How do Planets generate electromagnetic waves?


by junk0
Tags: electromagnetic, generate, planets, waves
junk0
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#1
Dec1-12, 11:01 AM
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We can determine what kind of elements exist in different planets by observing electromagnetic waves from planets' direction, but does anyone have any suggestion on how planets generate electromagnetic waves at the first place?

Does anyone have any suggestions?
Thanks in advance for any suggestions
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Ibix
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#2
Dec1-12, 12:02 PM
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They don't. But they tend to be in orbit around stars that do, and they reflect. Much the same as Venus, Mars, Jupiter and the Moon.

Edit: actually, planets also absorb and re-radiate electromagnetic radiation from the star. This isn't as significant as straightforward reflection from the star, but it is a factor. This is called blackbody radiation, and the spectrum depends only on the temperature. It's exactly the same effect as iron glowing after it's been in a forge and, strictly speaking, it's emission of radiation rather than reflection. But it is still driven by energy from stars, not any internal process.
Drakkith
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#3
Dec1-12, 01:28 PM
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EM radiation can be created by several different mechanisms. The first and by far most dominant is called "Black Body Radiation". This simply means that a perfect "black body", which is something that would absorb all wavelengths of light that hit it, would emit radiation with a spread of wavelengths and intensities that varies directly with its temperature. The higher the temperature of an object, the higher the frequency of the radiation it emits and the more of it it emits. We don't have any perfect black bodies in reality, but most objects are close enough that we can ignore most of the small differences.

While all objects in the universe emit EM radiation because they are above absolute zero, planets aren't usually hot enough to emit visible light, since that takes a temperature of about 3,000 kelvin before we get much of anything in the visible range. Instead they emit microwave and infrared radiation mostly, which is of a much lower energy and frequency.

The planets in the solar system can be seen because of the light that is reflected from their surfaces or atmospheres. Since different elements can absorb different frequencies of light, the color of an object can be used to determine what it is composed of. (Technically not the color, but the spectrum seen with a spectrograph)

junk0
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#4
Dec1-12, 06:59 PM
P: 40

How do Planets generate electromagnetic waves?


Quote Quote by Ibix View Post
... planets also absorb and re-radiate electromagnetic radiation from the star. This isn't as significant as straightforward reflection from the star, but it is a factor. This is called blackbody radiation, and the spectrum depends only on the temperature. It's exactly the same effect as iron glowing after it's been in a forge and, strictly speaking, it's emission of radiation rather than reflection. But it is still driven by energy from stars, not any internal process.
Quote Quote by Drakkith View Post
... which is something that would absorb all wavelengths of light that hit it, would emit radiation with a spread of wavelengths and intensities that varies directly with its temperature.

... The planets in the solar system can be seen because of the light that is reflected from their surfaces or atmospheres. Since different elements can absorb different frequencies of light, the color of an object can be used to determine what it is composed of. (Technically not the color, but the spectrum seen with a spectrograph)
So will the amount of reflected radiation be increased by which following items?
  • Increasing the surface area of planets?
  • Increasing the temperature of planets?
  • Increasing the mass of planets?
  • Increasing the volumn of planets?
  • Increasing the density of planets?
  • Increasing the rotation period of planets?
  • Other factors ... ?

Does anyone have any suggestions?

Thanks in advance for any suggestions
Chronos
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#5
Dec2-12, 02:05 AM
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The 'radiation' you appear to be referring to are absorption lines in planetary atmospheres. This requires an external light source [eg, a star]. When light passes through a planetary atmosphere certain wavelengths are absorbed by elements present in that atmosphere. Light reflected off the surface suffers a similar effect. Certain wavelengths are preferentially absorbed based on the chemical composition of the surface.
Ibix
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#6
Dec2-12, 04:22 AM
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Blackbody radiation isn't really a significant factor for planets, at least in the visible wavelengths. That said, there are two things that affect the brightness of blackbodies - temperature and size. A hotter object emits more radiation per square meter, but the colour (or, more precisely, the spectrum) also changes. Two blackbodies at the same temperature have exactly the same spectrum and emit exactly the same amount of energy per unit area. However, a bigger object has more surface area, so is brighter overall.

