APOD: The Dancing Auroras of Saturn (2010 Sep 27)

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APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by APOD Robot » Mon Sep 27, 2010 3:59 am

Image The Dancing Auroras of Saturn

Explanation: What drives auroras on Saturn? To help find out, scientists have sorted through hundreds of infrared images of Saturn taken by the Cassini spacecraft for other purposes, trying to find enough aurora images to correlate changes and make movies. Once made, some movies clearly show that Saturnian auroras can change not only with the angle of the Sun, but also as the planet rotates. Furthermore, some auroral changes appear related to waves in Saturn's magnetosphere likely caused by Saturn's moons. Pictured above, a false-colored image taken in 2007 shows Saturn in three bands of infrared light. The rings reflect relatively blue sunlight, while the planet itself glows in comparatively low energy red. A band of southern aurora in visible in green. Inspection of many more Saturnian images may well lead to an even better understanding of both Saturn's and Earth's auroras.

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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by mexhunter » Mon Sep 27, 2010 4:05 am

A photograph of very psychedelic tones, but of extraordinary sharpness and resolution.
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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by bystander » Mon Sep 27, 2010 10:53 am


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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by biddie67 » Mon Sep 27, 2010 12:40 pm

This picture looks more like a display found in a museum - amazing clarity. Couldn't/Wouldn't all the planets in our solar system be capable of having auroras?

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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by orin stepanek » Mon Sep 27, 2010 1:07 pm

Click to play embedded YouTube video.
Neat movie from today's links 8-)
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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by Chris Peterson » Mon Sep 27, 2010 2:00 pm

biddie67 wrote:This picture looks more like a display found in a museum - amazing clarity. Couldn't/Wouldn't all the planets in our solar system be capable of having auroras?
You need a planetary magnetic field and an atmosphere. That cuts down the possible planets.
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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by neufer » Mon Sep 27, 2010 2:30 pm

Chris Peterson wrote:
biddie67 wrote:
Couldn't/Wouldn't all the planets in our solar system be capable of having auroras?
You need a planetary magnetic field and an atmosphere. That cuts down the possible planets.
Well...you need an atmosphere at least:
  • "Because Venus has no intrinsic (planetary) magnetic field, Venusian auroras appear as bright and diffuse patches of varying shape and intensity, sometimes distributed across the full planetary disc. Venusian auroras are produced by the impact of electrons originating from the solar wind and precipitating in the night-side atmosphere."
http://en.wikipedia.org/wiki/Aurora_%28astronomy%29 wrote:
<<Both Jupiter and Saturn have magnetic fields much stronger than Earth's (Jupiter's equatorial field strength is 4.3 gauss, compared to 0.3 gauss for Earth), and both have large radiation belts. Aurora has been observed on both, most clearly with the Hubble Space Telescope. Uranus and Neptune have also been observed to have auroras.
Image
The auroras on the gas giants seem, like Earth's, to be powered by the solar wind. In addition, however, Jupiter's moons, especially Io, are powerful sources of auroras on Jupiter. These arise from electric currents along field lines ("field aligned currents"), generated by a dynamo mechanism due to the relative motion between the rotating planet and the moving moon. Io, which has active volcanism and an ionosphere, is a particularly strong source, and its currents also generate radio emissions, studied since 1955. Auroras have also been observed on Io, Europa, and Ganymede themselves, e.g., using the Hubble Space Telescope. These are generated when Jupiter's magnetospheric plasma impact their very thin atmospheres.

Auroras have also been observed on Venus and Mars. Because Venus has no intrinsic (planetary) magnetic field, Venusian auroras appear as bright and diffuse patches of varying shape and intensity, sometimes distributed across the full planetary disc. Venusian auroras are produced by the impact of electrons originating from the solar wind and precipitating in the night-side atmosphere. An aurora was also detected on Mars, on August 14, 2004, by the SPICAM instrument aboard Mars Express. The aurora was located at Terra Cimmeria, in the region of 177° East, 52° South. The total size of the emission region was about 30 km across, and possibly about 8 km high. By analyzing a map of crustal magnetic anomalies compiled with data from Mars Global Surveyor, scientists observed that the region of the emissions corresponded to an area where the strongest magnetic field is localized. This correlation indicates that the origin of the light emission actually was a flux of electrons moving along the crust magnetic lines and exciting the upper atmosphere of Mars.>>
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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by Chris Peterson » Mon Sep 27, 2010 2:52 pm

neufer wrote:Well...you need an atmosphere at least...
I did consider the possibility of direct impact on atmospheres by unfocused charged particles. So yes, I guess this can happen with enough energy to produce something like a visible aurora. But I'd still argue that you need a magnetic field to produce a true aurora as we generally understand the term. What you get on Venus and Mars seems to me more like a type of skyglow than an actual aurora. We get something similar here on Earth, as well, although our magnetic field attenuates it.
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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by gg » Mon Sep 27, 2010 3:16 pm

