Sky and Telescope - November 2015 - 27

SOLAR BEAUTY MARK In 1947 a gigantic sunspot group
marred the solar surface for several months. At its largest, the
group spanned an area roughly twice that of Jupiter. This photo
shows the Sun as it appeared on April 6th of that year, right
around the time the feature was at its maximum size.

such behavior might crop up again in the future.
Furthermore, solar superﬂares are theoretically possible. Calculations by Kazunari Shibata (Kyoto University) and colleagues show that in just one solar cycle, the
Sun could theoretically build up enough magnetic ﬁeld
to power a solar ﬂare 10 times larger than the 2003 ﬂare.
It would take 40 years to build up the magnetism needed
to power a ﬂare 100 times larger.

The Road Ahead
So, will the Sun ever superﬂare?
In truth, we aren't sure. We don't yet understand how
closely the Sun's behavior mimics that of other stars like
it. Although we detected gigantic stellar ﬂares before the
Kepler mission, until its advent we were unable to catalog
hundreds of them at a time. The wealth of information in
Kepler's data has raised more questions than it answered.
We see solar-like stars with a variety of rotation periods, temperatures, and diameters. We see ﬂares 10 times
larger than ones observed on the Sun, and we see ﬂares
10,000 times that large, too. We infer that some solarlike stars have massive spots or are covered in spots, and
some don't have any spots at all. Do all these stars have
the same mechanism for generating magnetic ﬁelds?
And how many superﬂares versus garden-variety ﬂares
do solar-like stars produce?
Thankfully, there are more data to comb through
for answers. Promising information exists in recently
digitized historical records from the Song Dynasty, as
well as carbon-14 measurements from Chinese corals.
The Solar Dynamics Observatory will continue taking
almost-constant images of the Sun, ensuring we never
miss another sunspot as it rolls across the solar disk.
And observations by larger telescopes, better designed to
study stellar ﬂares, may allow us to answer some of these
questions by observing fainter signals in more spectral
lines. The last few years have been rife with discoveries,
and the next few will likely be the same. ✦
Monica Bobra is a solar physicist at Stanford University.

S&T ARCHIVE / MOUNT WILSON OBSERVATORY

Aulanier (Paris Observatory) and colleagues recently
used a numerical model to calculate the largest possible
ﬂare this sunspot could power under realistic solar conditions. The team discovered that even this largest-ever
sunspot couldn't produce a ﬂare more than a few times
larger than the one in 2003.
Other groups have come up with similar results. Carolus Schrijver (Lockheed Martin Solar and Astrophysics
Laboratory) and colleagues recently calculated that 10%
of the Sun would have to be covered in spots to power a
ﬂare 10 times larger than the largest one observed. And
we've never seen the Sun look like that. By statistically
analyzing the size of solar ﬂares - the vast majority
of which are miniscule - Schrijver's team estimates
that there's at most a 10% chance we'll see a superﬂare
within the next 30 years.
Hugh Hudson (University of California, Berkeley) has
made a similar calculation by analyzing supergranules,
giant convective bubbles in the photosphere. Sunspot
umbrae are not usually larger than the area of a supergranule, he observed. Thus, the average-size sunspot
can contain only so much magnetic ﬁeld. And the ﬁeld
can release its energy only so fast. After calculating these
numbers, Hudson also concludes that the average sunspot
can't produce ﬂares much larger than the one in 2003.
All of these calculations, however, don't deﬁnitively
exclude a solar superﬂare from ever happening. They
simply point to the fact that the Sun, as it behaves right
now, is unlikely to produce a superﬂare. But these data
are only from recent historical records, and our middleaged Sun has been around for 4.5 billion years. During
that time, it has displayed some erratic, unpredictable
behavior - such as the Maunder Minimum, between
1645 and 1715, when the Sun went nearly spotless - and

Sk yandTelescope.com November 2015

http://www.SkyandTelescope.com

Sky and Telescope - November 2015

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