Lines of black aurora (29-3-2006)

[Axel]

Woa, I am confused. The caption explaining why there are black gaps in auroras says "negatively charged particles may be sucked out from the Earth's ionosphere along adjoining magnetic field lines." It's the "adjoining magnetic field lines" that get me. Sure enough, large parts of the aurora in the picture (especially to the left) show regular striations that imply the existence of adjoining lines. And I have noticed such striations in many other auroras.

But I was taught that magnetic lines of force are either a convention respresenting a smooth gradient, like lines of equal elevation on a map, or else an artefact of the iron filings in the high school lab. Are there really lines? If not, why do the striations in auroras sometimes look so orderly and evenly spaced?

[harry]

Hello
I usually collect links with specific topics here are some


Re: Link
http://antwrp.gsfc.nasa.gov/apod/ap060329.html

go to this link for further info

http://www.phys.ucalgary.ca/~trondsen/p ... tures.html
http://www.gi.alaska.edu/ScienceForum/ASF4/422.html
http://sci.esa.int/science-e/www/object ... ctid=29100

http://www-istp.gsfc.nasa.gov/Education/whmfield.html

http://www-istp.gsfc.nasa.gov/Education/wposion.html

http://liftoff.msfc.nasa.gov/academy/sp ... phere.html

http://www.haarp.alaska.edu/haarp/ion1.html
these are nice images
http://spaceweather.com/aurora/gallery_01mar06.htm

Aurora search: wow niceeeeeeee
http://antwrp.gsfc.nasa.gov/cgi-bin/apo ... rch?aurora

Sorry for so many images, its just that its so interesting

[Axel]

Thanks for the links. I don't think any of them answers my question, although the ESA page on the cluster quartet probes is very interesting. Two supplementary questions: (1) is anyone mapping/has anyone mapped the distribution of those "holes" in the ionosphere at any given time? (2) while the black spaces in auroras are described as "abnormally black", that could mean many things: for instance, are they simply patches of normal night sky interrupting the aurora, or are stars in those patches actually dimmed?

Sorry to be adding more questions. Feel free to give me a few billion dollars for further research.

[Qev]

But I was taught that magnetic lines of force are either a convention respresenting a smooth gradient, like lines of equal elevation on a map, or else an artefact of the iron filings in the high school lab. Are there really lines? If not, why do the striations in auroras sometimes look so orderly and evenly spaced?

The whole usage of the term 'magnetic field-lines' in astrophysics has always bothered me for the same reason. A great example of where this confuses me is in descriptions of 'field-line reconnection' during solar magnetic explosions. It's probably just a case of analogy-confusion on my part.

The anti-auroras, as they call them, aren't actually darker than the nighttime sky, they're more-or-less non-glowing gaps in the aurora. I'm pretty sure that stars would shine through them as they would through an empty, non-auroral sky.

[astro123]

Magnetic field lines are perfectly real. They occur in many situations such as the aurora, and the good old iron-filings experiment. Look at solar flares, and SOHO images of the sun's corona -you see the looped field lines.
Spacecraft can determine the lines' positions by monitoring the density of charged particles as they fly through. Under the right conditions the field becomes "frozen-in" to the surrounding plasma. Bulk motion of the plasma can then push the field-lines around. Notice the aurora is always dancing around? those field-lines are literally blowing in the (solar) wind. Reconnection occurs when a line is bent too far, doubling back on itself until two portions with the same polarity come into "contact". The line then re-connects at this knot-point eliminating the stress by releasing all the mechanical energy that had been bending the line. This energy is expended in accelerating the plasma particles attached to the line.

[Qev]

I always understood 'fields' to be continuous, with the 'field lines' being an artifact of how we represent them, defining surfaces of constant field density analogous to the lines on a topographical map defining areas of constant height. I've never been so good with field theory though.

[astro123]

Hi Qev, Field lines may be analagous to contours on a map, but they are not the same thing. Think about contours, as you move from one line to the next the height changes smoothly. This is not the case with field lines. The lines are places of high magnetic field, the space between the lines are regions of low field strength. The lines repel each other and the density of lines increases as the field strength increases. Often you will see references to magnetic field "density" which is roughly the number of field-lines per some volume or area. In the earth's magnetosphere each field line has charged particles orbiting it (or gyrating), the radius of the orbit is called the gyroradius, and the time taken to complete an orbit is described by the gyrofrequency -both depend on the particle's charge and momentum. The orbiting particles drift along the field-lines with time. Where are the ends of this field line? they are tied to the earth's poles. This is why the aurora happen around the poles and not elsewhere -those gyrating particles are bouncing back and forth between the poles and slamming into the atmosphere each time. Those dark regions mentioned in the beginning of this post are regions of low field density. Hope this helps! its a fascinating subject, I'm always amazed how much is known!

[Pete]

The lines are places of high magnetic field, the space between the lines are regions of low field strength.

If this were true, a compass needle would bounce all over the place as you traversed the globe. Field lines are NOT physically real; they're conceptual aids. A magnetic field does not consist of discrete lines of influence with empty spaces between them in which the field is weaker. Spiral motion of charged particles "along" magnetic field lines is caused by the Lorentz force which acts perpendicular to both the magnetic field and the component of motion perpendicular to the field. Motion parallel to a magnetic field produces no force; particles trace out a spiral path if their speed has a component in line with the magnetic field. If its initial speed is entirely perpendicular to the field, the particle traces out a circle. This page has a nice diagram and explanation: http://www.jupiterradio.com/jove-emission.php

[astro123]

If field lines were purely conceptual aids, one could pick any path parallel to the field and all such paths would be equivalent. But this is not right either!
We see charged particles (and magnetic particles) concentrated along specific lines. These lines appear to be relatively permanent, they move with the plasma, and particles attached to a given field line do not move between lines. This argument over reality is really an abstract philosophical one, as many phenomena can be described as fictitious when viewed in the context of a particular paradigm. However the plasma particles aren't capable of "concepts" so they must be responding to some property of the potential. Whether you view the resulting "field line" or flux tube (which you can clearly see and measure, along with its effects and interactions with the surroundings) as a "real" object or not, is up to you! Real-life magnetic fields encountered in astrophysics are not smooth well behaved things, they are tangled and complex, the everyday laws of physics are only typically valid over scales on which the field is smooth. The theory of magnetohydrodynamics (MHD) takes over in these situations. Unfortunately its not easy to comprehend. Check out the SOHO website and science.nasa.gov/ssl/pad/solar/the_key.htm

(thanks for pointing out my mistake in oversimplifying)

[Qev]

I think I've been misinterpreting field lines, actually. They indicate the local direction of the magnetic field, don't they. The density of field lines at a particular location indicates the strength of the field there. Right?

I'm tending to agree with Pete on this one... at least, his description most closely matches how I understood fields to be. Of course, field interactions with charged plasmas and whatnot can probably lead to some interesting structures, what with field pinning and stuff like that going on.