (University of Hawai'i), who was
not involved in the study. "Multiple
approaches to the investigation of deep
minima are paramount," he says.
If we are heading for a deep minimum, we'll perhaps see fewer northern
lights - and those auroras that do
appear will concentrate more around
the poles than they do right now,
Owens says.
■ MONICA YOUNG
B0355+54

SUN: SDO / HMI; PULSA RS: G EMING A X-R AY: N ASA / CXC / PSU / B. POSSELT E T A L., B 0355+54
X-R AY: N ASA / CXC / GW U / N. K LING ER E T A L., INFR A RED BACKG ROUND: N ASA / JPL- CA LTECH,
ILLUSTR ATIONS: N A HKS TREHNL

Two Pulsars Blowing
in the Wind
THESE X-RAY IMAGES of two pulsars
(top two panels) show beautiful, complex clouds of charged particles created
as the spinning neutron stars move
through space. Although a pulsar's beat
is its most deﬁning characteristic, it's
not all we see. Huge electric forces, generated by the spinning magnetic ﬁeld,
rip charged particles off the pulsar's
surface. Some of these particles escape
as jets from the poles. Others form a
donut-shaped cloud around the pulsar,
which sweeps around as the star rotates.
Using the Chandra X-ray Observatory, Bettina Posselt (Penn State
University) and colleagues studied two
very different pulsars, Geminga (left)
and B0355+54, both of which are surrounded by clouds of X-ray-emitting
particles, called pulsar wind nebulae,
that are swept back by the pulsars'

passage. The observations, paired here
with artist's concepts for clarity, reveal
the pulsars' orientations and explain
why astronomers see Geminga pulse
in gamma rays but not B0355+54:
Gamma rays come from near a pulsar's
equator, and while we see Geminga
from the side, B0355+54 has its poles

pointed at us. Conversely, radio emission comes from the star's magnetic
poles, explaining why Geminga seems
to be radio quiet when B0355+54 isn't.
The papers appear in the December
20th and January 20th issues of the
Astrophysical Journal.
■ TIARA DIAMOND
s k y a n d t e l e s c o p e .c o m

* JUNE 2017

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Sky and Telescope - June 2017

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