None of this is relevant to working out what chemicals are present in the planet. That information comes from looking at the starlight reflected off the planet. As Chronos says, light at certain wavelengths is absorbed by the atmosphere and surface of the planet. Those wavelengths are characteristic of the chemicals present. We basically look for dark lines in the spectrum (where light has been absorbed) and compare them to a database of dark lines produced by different chemicals.
junk0
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#7
Dec2-12, 04:26 AM
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Quote Quote by Chronos View Post
The 'radiation' you appear to be referring to are absorption lines in planetary atmospheres. This requires an external light source [eg, a star]. When light passes through a planetary atmosphere certain wavelengths are absorbed by elements present in that atmosphere. Light reflected off the surface suffers a similar effect. Certain wavelengths are preferentially absorbed based on the chemical composition of the surface.
Your suggestions seem to me that larger surface area will increase the chance of light passing through a planetary atmosphere, will it be only factor to increase the amount of certain elements' wavelengths to be absorbed in atmosphere? does it have any other factors?
  • Increasing the surface area of planets [Confirmed]
  • Increasing the temperature of planets?
  • Increasing the mass of planets?
  • Increasing the volumn of planets?
  • Increasing the density of planets?
  • Increasing the rotation period of planets?
  • Other factors ... ?

Does anyone have any suggestions?

Thanks in advance for any suggestions
Bandersnatch
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#8
Dec2-12, 07:09 AM
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Junk0, your original question regarded detection of chemical composition of the planetary surfaces(or atmospheres). As the others tried to explain to you, this has got nothing to do with how bright a planet is.
Light coming from a very faint, faraway, small planet will have the same spectrum as light coming from a bright, close one - as long as their chemical composition is the same.

Here's an example of what it looks like:

You just look for dips in the spectrum, and compare them with the amount of light you receive in other wavelenghts. Since you're using ratios, it's independent of the total amount of light you get.

The key words for further reading are "planetary spectroscopy", or just spectroscopy.

How bright a planet is on the sky, on the other hand, depends on:
1. how far away from the source of light(the Sun) it is
2. how large it is - meaning surface area that reflects the light*
3. what is its reflective surface made of, aka its albedo - defined as the ratio of incident light and reflected light
4. how far away from the observer it is

Black body radiation is another kettle of fish.
Its spectrum AND "brightness"(for planets its in far infrared, so invisible to a naked eye) depends on Temperature, so even though the brightness falls with distance from the observer, the blackbody spectrum stays the same, allowing us to deduce the temperature.
This is the only EM radiation that planets actually produce themselves.
Since its wavelengths are very long in the case of planets, they're not very useful for detecting chemical composition by looking for dips in the spectrum, as most elements absorb light of much shorter wavelenghts.


It's important to remember that the blackbody spectrum and absorbtion spectrum are two different things. They are connected with completely different phenomena, and in our case occupy completely different wavelenghts.


*So this means half of the sphere surface area(which is 4*∏*R2). Notice that larger area means larger radius, and that in turn means that the larger planet has got larger volume, AND providing the density is the same between the two bodies, larger mass. So the mass, surface and volume points are closely interconnected, and it makes little sense to separate them like you did. The basic variables are density and radius, and in our case only radius matters.
junk0
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#9
Dec2-12, 07:21 AM
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Quote Quote by Bandersnatch View Post
...*So this means half of the sphere surface area(which is 4*∏*R2). Notice that larger area means larger radius, and that in turn means that the larger planet has got larger volume, AND providing the density is the same between the two bodies, larger mass. So the mass, surface and volume points are closely interconnected, and it makes little sense to separate them like you did. The basic variables are density and radius, and in our case only radius matters.
What about increasing the rotation period of planets? will it increase the surface area as well in term of time?
For example

Surface area is 4*∏*R2

The rotation period of planet A is 1 day and radius is R,
The rotation period of planet B is 0.5 day, and radius is R, will Planet B has double the surface area of Planet A? because it rotates faster.

