Sky and Telescope - August 2018 - 16

2017 Eclipse Results

As the Moon's 110-km-wide shadow
raced at supersonic speed across
25%
the U.S., any given location
50%
75%
75%
75
5%
within the path had at most
90%
162 seconds of totality. That's
90%
90
0%
75%
not much time to make
50%
0%
%
25%
detailed and often intricate
observations of the corona's
faint, complex structure.
Some of this work focused on
the corona's temperature. Shadia
Habbal (University of Hawai'i) and her
team isolated emissions from highly ionized
atoms, especially iron, that yield the temperature, density,
and motion within the corona. They identiﬁed pockets of
material from prominences that were relatively cool (at
temperatures typically found in the chromosphere) despite
having been pushed up into much hotter coronal regions.
This ﬁnding addresses a longstanding question about the
fate of such material during coronal mass ejections: Apparently it survives the corona's searing environment, even as it
travels outward into interplanetary space. This result places
new, strong constraints on physicists' models of CMEs and
the role they play in space weather.
Meanwhile, a Williams College team led by Jay Pasachoff
also looked at both the white light scattered off coronal electrons as well as emission from highly ionized atoms. Comq SOLAR SWEET SPOT Spacecraft like NASA's GOES 16 can observe
the Sun's brilliant photosphere (center), and others like the venerable
Solar and Heliospheric Observatory can record streamers in its outer
corona (shown in red). But to study the innermost corona, the complex
region within about 1½ million km of the Sun, researchers must use the
fleeting views available during totality - such as the telescopic view
shown here, taken from Salem, Oregon.

AUGUST 2 018 * SK Y & TELESCOPE

t SHADOW ZONE The Moon's
umbral shadow crossed the
contiguous United States
from coast to coast during the
total solar eclipse on August
21, 2017. Lines parallel to the
central path denote the fraction
of the Sun's area covered by
the Moon. Dark spots along the
path mark the umbra's location
at 10-minute intervals.

q VIEW FROM
SPACE The
Moon's umbral
shadow and
surrounding penumbra created a
distinct dark spot
on North America
during the August 2017 total
solar eclipse.
These views are
from NASA's
Deep Space
Climate Observatory, positioned
1.5 million km
from Earth.

bining these observations with
those from Habbal's team provides observations at six different
locations along the path of totality
and covers a span of 34 minutes.
These observations conﬁrm that coronal temperatures cluster around two distinct values. The
1,000,000K corona consists of plasma that's expanding
away from the Sun, while the 2,000,000K corona is plasma
conﬁned by magnetic arches or loop-like structures that are
anchored to sunspot regions in the photosphere.
Several teams of solar physicists recorded complementary
observations from the ground and aircraft in a spectral range
only feasible near and during totality: infrared wavelengths
from around 1 to 6 microns. Two main goals were observing the chromosphere in visible light and measuring infrared spectral lines from coronal plasma. The latter serve as
ﬁngerprints of the magnetic ﬁeld that permeates and shapes
the solar corona. We can measure magnetic ﬁelds in the solar
photosphere quite well but doing so in the corona is notoriously difﬁcult.
To observe the chromosphere, a team led by Paul Bryans,
Phil Judge, and Steven Tomczyk (High Altitude Observatory)
measured what is called the chromospheric ﬂash spectrum. Seconds before and after the corona's appearance, the chromosphere brieﬂy becomes visible as a brilliant crimson arc along
the Sun's limb. The Moon was moving across the solar disk at
211 km per second (472,000 mph) as projected onto the Sun,
so by taking observations 100 times per second, the researchers could record a spectrum for every 2 to 4 km of movement

M A P: N ASA / SVS / ER NEST T. WRIG H T; SH A DOW SERIES: N ASA / DSCOV R / EPIC; SUN L AY ERS: SUN: N ASA / GOES 16 /
SU VI / DA N SE ATON; INNER CORON A: JAY PASACHOFF / RON DA N TOWIT Z / WILLIA MS COLLEG E ECLIPSE E X PEDITION /
N ATION A L G EOG R A PHIC / NSF / N ASA SPACE G R A N T (M A); OU TER CORON A: N ASA / SOHO / L ASCO CONSORTIU M

Probing the Corona's Light

Sky and Telescope - August 2018

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