Sky & Telescope - November 2023 - 19

NSO / AURA / NSF & NASA / SDO
Height above visible surface
(not to scale)
until they could sort out mechanical problems. " I remember
one day the images were very jittery, " Rimmele says. " It
turned out new cooling fans were installed in an electronic
cabinet, and it introduced vibrations all through the mechanical
structure into the optics. There is a continuous stream
of problems like this that we are still addressing and fixing,
almost on a daily basis. "
Nevertheless, DKIST has already provided valuable data.
The upcoming results of the first round of observations
promise to tell us more than ever before about our host
star. Scientists are especially optimistic that investigations
into flares and mass ejections, the solar wind, and heating
processes in the Sun's atmosphere will soon yield answers
to looming questions. They may even enable better spaceweather
predictions.
Heat Wave
For some unknown reason, the corona is millions of degrees
hotter than the layers directly below it. You might think that
the temperature would just get cooler and cooler as you move
outward from our star's center, but it's not that simple.
The core is by far the hottest part of the Sun, at 15 million
degrees Celsius. Subsequent layers do cool off, with the
photosphere measuring only 5500°C. But after that, things
change. Just above the photosphere, the chromosphere averages
a bit hotter, 6000°C. Then in the thin transition region
between the chromosphere and the corona, there is a sudden,
significant rise in temperature to 1 million degrees. The
corona itself ranges between 1 and 2 million degrees.
There's no straightforward explanation - certainly there
are no nuclear reactions, like in the core. Solar scientists have
several theories, all connected in one way or the other to
magnetic fields, which can carry and release energy that heats
surrounding plasma. Wave heating and magnetic reconnection
are two favored mechanisms.
Wave heating works via the convective
motions of plasma in the
photosphere, which propel magnetic
" disturbances " all the way
through to the corona. There,
the wave energy dissipates, heating
the atmosphere. Magnetic
reconnection, on the other
hand, involves the way that the
Sun's magnetic field lines tangle
and snap into new arrangements.
Heliophysicists think that
5,000
km
1,000
km
uCHANGING VIEW This series
of images shows how a sunspot
looks in different layers in the solar
atmosphere. The photosphere and
chromosphere images come from the
Dunn Solar Telescope in New Mexico;
the corona image comes from NASA's
Solar Dynamics Observatory.
the energy released in this process could result in small-scale
heating events called nanoflares. Astronomers have observed
both waves and reconnection events, but drawing a straight
line between these phenomena and coronal heating is not a
simple task. It is also possible that these are just two of many
mechanisms that contribute to coronal heating.
This is where DKIST comes in.
Astronomers took first-light images of the Sun with
DKIST's Visible Broadband Imager (VBI), a high-speed camera
that can resolve areas of the solar surface and lower atmosphere
as small as 16 kilometers across. Researchers designed
it specifically to study sunspots and other features on the
Sun's surface over time. These inaugural images show a
distinctly granulated pattern of boiling plasma, which rises
in the cells' centers and then cools off and sinks below the
surface in the darker sections between the granules. The cells
are each about the size of Texas, and the dark lanes between
bear tiny, bright markers of magnetic fields. Never before seen
at such high resolution, these glowing sparks appear to show
energy being channeled up from the surface into the corona,
perhaps acting as wave guides - structures formed by concentrated
magnetic fields that shepherd waves into the Sun's
atmosphere. More observations and study of this phenomenon
could help to support wave-heating theories.
Meanwhile, other DKIST work may support reconnection's
role instead. Research by Ryan Campbell (Queen's University
Belfast) and others in his group suggests that magnetic
reconnection and subsequent heating of the corona could be
facilitated by tiny little energy loops called magnetic bipoles,
which constantly cover the whole surface of the Sun. When
the loops' fields reconnect, magnetic energy converts into
radiation and kinetic energy, producing nanoflares that might
help heat the solar atmosphere. The telescope's microscopic
capabilities are key to these investigations.
Corona
Chromosphere
17.1
nm
656
nm
Photosphere
(visible
surface)
km
600 nm continuum
sk yand tele scope .o r g * NOVEMBER 2023 19
854
nm
543
nm
http://skyandtelescope.com
Sky & Telescope - November 2023

Table of Contents for the Digital Edition of Sky & Telescope - November 2023
