Sky and Telescope - November 2015 - 22

Solar Power Unleashed

Superflares
Astronomers have discovered many Sun-like stars that unleash titanic ﬂares.
Could our Sun produce such a ﬂare? Has it already?

The largest solar ﬂare in modern history happened
only 12 years ago. On November 4, 2003, a sunspot
group on the western limb of the Sun hurtled a massive
blast of particles in a direction away from Earth. A week
before, the same sunspot group had cranked out eruptions that spawned auroras as far south as Florida. This
ﬂare was likely as big as the ﬁrst one ever recorded,
using ink and paper, in 1859.
But both these eruptions, monsters by our standards,
are tiny compared with superﬂares - ﬂares roughly tens
to thousands of times more energetic than the largest
solar ﬂare ever observed. In fact, many yellow, middlingmass G-type stars like the Sun produce superﬂares.
Astronomers wonder why this is. As such, they're beginning to ask: Could a superﬂare ever occur on the Sun?
That, it turns out, is a controversial question.

Flare Mechanics
The story of a solar ﬂare begins deep inside the Sun.
There, energy from the seething, boiling interior gets
converted into magnetic energy, giving rise to the solar
magnetic ﬁeld. When strongly concentrated bits of ﬁeld
poke out of the solar surface (called the photosphere),
they choke the motion of the photosphere's hot gas,
making those locations appear dark. We call these dark
features sunspots. In most cases, sunspots travel in
pairs. Each pair acts like a tiny bar magnet worming its
way across the solar disk, with one spot leading while
the other follows.
Like long stalks of grass swaying in the breeze,
the magnetic ﬁeld is anchored ﬁrmly to sunspots but
moves freely in the upper solar atmosphere, or corona.
There, the ﬁeld can twist, snap apart, and fuse back
together, and when it does it releases energy. We call
this burst of energy a ﬂare.
To happen, solar ﬂares need a magnetic ﬁeld that's
22

November 2015 sky & telescope

both freely moving and strong. That's why ﬂares release
most of their energy in the corona (where the ﬁeld moves
like billowing meadow grass), directly above sunspots
(where the ﬁeld is strongest).
During a ﬂare, particles from the Sun head every
which way. Some travel out into space. Under the right
conditions, they ﬂow seamlessly from the solar to terrestrial magnetic ﬁeld, hit our ionosphere, and create auroras.
But our Sun's ﬂares are nothing compared with
superﬂares. In 2012, Hiroyuki Maehara (then at Kyoto
University, Japan) and colleagues discovered 365 superﬂares on 148 solar-like stars, using data from NASA's
Kepler space telescope in a landmark study of the
largest sample of superﬂares compiled for these stars.
The Kepler satellite, which observed more than 100,000
stars on a ﬁ xed patch of sky over a four-year period, was
designed to detect exoplanets, but astronomers soon discovered that careful processing could uncover thousands
of superﬂares. (The signal from an average-size solar
ﬂare is too faint for Kepler to detect.)
While Kepler cannot directly image these distant
stars, it can detect how brightly a star shines over time.
Astronomers compile this information in diagrams called
light curves. By analyzing peaks in these light curves,
Maehara's team and, soon after, many others discovered
hundreds of superﬂares on solar-like stars.
In addition, from dips in these same light curves,
Maehara's team also inferred that these stars' surfaces are
marred with massive starspots, the likes of which we've
never seen on the Sun. These starspots cover huge swaths
of the stellar surface and can survive for months on end.
This summer, Yuta Notsu (Kyoto University) and
colleagues used the 8.2-meter Subaru Telescope atop the
summit of Mauna Kea to track down some of the stars
reported in Maehara's study. In particular, they looked
at the stars' spectra in the ionized calcium (Ca II) and

NASA / SDO / GENNA DUBERSTEIN

Monica Bobra

Sky and Telescope - November 2015

Table of Contents for the Digital Edition of Sky and Telescope - November 2015