Sky and Telescope - November 2015 - 25

Signs of a Superﬂare?
While observations of other solar-like superﬂaring stars
cannot provide all of the answers, there are other places
to look. One place is right here, on planet Earth.
When high-energy particles impact Earth's atmosphere, they create a type of radioactive carbon called
carbon-14, which is then incorporated into atmospheric
carbon dioxide. During an extreme solar ﬂare, these
particles come from the Sun and bombard Earth, creating higher-than-normal levels of carbon-14. Trees ingest
this carbon, preserving a historical record of the particle
surge in their rings. But no such carbon-14 spikes were
observed until 2012, when Fusa Miyake (Nagoya University, Japan) and colleagues unearthed a sharp increase

10%
8%
6%
4%
2%
0
-2%
July 12

Aug 1

SUPERFLARE SPOTTED Hiroyuki Maehara's team used Kepler
data to discover 365 superﬂares on Sun-like stars, including this
ﬂare on the star KIC 6034120. The superﬂare lasted 5½ hours and
had a total estimated energy of 3 × 1035 ergs, or about a hundred
times larger than the largest ﬂare ever observed on the Sun. The
team infers that the periodic variations in the light curve likely
come from starspots rotating in and out of view as the star spins.

in carbon-14 content in rings formed by Japanese cedar
trees around AD 775. Since then, many groups have discovered similar increases from around the same year in
bristlecone pines from the White Mountains of California, oaks from Germany, larches from northern Siberia,
and, less than a year ago, kauri trees from New Zealand.
But trees aren't the only history books around. Highenergy particles impacting Earth's atmosphere also

Plasma
ﬂows

Flare

DIAGRAMS: S&T: GREGG DINDERMAN; FLARES: NASA / SDO
AND THE AIA, EVE, AND HMI SCIENCE TEAMS

Magnetic loop
in corona

re
sphe
oto
Ph
Footpoints
of coronal loop

Earth date (2009)

S&T: LEAH TISCIONE, SOURCE: H. MAEHARA ET AL. / NATURE 2012

Change in stellar brightness, relative to average

ﬂare duration. On the Sun, the white-light emission from
a ﬂare usually lasts less than 10 minutes. On solar-like
stars, the white-light emission from a superﬂare lasts
for almost half a day. There is a plausible explanation for
this discrepancy: perhaps the superﬂare is composed of
many smaller - albeit still quite large - ﬂares superimposed atop one another, creating a gargantuan signal.
This idea is not new. Observations from the Solar
Dynamics Observatory and STEREO spacecraft clearly
show that when the magnetic ﬁeld rearranges itself during a solar ﬂare, it can aﬀect already-stressed magnetic
ﬁelds elsewhere on the Sun. In some cases, this causes
a domino eﬀect, triggering ﬂares that might not erupt
otherwise. Recent numerical models, notably by Tibor
Török (Predictive Science Incorporated) and colleagues,
can reproduce such observations.

ONE WAY TO MAKE A FLARE Astronomers aren't quite sure of the mechanics behind solar ﬂares, but one scenario is the pinching oﬀ of a magnetic ﬁeld loop. In this scenario, the loop has its footpoints in the photosphere and extends into the corona (A).
Plasma ﬂows pinch the magnetic loop (B); these ﬂows might be part of the natural movements in the solar atmosphere, or inherent
to the plasmoid eruption's dynamics. The magnetic ﬁeld lines then reconnect and the lower loop snaps back toward the photosphere (C). Plasma ﬂows away from the reconnection point, and shock waves within the plasma heat it, creating the intense burst
of emission we observe at multiple wavelengths. In simple 2D models like this one, the upper loop carries away ions and can evolve
into a coronal mass ejection. But many ﬂares don't show the pinch-oﬀ and plasma blob, leading researchers to suspect that, while
magnetic knotting and reconnection are fundamental to ﬂ are creation, the loop pinch-oﬀ process is not.

Sk yandTelescope.com November 2015

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Sky and Telescope - November 2015

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