Tuesday, March 06, 2007

Jupiter Poles

This photo of Jupiter combines a visual image from Hubble with X-ray data from the Chandra spacecraft. The North and South poles are alive with X-rays. Like Earth's aurora, this display occurs in both poles simultaneously 140,000 km distant! Something in Jupiter's core links the poles. Jupiter has an immense magnetic field, the strongest in the solar system. Presence of a singularity would explain Jupiter’s field, and why Jupiter gives off nearly twice as much heat as it receives from the Sun.

Cassini has seen a huge hurricane at Saturn's South Pole. The largest planet does not have a storm centred at the South Pole because Jupiter’s magnetic poles do not line up with the geographic poles. The magnetic field is angled 9.5 degrees, indicating that Jupiter’s singularity is offset like that of Earth. Jupiter completes its day in less than 10 Earth-hours. A southern jet radiating from Jupiter’s core would be bent further outward by the rapid rotation. Magnetic fields would cause the storm to rotate in an anti-clockwise direction as seen from above. For signs of such a storm, we can look at the latitude of Jupiter’s Great Red Spot.

Astronomers like Geoffrey Marcy and Debra Fisher have found "hot Jupiters" in other solar systems, giant planets orbiting very close to stars. Old ideas of planet formation can not explain these objects. They could not have formed solely from condensing gas, for solar radiation would cause the gas to evaporate. Presently astronomers speculate that they formed farther from their suns, and then drifted closer after formation. All of them? If that is true, why haven't our giant planets drifted into the Sun?

If giant planets formed around singularities, a Black Hole's gravity would prevent them from dissipating. Hot Jupiters could then form extremely close to stars. Rotating Black Holes could also account for the magnetic fields of Uranus and Neptune, and why those fields are far offset from the planets’ rotation axes. Jupiter's X-rays could result from the polar jets of a Black Hole.

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Blogger Rae Ann said...

Fascinating! Watched a show about Earth's auroras last night. I didn't realize that they were still quite mysterious.

3:21 AM  
Blogger L. Riofrio said...

Glad you are enjoying the posts. The auroras are fascinating. I grew up used to tropical climates. Earth's poles are like another world.

7:59 AM  
Blogger nige said...

This comment has been removed by the author.

2:12 PM  
Blogger nige said...

It is a wonderful composite picture of x-ray aurora, but the mechanism might be simple.

Jupiter has a massive magnetic field with many times the van Allen belt effect of Earth.

You may recall the story. Gilbert in 1600 AD worked out that Earth is a giant magnet. Then in 1958, van Allen sent a geiger counter into space in one of the early "Explorer" satellite series.

The count rate, radioed back, increased exponentially to a vast extent and then suddenly went to zero as the geiger counter saturated.

Clearly people knew before 1958 that there is cosmic radiation in space which is more intense than on the Earth because we're shielded by the atmosphere (equivalent to 10 metres of water shielding you) and a space probe has no such protection.

But the increase was far more than could be accounted for by cosmic radiation. So van Allen deduced that cosmic radiation, particularly electrons, had become trapped by the Earth's magnetic field.

A moving electron in space, unless it happens to be moving parallel to the Earth's magnetic field lines, is made to spiral around the Earth's magnetic field lines because of the Lorentz force, F = qvB.sin{theta}

Electrons and protons are lost by the sun, and the solar wind fires this radiation at the earth.

So charged particles get trapped in space in massive shells out to many Earth radii, spiralling around the magnetic field lines and travelling between poles.

When they approach a pole, the magnetic field strength increases (i.e., the magnetic field lines come closer together, so that the number of imaginary field lines passing through a given area is increased).

This increase in magnetic field strength where the field lines approach close to the Earth's poles makes the radiation spirals flatten out, slowing the forward speed of particles until they either stop and then reflect back (at a "conjugate" point above the atmosphere), or else leak into the atmosphere around the poles, striking air molecules and ions, which release light (causing the aurora) while falling back to their ground state energy levels.

In the case of Jupiter it is exactly the same, except for a bigger planet and more trapped radiation, and also a denser atmosphere.

