It's Birkeland's Birthday (TPOD Dec 13, 2010)

[Nereid]

It's Birkeland's Birthday (TPOD Dec 13, 2010), by Stephen Smith

The image is taken from the Photos et films page in the Planeterrella website; there are many cool images, and it's a great site too!

The University College London's Department of Physics and Astronomy (Astrophysics Group) is one of several groups (or teams) around the world working on the Saturn plasmasphere (the "version" referred to is of STIM, the Saturn Thermosphere Ionosphere Model):

A simplified 2-d and 1.5-d version has recently been used to examine the contributions of Joule heating and ion drag to the thermospheric body temperature (Nature, 445, 399 (2007)). Another version of this model - called KIM (Kronian Ionosphere Model) - uses a plasma tube based thermospheric code (derived from the CTIP plasma tubes) to extend the ionosphere out into the Saturnian plasmasphere - this is being used to develop the chemistry of the interaction with the Saturnian rings.

[mharratsc]

We need a model that concisely demonstrates the polar hexagons... :\

[shadowmane]

I just read this one, and got to thinking. This is something I've been thinking on for a while now. If the currents go from the sun to the planets, could that current be followed by an electrified probe for faster travel between planets, or even stars? The question then becomes, how do you get to the pole of the sun in order to find the current leading out of the solar system? And if our sun is connected to another star, just which star is it?

[Nereid]

I just read this one, and got to thinking. This is something I've been thinking on for a while now. If the currents go from the sun to the planets,

As far as I know, no such currents have yet been observed, by spacecraft travelling around in the space between the Sun (well, ~Mercury's orbit) and any of the planets.

At least no currents that have any significant amount of oomph.

could that current be followed by an electrified probe for faster travel between planets, or even stars? The question then becomes, how do you get to the pole of the sun in order to find the current leading out of the solar system? And if our sun is connected to another star, just which star is it?

Don't you first have to show, quantitatively, that such a current exists? And if so, what its properties are?

[Jarvamundo]

Nereid: what would you say causes the auroras?

[solrey]

Mike said:

We need a model that concisely demonstrates the polar hexagons... :\

I'm working on something to that effect.
My research indicates the "drift-Alfven model" should provide the correct mathematical treatment, with in-situ data from Saturn (primarily from Cassini), to simulate the polar polygonal vortices. The model describes a rather broad range of stability for six lobed hexagons as well as current "streets" between the lobes. It also fits the south polar vortex as a two lobe configuration.

The Alfven equations take the form of three coupled Lagrangian equations describing the conservative evolution of three generalized vorticities.

That's about as concise as it gets.

Here's a paper describing the drift-Alfven model:
Vortex interactions in magnetized plasma


cheers

[mharratsc]

Pretty cool stuff, bud! Keep us posted on it, willya?

[Nereid]

Nereid: what would you say causes the auroras?

I assume you mean the ones on Saturn ... if so, then, in the words of the UCL webpage, solar wind-magnetosphere interaction.

[Influx]

I just read this one, and got to thinking. This is something I've been thinking on for a while now. If the currents go from the sun to the planets,

As far as I know, no such currents have yet been observed, by spacecraft travelling around in the space between the Sun (well, ~Mercury's orbit) and any of the planets.

At least no currents that have any significant amount of oomph.

And what exactly is the solar wind? Wind?

"The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 10 and 100 keV. The stream of particles varies in temperature and speed over time. These particles can escape the Sun's gravity because of their high kinetic energy and the high temperature of the corona."

"Electric current means, depending on the context, a flow of electric charge (a phenomenon) or the rate of flow of electric charge (a quantity). This flowing electric charge is typically carried by moving electrons, in a conductor such as wire; in an electrolyte, it is instead carried by ions, and, in a plasma, by both."

WHAT IS PLASMA?

"In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. The basic premise is that heating a gas dissociates its molecular bonds, rendering it into its constituent atoms. Further heating leads to ionization (a loss of electrons), turning it into a plasma: containing charged particles, positive ions and negative electrons.[1]

The solar wind is a charged flowing plasma, therefore it is a current.