Do you have any suggestions?
Thanks everyone very much for any suggestions
Bandersnatch
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#10
Dec2-12, 07:39 AM
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I must say that I'm not quite sure where did you get that idea from.

A planet is basically a ball. It doesn't matter if you rotate it or not, its surface stays the same.
A basketball won't grow larger just because you start spinning it on your finger.

Remember, size(i.e.radius), surface area, and volume are interconnected. You can't increase one without increasing all the others.
junk0
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#11
Dec2-12, 07:49 AM
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I think the surface area of planet B will be double comparing to planet A, because the rotation period of planet B is twice faster than planet A's.

Because the amount of electromagnetic waves [INPUT] keep unchanged for both planets, the amount of reflection [OUTPUT] will also keep unchanged, which make both amount of reflection [OUTPUT] the same from planet A and B.

Will it be correct?

Thanks everyone very much for any suggestions
Astronuc
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#12
Dec2-12, 09:51 AM
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Jupiter radiates more energy than it receives from the sun.

Jupiter is too far from the Sun and receives very little energy. The energy needed to power all the turbulence in Jupiter's atmosphere comes from heat released from the planet's core.
http://abyss.uoregon.edu/~js/ast121/lectures/lec19.html

If you measure how much total light it emits, it is more than the amount of sunlight that reaches the planet. Like the persistence of the Great Red Spot, which suggests some internal mechanism is providing energy to the cloud layers, the fact that Jupiter is giving off more light than it receives also suggests there is some internal energy generation.
https://www.e-education.psu.edu/astr...nt/l11_p5.html

. . . .
Jupiter's rate of internal energy generation is 4 X 1017 watts. This is about the same as the rate at which Jupiter absorbs energy from the Sun. Thus Jupiter's atmosphere is sort of intermediate between a normal planetary atmosphere which obtains most of its energy from the Sun, and a star's atmosphere which is entirely heated from below. The bulk of this internal energy of Jupiter is primordial.

Saturn's internal energy source is about half of Jupiter's. Since Saturn has only a quarter the mass of Jupiter, this implies a greater rate of internal energy generation. With its lower mass Saturn should also have less primordial heat left over from its formation. The source is thought to be due to the separation of helium from hydrogen in the interior. In the liquid hydrogen mantle, heavier helium drops form which then sink toward the core of the planet, releasing gravitational energy. That is, Saturn is still in the process of differentiating! This process occurs in Saturn and not Jupiter due to the cooler temperatures inside Saturn. In Jupiter the higher temperature keeps the hydrogen and helium well-mixed.

Neptune has a small internal heat source, while Uranus has no measurable internal source. Thus these two planets have about the same effective temperatures in spite of their different distances from the sun. What causes this difference between the two otherwise very similar planets is unknown.

. . . .
http://www.oglethorpe.edu/faculty/~m...ture_notes.htm

http://astro.wsu.edu/allen/courses/notes/note12.html
goldsax
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Dec4-12, 09:59 AM
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there is also heat from radiogenic material ,although its conribution is small.
davenn
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Dec5-12, 07:35 PM
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Quote Quote by junk0 View Post
I think the surface area of planet B will be double comparing to planet A, because the rotation period of planet B is twice faster than planet A's.

Because the amount of electromagnetic waves [INPUT] keep unchanged for both planets, the amount of reflection [OUTPUT] will also keep unchanged, which make both amount of reflection [OUTPUT] the same from planet A and B.

Will it be correct?

Thanks everyone very much for any suggestions
no thats not correct

as a previous poster said it doent matter if the planet is spinning or stationary, the surface area isnt going to change

Dave
Dotini
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Dec7-12, 07:21 AM
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Electromagnetic waves are one of the theories being being investigated by NASA as to the origin of the extreme energy and speed of particles in the outer Van Allen belt.