Hence, the x-ray aurora at Jupiter's poles is due to high energy electrons at the poles hitting a denser atmosphere than Earth's. X-rays are produced by decelerating electrons, or by line spectra like ordinary light emission but at higher energy. The x-ray mechanism by electrons getting decelerated is due to Coulomb scatter which stops the electrons and reflects them, with some emission of braking radiation (bremsstrahlung) of x-ray frequency.

Louise, do you know if the Chandra spacecraft has resolved the energy spectrum of the x-rays from the aurora at Jupiter's poles? This would be one direct way to find out what is the auroral mechanism for the x-rays.

2:17 PM  
Blogger L. Riofrio said...

Nige, the Chandra data is pretty sketchy because the spacecraft was designed to look at more distant objects. I would love to know the detailed spectrum, or if there are gamma rays as well.

Auroras and the Van Allen Belt are caused by Earth's magnetic field, which still doesn't explain what causes the field. Gilbert thought Earth was a permanent magnet, but minerals would lose their magnetism in the high temperatures of Earth's interior. This quandary led to the "dynamo theory" of the interior. The power source spinning the dynamo has long been a mystery.

5:00 PM  
Blogger nige said...

Hi Louise,

Well, we know that the Earth's magnetic field flips suddenly every so often:

"Sometimes the field completely flips. The north and the south poles swap places. Such reversals, recorded in the magnetism of ancient rocks, are unpredictable. They come at irregular intervals averaging about 300,000 years; the last one was 780,000 years ago. Are we overdue for another? No one knows."

So the source of the dynamo effect is provably not a stable effect.

It's evident that earth's rotation and the meteorological effects of that rotation (Corolis force) in the atmosphere give some clues about how the dynamo effect is set up. There is convection of hot plumes in the mantle, and that may be analogous over thousands of years to processes taking place in the atmosphere over a shorter time scale.

Hence, convection currents and the Corolis force effects on them would create very "slow motion" storm effects in the mantle, taking thousands of years to rotate.

The overall effect would be a magnetic field. There is probably a slight radial electric field between the iron/nickel core of the earth and the mantle, as you get some electron transfer when rubbing together two dissimilar materials. The charge separation, affected by the earth's rotation, would cause magnetic fields:

"At the heart of our planet lies a solid iron ball, about as hot as the surface of the sun. ... The inner core is 70% as wide as the moon. It spins at its own rate, as much as 0.2° of longitude per year faster than the Earth above it, and it has its own ocean: a very deep layer of liquid iron known as "the outer core."

"Earth's magnetic field comes from this ocean of iron, which is an electrically conducting fluid in constant motion. Sitting atop the hot inner core, the liquid outer core seethes and roils like water in a pan on a hot stove. The outer core also has "hurricanes"--whirlpools powered by the Coriolis forces of Earth's rotation. These complex motions generate our planet's magnetism through a process called the dynamo effect."

Maybe the magnetic fields of Jupiter are created in a way similar to those of Earth, just with a hot (ionized) plasma core rotating in place of Earth's iron/nickle core.

5:32 AM  
Blogger nige said...

Of course the "dynamo effect" isn't a helpful way to describe the mechanism whereby the Earth's magnetic field arises.

In a dynamo, motion of coils of wire with respect to magnets causes currents to be induced in the wire.

In the Earth, it's surely an electric motor effect, not a dynamo effect.

The motion of electric charge (i.e., a current) creates a magnetic field.

5:37 AM  
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I have studying about Jupiter and I think that this information can help me to learn more about it, I think that the magnetism is a great topic.

5:33 AM  
Anonymous Sildenafil said...

I was study something related with the changes in the magentic fields in the Earth, but now result that is not only the Earth, all the planets of the solar system has changed it magnetic fields, this in order to obtain a new position in a solar perspective.

4:24 AM  
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You may recall the story. Gilbert in 1600 AD worked out that Earth clockwise direction as seen from above. For signs of such a storm, we can look at the latitude of Jupiter’s Great Red Spot.

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