I assume you mean the ones on Saturn ... if so, then, in the words of the UCL webpage, solar wind-magnetosphere interaction.

At least no currents that have any significant amount of oomph.

All of the aurorae in the solar system are caused by this "current", I'd say that is plenty of oomph.

"HOW MUCH POWER DO AURORAE PRODUCE?"

"Typically they produce tens of billions of watts of electrical power, but there is no known way to harness this energy because of the vast scales over which it is produced, and the very low density of this power. It would be like trying to harness the mechanical energy of a flock of mosquitoes. If you want to keep up with the actual daily/hourly power output of aurora, have a look at the NOAA/POES satellite website. They produce plots like the one above which give the total equivalent power of auroral activity in the north and south hemisphere. The numbers in their archive show a range from 4 to over 300 billion watts of power in the northern hemisphere alone. The total world production would be about twice this amount because there are nearly identical aurora in the southern hemisphere too."

Granted the power is fairly defused, but the totals are staggering, especially if you factor the rest of the solar system in. And even then the planets "intercept" a tiny fraction of the power.

[Nereid]

Actually, Influx, current is defined as the net flow of charge (there's also stuff about direction, etc).

So, the current through a surface (however defined) is the number of + charges, and the number of - charges, in a given time, suitably summed (the direction of any current is that orthogonal, or normal, to the surface). If the total charge through the surface is zero, then there's no current.

Perhaps another example might help?

Salt water contains negative ions (of chlorine, for example) and positive ones (of sodium, for example). The numbers usually balance, to within one part per thousand, million, or whatever. So, if you pick up a (plastic) bucket of salt water and carry it across the room, is there a current? After all, charges have been moving!

The answer is no; can you see why?

[Influx]

So, if you pick up a (plastic) bucket of salt water and carry it across the room, is there a current? After all, charges have been moving!

The charges were moved, big difference between moving a bucket of salt water and the flow of plasma! "Solar wind."

[Nereid]

OK, now put the salty water into a (plastic) pipe, and connect a (mechanical) pump. The water now flows in the pipe.

That is certainly a current (moving water), but is it an electrical current? After all, there are moving charges, aren't there?

The answer is no, there's no electrical current (well, there may be a teensy weensy one; cosmic rays may produce temporary imbalances of electrical neutrality, for example).

Can you see why (there is no electrical current)?

[Influx]

"Electric current means, depending on the context, a flow of electric charge (a phenomenon) or the rate of flow of electric charge (a quantity). This flowing electric charge is typically carried by moving electrons, in a conductor such as wire; in an electrolyte, it is instead carried by ions, and, in a plasma, by both."

Both refers to ions and electrons.

http://en.wikipedia.org/wiki/Electric_current

[Nereid]

"Electric current means, depending on the context, a flow of electric charge (a phenomenon) or the rate of flow of electric charge (a quantity). This flowing electric charge is typically carried by moving electrons, in a conductor such as wire; in an electrolyte, it is instead carried by ions, and, in a plasma, by both."

Both refers to ions and electrons.

http://en.wikipedia.org/wiki/Electric_current

I'm a little surprised, Influx, to find you quoting Wikipedia as an authoritative reference, especially in light of the content of this Thunderbolts Forum thread.

Salty water contains both negative and positive ions, and if the salty water is flowing, the charges are, by definition, also flowing ... yet there is no electric current.

In any case, as physics is quantitative, we need a quantitative definition of electric current, if we're going to discuss (plasma) physics, don't we?

[Influx]

I'm a little surprised, Influx, to find you quoting Wikipedia as an authoritative reference, especially in light of the content of this Thunderbolts Forum thread.

Well, I don't represent the views of Thunderblog, and they mine. This is, after all, a forum, not a classroom.

Salty water contains both negative and positive ions, and if the salty water is flowing, the charges are, by definition, also flowing ... yet there is no electric current.

Sigh...pay attention to charge carriers!

The plasma is the charge carrier. The plasma is not being carried along by "water."