Sky and Telescope - August 2018 - 19

p TO THE STRATOSPHERE A flight crew prepares one of two NASA-owned WB-57
aircraft for a test run from Ellington Field in Texas. The planes tracked the eclipsed
Sun with visible-light and infrared cameras.
tp SHADOW CHASERS Based at Ellington Air Force Base north of Houston, two of
NASA's WB-57 aircraft chased the Moon's shadow along staggered flight paths that
provided 7½ minutes of corona observations.

FLIG H TPL A N: N ASA; THER M A L-INFR A RED IM AG ES: N ASA / SWRI / A MIR CASPI /
DA N SE ATON; JE T: N ASA / SWRI / A MIR CASPI

t WARMTH OF THE SUN Using specially equipped NASA planes flying at high
altitude, investigators captured the totally eclipsed Sun in visible light (at left, filtered
green to improve coronal contrast) and at thermal-infrared wavelengths.

example, they found signiﬁcant differences in how the ionosphere responded in longitude and latitude - and even saw
changes in the propagation characteristics in magnetically
connected ionospheric regions over South America.
One of the most exciting results was conﬁ rmation of
a prediction made almost 50 years ago. In 1970, George
Chimonas (then at University of Toronto) deduced that a
bow wave would form in the ionosphere - much like the
wave created by a boat moving through water - as the
Moon's supersonic shadow sped across Earth. Chimonas
reasoned that electrons would rush in to ﬁ ll the ionospheric
hole, altering the total electron content just behind the
umbral shadow.
Observations during the 2017 eclipse conﬁrm this bow
wave's creation unambiguously for the ﬁrst time. Researchers measured changes in the ionopshere's TEC as the shadow
raced along that matched what would happen if a bow wave
had sped through. Furthermore, Earle's team detected a corresponding wave much deeper in Earth's atmosphere, in the
thermosphere layer just above the surface.
All of these eclipse-induced phenomena have given scientists tools to probe the nooks and crannies of the complex
ionospheric pie with keen new insight. The ionosphere has a
substantial impact on our technological way of life, and the
shadow cast during last year's eclipse has brought us closer
than ever before to understanding this critical region.

Effects Near the Ground
The environment closest to us, from the stratosphere to the
ground, was also "illuminated" by the Moon's shadow, creating

changes in the intensity of sunlight in the lower atmosphere.
Earth scientist Jay Herman (University of Maryland) and
colleagues took the opportunity to carefully measure the role
of clouds as regulators and reﬂectors of solar energy by using
the Moon as a giant, impenetrable cloud. Using images from
NASA's Deep-Space Climate Observatory (DSCOVR) spacecraft, like the ones shown on pages 16 and 17, they found
that the amount of sunlight on Earth fell by 10% during the
eclipse. By comparison, on any given day the global variation
is less than 1%. The team is a long way from explaining this
drop; the researchers think it'll take them a couple of years to
understand what's going on.
"Big-picture" views of how Earth looked during totality came from a novel effort called the Eclipse Ballooning
Project. Under the leadership of Angela Des Jardins (Montana State University), 55 teams of high-school and college
students across the country launched instrumented weather
balloons that climbed to altitudes of roughly 30 km (100,000
feet) in the stratosphere.
The cameras they carried aloft provided the ﬁrst-ever live
eclipse video from that altitude and showed that the top of
the planetary boundary layer, the region of the atmosphere
that interacts with Earth's surface, dropped nearly to its
nighttime altitude during totality.
Meanwhile, Kevin Knupp led a team from University of
Alabama, Huntsville, that monitored a pronounced reduction in turbulence, wave motions, and wind speed within the
boundary layer. These eclipse-induced changes were similar to
what occurs during the slower afternoon-to-evening transition near and after sunset.
sk yandtelescope.com * AUGUST 2 018

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Sky and Telescope - August 2018

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