Sky and Telescope - August 2018 - 17

waves propagate upward
into the corona and
drive its temperature to
million-degree levels. These
waves move rapidly and so require
observations with very high time resolution and very low
background light to record any faint changes in their
structure and strength. The white-light and near-infrared
cameras in the WB-57 provided that, taking images 30
times per second. The result was a huge amount of data
that have yet to be analyzed fully. But Caspi has promised
that if the waves are there, his team will ﬁnd them.

G R A PH: N ASA

The Shadowed Ionosphere
Standing in the Moon's umbral shadow offers an otherwise unobtainable view of the Sun's atmosphere. Closer
to home, that same shadow provides a unique laboratory

Temperature (Kelvin)
20,000

1,000

10,000

2 × 106

100,000

Density

Altitude (kilometers)

by the Moon's limb across the chromosphere during its ﬂeeting visibility before and after totality. These infrared observations reveal structures not observed by the wavelengths seen
with space-based observations.
To measure coronal emission lines with sufﬁcient precision and sensitivity required a different kind of approach,
because our atmosphere's light absorption and its own heat
interfere with these observations. To get good data, a team led
by Jenna Samra (Harvard) and Edward DeLuca (Smithsonian Astrophysical Observatory) placed their spectrometer in
a Gulfstream V jet that carried it to an altitude of 13.7 km
(45,000 feet). There they measured - for the ﬁrst time -
spectral lines from 1.4 to 4 microns that are sensitive to the
strength of coronal magnetic ﬁelds. In fact, two of the lines
had never been recorded before.
Also working in the stratosphere were Caspi and his team,
who used two WB-57 aircraft to observe the eclipse. This
wasn't the ﬁrst time that researchers literally chased the
Moon's shadow - for example, a supersonic Concorde ﬂight
in 1973 immersed itself in totality for 74 minutes! Although
more modest, the two NASA planes ﬂew in a staggered formation that offered an uninterrupted view of the corona for 7½
minutes from an altitude of 15.2 km,
putting them above more than 85%
of Earth's atmosphere.
Solar physicists suspect that
different types of magnetic

Temperature
Corona

3,200

Transition region

3,000

Chromosphere
500

Photosphere
0
10-11

10-9

10-7

10-5

10-3

Electron density (electrons per cm )
p COMPLEX LAYERS The zone from the Sun's photosphere to its
lowermost corona is less than 10,000 km deep, yet it is a region of
remarkable transitions in pressure and temperature that solar physicists
do not yet fully understand. Only during total solar eclipses can all of
this important region be studied.

that lets us study energy ﬂow from the Sun to our planet's
surface and the dynamics of Earth's upper atmosphere.
Imagine being in space above Earth, looking down on our
planet during last August's eclipse. From this vantage, the
Moon's shadow plays a different role. The day-and-night cycle
of incident sunlight is a main driver of our atmosphere's daily
changes - from the very edge of space down through the
mesosphere and stratosphere to the troposphere, where air
ﬁnally interacts with land and water. During the day, the
Sun's ultraviolet radiation breaks
down neutral atoms in the
upper atmosphere, creating the electriﬁed soup of

electrons and ions called the
ionosphere. This broad layer
reacts to changes in the intensity
of sunlight, the encroaching solar
wind above (space weather), and Earth
below (terrestrial weather).
The ionosphere both refracts and transmits radio waves,
sk yandtelescope.com * AUGUST 2 018

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Sky and Telescope - August 2018

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