"There are two broad theories on how the particles get energy: from radial transport or in situ. In radial transport, particles move perpendicular to the magnetic fields within the belts from areas of low magnetic strength far from Earth to areas of high magnetic strength nearer Earth. The laws of physics dictate that particle energies correlate to the strength of the magnetic field, increasing as they move towards Earth. The in situ theory posits that electromagnetic waves buffet the particles -- much like regular pushes on a swing -- successively raising their speed (and energy)."

http://www.nasa.gov/mission_pages/rb...tmosphere.html

"a fully kinetic electromagnetic model to study instabilities and waves in planetary plasma environments"
http://www.ann-geophys.net/28/743/20...-743-2010.html

Energy transport by kinetic-scale electromagnetic waves in fast plasma sheet flows
http://www.agu.org/pubs/crossref/201...JA017863.shtml

Respectfully submitted,
Steve
Dotini
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#16
Dec2-13, 06:45 AM
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http://prl.aps.org/accepted/24079Y9f...6d04b02dc48c11
Megavolt parallel potentials arising from double-layer streams in the Earth's outer radiation belt

F. S. Mozer, S. D. Bale, J. W. Bonnell, C. C. Chaston, I. Roth, and J. Wygant

Accepted Wednesday Oct 23, 2013

Huge numbers of double layers carrying electric fields parallel to the local magnetic field line have been observed on the Van Allen Probes in connection with in-situ relativistic electron acceleration in the Earth's outer radiation belt. For one case with adequate high time resolution data, 7000 double layers were observed in an interval of one minute to produce a 230,000 volt net parallel potential drop crossing the spacecraft. Lower resolution data show that this event lasted for six minutes and that more than 1,000,000 volts of net parallel potential crossed the spacecraft during this time. A double layer traverses the length of a magnetic field line in about 15 seconds and the orbital motion of the spacecraft perpendicular to the magnetic field was about 700 km during this six minute interval. Thus, the instantaneous parallel potential along a single magnetic field line was the order of tens of kilovolts. Electrons on the field line might experience many such potential steps in their lifetimes to accelerate them to energies where they serve as the seed population for relativistic acceleration by coherent, large amplitude whistler mode waves. Because the double layer speed of 3,100 km/sec is the order of the electron acoustic speed (and not the ion acoustic speed) of a 25 eV plasma, the double layers may result from a new electron acoustic mode. Acceleration mechanisms involving double layers may also be important in planetary radiation belts such as Jupiter, Saturn, Uranus, and Neptune, in the solar corona during flares, and in astrophysical objects.


http://www.space.com/23747-earth-rad...electrons.html
Huge electric fields in the radiation belts around Earth may help explain how electrons surrounding the planet can be accelerated to speeds near that of light, researchers have found in a new study.

These findings, detailed Dec. 2 in the journal Physical Review Letters, could help shed light on the radiation belts of planets such as Jupiter, Saturn, Uranus and Neptune, as well as the behavior of the sun during flares and of bodies beyond the solar system, such as stellar nurseries, neutron stars and incredibly energetic black holes known as quasars.
===========
The double layers can in combination generate strong electric fields, ones more than 1 million volts strong. The researchers suggest the combinations of double layers seen in the outer belt are powerful enough to drive electrons to relativistic speeds, ones near the speed of light.

"It has been surprising that there are mechanisms that accelerate electrons to relativistic energies in Earth's radiation belt and throughout all of astrophysics," Mozer said. "There have been a lot of theories about what those mechanisms are, but many of them have shown not to work."

These findings suggest double layers may help drive electrons to relativistic speeds elsewhere in the cosmos.


Edit:
http://physics.aps.org/articles/v6/131 <--- Link to illustration, PDF of paper, explanation of double layers, implications
Solon
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#17
Dec3-13, 03:43 PM
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Hannes Alfven, 1986

"Double layers in space should be classified as a new type of celestial object."

DOUBLE LAYERS AND CIRCUITS IN. ASTROPHYSICS (16 meg pdf)
http://www.diva-portal.org/smash/get...T01.pdf‎


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