This week’s “guess the planet” image shows a network of
polygonal patterns. Polygons are common across the solar system, being found on
Earth, Mars and Pluto. Similar looking patterns can appear in networks of
fractures, including the desiccation cracks in drying mud, which you might see
in your garden at this time of year. On a different scale the thermal
contraction cracks that cover much of the northern region on Mars have a very
similar pattern of polygonal cracks, surrounding open spaces. On Pluto the
plains of the Sputnik Planum are covered with a network of roughly hexagonal
cells, which we’ve looked at on this blog before.
However the polygons in this week’s image don’t come from
any of these planets, but rather from a different type of solar system body
entirely. This is an image of the surface of the Sun.
This image was captured by the Hinode (ひので, Literally “Sunrise”) spacecraft,
a satellite designed to study the sun’s magnetic field. It was launched by the
Japan Aerospace Exploration Agency (JAXA,宇宙航空研究開発機構) in collaboration with several other
space agencies. The satellite orbits the Earth. However it is in a sun
synchronous orbit. This means that it circles the Earth at just the right rate
that the sun is always directly overhead. Essentially ensuring that it is
always the same time of day regardless of which part of the Earth it is
currently above. This gives it an uninterrupted view of the sun, which is a
massive advantage over ground based observatories. This image from Hinode shows
the change in temperature across a region of the sun’s surface. The description
for this image doesn’t indicate how large an area it covers. However these
solar “granules” can be more than 1000 km across, so we can get a rough sense
of the scale of the image.
This field of polygonal cells are the surface expression of
a very important process called convection. This is the process by which a hot
material rises, while cooler material sinks, leading to a circulation through a
fluid, and a redistribution of heat. This is because the density of a fluid is
strongly dependant on its temperature. As a region of the fluid heats up it
becomes much less dense than the surrounding material, causing it to rise buoyantly.
In our day to day lives we are most familiar with this process from the fact that
hot air rises, while cool air sinks. Convection processes can occur in any
fluid, from the atmospheres of planets, to the water in a saucepan.
In the case pictured here it is the plasma that makes up the
suns surface which is circulating. It is massively hotter than any air or water
on earth, but it is the change in density which is significant. The bright
areas in the centre of the polygonal cells are hot regions, where superheated
plasma is rising from the interior of the sun, and bringing energy with it. This
energy is then lost to space, cooling the material, which then sinks back into
the sun’s interior in the dark regions around the edges of the cells.
Along with conduction through solid objects, and radiation
through space Convection is one of the main ways in which energy can be transferred
from one place to another. It is very common, since it can occur in any
substance where a change in the density of the material can cause it to flow. This
gives us a handy short cut for understanding what is going on in the surface of
the sun. we understand convective processes at low temperatures, so can apply
this understanding to high temperatures in order to figure out how the heat
flow through the sun works.
Image credit:
Hinode JAXA/NASA/PPARC
https://www.nasa.gov/mission_pages/hinode/solar_013.html
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