I am puzzled by the shadow band on the surface. If the red is really glow from the planet, why is there black band where the rings cast a shadow. Shouldn't the red glow still be visible but perhaps more lower in intensivity?

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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by biddie67 » Tue Sep 28, 2010 12:21 am

http://en.wikipedia.org/wiki/Aurora_%28astronomy%29 wrote:
..... Venus has no intrinsic (planetary) magnetic field, .......

Is Venus without a magnetic field because of it location near the Sun -or- because it's not made up of an iron-based core? What about Mercury?

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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by Chris Peterson » Tue Sep 28, 2010 1:23 am

biddie67 wrote:Is Venus without a magnetic field because of it location near the Sun -or- because it's not made up of an iron-based core?
This is not well understood. The most popular theory is that Venus has a molten iron core like the Earth, but it doesn't have a large enough thermal gradient from the interior to the exterior to produce convection, which is required for a dynamo-driven magnetic field. The lack of a large thermal gradient may be related to some sort of resurfacing that joined any plates, effectively halting tectonic processes.
What about Mercury?
Mercury has a moderately strong magnetic field. It is probably produced by the same sort of dynamo effect that drives Earth's field. Mercury is assumed to have a liquid iron core, which is probably kept liquid from tidal forces (Mercury has a very eccentric orbit).
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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by neufer » Tue Sep 28, 2010 2:02 am

gg wrote:
I am puzzled by the shadow band on the surface. If the red is really glow from the planet, why is there black band where the rings cast a shadow. Shouldn't the red glow still be visible but perhaps more lower in intensivity?
Atmospheric bands in the ring shadow never receive direct sunlight and therefore remain colder than the rest of the planet.
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Re: APOD: The Dancing Auroras of Saturn (2010 Sep 27)

Post by neufer » Tue Sep 28, 2010 2:14 am

Chris Peterson wrote:
biddie67 wrote:Is Venus without a magnetic field because of it location near the Sun -or- because it's not made up of an iron-based core?
This is not well understood. The most popular theory is that Venus has a molten iron core like the Earth, but it doesn't have a large enough thermal gradient from the interior to the exterior to produce convection, which is required for a dynamo-driven magnetic field. The lack of a large thermal gradient may be related to some sort of resurfacing that joined any plates, effectively halting tectonic processes.
It is hard to believe that the slow rotation isn't a major factor.
http://en.wikipedia.org/wiki/Venus#Magnetic_field_and_core wrote:
<<Venus rotates once every 243 Earth days—by far the slowest rotation period of any of the major planets.

In 1980, the Pioneer Venus Orbiter found that the Venusian magnetic field is much weaker than that of Earth's. This magnetic field is induced by an interaction between the ionosphere and the solar wind, rather than by an internal dynamo in the core like the one inside the Earth. Venus's small induced magnetosphere provides negligible protection to the atmosphere against cosmic radiation. This radiation may result in cloud-to-cloud lightning discharges.

The lack of an intrinsic magnetic field at Venus was surprising given that it is similar to Earth in size, and was expected also to contain a dynamo at its core. A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive and, while its rotation is often thought to be too slow, simulations show that it is adequate to produce a dynamo. This implies that the dynamo is missing because of a lack of convection in the Venusian core. On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much hotter than the top. On Venus, a global resurfacing event may have shut down plate tectonics and led to a reduced heat flux through the crust. This caused the mantle temperature to increase, thereby reducing the heat flux out of the core. As a result, there is not an internal geodynamo that can drive a magnetic field. Instead the heat energy from the core is being used to reheat the crust.

Another possibility is that its core has already completely solidified. The state of the core is highly dependent on the concentration of sulfur, which is unknown at present.>>
Art Neuendorffer

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