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Plasma physics

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The Physicist

Physicist: Generally speaking, by the time a gas is hot enough to be seen, it’s a plasma.

The big difference between regular gas and plasma is that in a plasma a fair fraction of the atoms are ionized.  That is, the gas is so hot, and the atoms are slamming around so hard, that some of the electrons are given enough energy to (temporarily) escape their host atoms.  The most important effect of this is that a plasma gains some electrical properties that a non-ionized gas doesn’t have; it becomes conductive and it responds to electrical and magnetic fields.  In fact, this is a great test for whether or not something is a plasma.

For example, our Sun (or any star) is a miasma of incandescent plasma.  One way to see this is to notice that the solar flares that leap from its surface are directed along the Sun’s (generally twisted up and spotty) magnetic fields.

A solar flare as seen in the x-ray spectrum.

A solar flare as seen in the x-ray spectrum.  The material of the flare, being a plasma, is affected and directed by the Sun’s magnetic field.  Normally this brings it back into the surface (which is for the best).

We also see the conductance of plasma in “toys” like a Jacob’s Ladder.  Spark gaps have the weird property that the higher the current, the more ionized the air in the gap, and the lower the resistance (more plasma = more conductive).  There are even scary machines built using this principle.  Basically, in order for a material to be conductive there need to be charges in it that are free to move around.  In metals those charges are shared by atoms; electrons can move from one atom to the next.  But in a plasma the material itself is free charges.  Conductive almost by definition.

Jacob's Ladder; for children of all ages

A Jacob’s Ladder.  The electricity has an easier time flowing through the long thread of highly-conductive plasma than it does flowing through the tiny gap of poorly-conducting air.

As it happens, fire passes all these tests with flying colors.  Fire is a genuine plasma.  Maybe not the best plasma, or the most ionized plasma, but it does alright.

Because the flame has a bunch of free charged particles it is pushed and pulled by

The free charges inside of the flame are pushed and pulled by the electric field between these plates, and as those charged particles move they drag the rest of the flame with them.

Even small and relatively cool fires, like candle flames, respond strongly to electric fields and are even pretty conductive.  There’s a beautiful video here that demonstrates this a lot better than this post does.

The candle picture is from here, and the Jacob’s ladder picture is from here.

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NASA’s Solar Dynamics Observatory, in orbit since 2010, is on a mission to help us understand the sun’s impact on Earth and nearby space, producing some the most dramatic images of the solar atmosphere. This image from March 13, 2012, shows a “medium-sized” solar flare (the bright spot on the right) which is big enough to cause radio blackouts in the Earth’s polar regions. Solar flares are bursts of radiation that come from a release of intense magnetic energy, and are our solar system’s largest explosive events. They can last from minutes to several hours. The image is taken in a wavelength of the electromagnetic spectrum which allows us to see what the sun’s atmosphere looks like.

NASA/SDO

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One of the biggest solar flares so far this year unleashed a strong geomagnetic storm Thursday that painted night skies with the shimmering hues of the Northern Lights. The wavering aurora light is caused by the electromagnetic interplay between the speeding particles of a coronal mass ejection from the sun and the Earth’s magnetic field. These solar flares are expected to increase in the months ahead as the sun ramps up to its solar maximum, which is expected to peak in late 2013.


Truckers left the paths of their tail lights below the bright night sky as they drove along the ice road on Prosperous Lake near Yellowknife, North West Territories on Thursday. (Bill Braden, The Canadian Press/Associated Press)


The Northern Lights were visible near Fáskrúðsfjörður on the east coast of Iceland, left, and near Yellowknife, North West Territories, right. (Jonina Oskardottir/Associated Press, left; Bill Braden, The Canadian Press/Associated Press, right)


The aurora borealis near Yellowknife, North West Territories. (Bill Braden, The Canadian Press/Associated Press)


The sky glowed over power lines at mile 9 on the Old Glenn Highway near Butte, Alaska. (Oscar Edwin Avellaneda/Reuters)

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NASA

Many auroral displays appear green, but sometimes, as in this Sept. 26 image from the International Space Station, other colors such as red can appear.

Alan Boyle writes

"Red sky at night, sailor's delight": That's one of the oldest sayings in the book when it comes to weather prediction, but this picture adds a new twist. The red sky is an aurora, seen from above by astronauts on the International Space Station. And the weather that's causing this phenomenon is space weather from the sun.

Auroras arise when electrically charged particles from the sun interact with atoms in the upper atmosphere, sparking emissions of light at various wavelengths. The displays are most likely to be visible around Earth's magnetic poles, where the interaction is strongest. The sun has been going through an upswing of activity over the past couple of months, which has generated a colorful series of northern and southern lights.

North or south, the most common shade of auroral light is green. That's the wavelength that's typically emitted when solar particles mix it up with oxygen atoms. But if there are lower-energy collisions with oxygen atoms or nitrogen atoms, the emissions edge toward the reddish end of the spectrum. That's what's happening in this picture, captured on Monday. You should be able to make out the space station's solar panels toward the upper left corner of the photo.

Space weather can create disruptions for satellite communication systems as well as electric grids on Earth, but so far the most noticeable effect from this year's solar storms has been a string of glorious auroras. We weathered the latest geomagnetic storm overnight, and SpaceWeather.com is offering up a glorious selection of snapshots from the event — including this red-and-green stunner from Russia's Kola Peninsula.

To learn more about the colors of the aurora, check out this "Causes of Color" explanation. And if you live in northern or southern climes, there's always a chance of seeing the lights for yourself. Last night, the aurora was visible from Minnesota, Germany and Poland in the north, as well as New Zealand in the south. The University of Alaska at Fairbanks provides this handy-dandy online guide to aurora-watching.

More auroral glories:

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding me to your Google+ circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.

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