Gegenschein (APOD 07 May 2008)

[JohnD]

All,
Re today's (7th May) APOD and the gegenshein.

I presume that this effect would be seen from anywhere in Earth's orbit, or indeed from anywhere in the Solar System, rather like a rainbow. But the rainbow is explained by the reflection/refraction of light in raindrops, that is a law of physics and depends on the relatively constant shape of rain drops.
In contrast, if (it may be presumed) interplanetary dust is composed of randomly shaped and orientated particles, how is it that the gegenshein reflects straight back towards the Sun? It 'should' be reflected in random directions.

The Wiki offers this explanation:
"The intensity of gegenschein is (relatively) enhanced because (a) each dust particle is seen in full phase, and (b) the backscattering geometry leads to constructive interference[citation needed]."

Please explain further!

John

[orin stepanek]

http://apod.nasa.gov/apod/ap080507.html
Maybe similar to a planetary ring?

[NoelC]

That's a great question!

There's something about it here, including a discussion on how the dust particles are illuminated in full phase when directly opposite from the sun, and also a note on constructive interference in the backscattering:

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

-Noel

[henk21cm]

In contrast, if (it may be presumed) interplanetary dust is composed of randomly shaped and orientated particles, how is it that the gegenshein reflects straight back towards the Sun? It 'should' be reflected in random directions.

The remark about the random directions is most certainly true if the particles are large compared to the wavelength of the light. When the sun shines on a sandy beach, the sand can be seen from all directions. What i do not know is what will happen if the particle size of this interplanetary dust is of the same as the wavelength of the light. Ordinary sandy grains, as found in slowly flowing rivers have a characteristic particle size of 0.2 mm. The particle size in tidal areas, where there is very little current, is much smaller: 1 μm. This material is called "Lutum". (In non Anglosaxon countries 1 μm is the upper limit for clay; in Anglosaxon countries the word clay is assigned to soil by its plasticity index, so i prefer Lutum). It is non organic, has minerals like Illite, Kaolite and Montmorolinite. These particles are highly anisotropic, i.e. small "plates". Reflection of light on these plates may be quite different from the light as reflected by roughly round grains of 200 μm.

"The intensity of gegenschein is (relatively) enhanced because (a) each dust particle is seen in full phase, and (b) the backscattering geometry leads to constructive interference."

Argument (b), well, i have some doubts. Interference of light is difficult to achieve. At school it took me many hours to perform a decent double slit interference experiment. Optical coherence length is the main reason why it is so difficult. An optical grating -this may be an incorrect English word, i mean a glass plate with a few hunderd of parallel equidistant lines per mm- is an easy way to demonstrate interference with light.

The only natural process in which interference occurs, is a maser, and that is in the micro wave region of the spectrum, at wavelengths at least thousand times larger.

If constructive interference is the correct explanation, i would expect other maxima in a slightly different direction.

If the particle size is the culpritt, the light of the Gegenschein must be (partly) polarized, due to the flat shape of the particles.

Both are open to be falsified, if the experiments are not in agreement with my predictions.

[neufer]

"The intensity of gegenschein is (relatively) enhanced because (a) each dust particle is seen in full phase, and (b) the backscattering geometry leads to constructive interference[citation needed]." Please explain further!

Think of each ~1 mm diameter dust grain as a close ensemble of Rayleigh scattering (dielectric) dipole antennae all in locked phase with the incoming sunlight. This results in a strong FORWARD Mie scattering (which is basically lost in the sunlight itself):



However, unless the grains are a precise (fractional wavelength) size and shape there will be some spill over into a moderately strong BACKWARD Mie scattering which is the gegenschein (and which is bright compared with dark space).

[JohnD]

Thank you henk and neufer!

But I still don't understand.
neufer, you invoke Mie scattering, and show an illustration of enhanced scattering distal to the light source. I've confirmd that I am not mistaken as that is shown in other articles on Mie scattering.
That would produce a darker area to anti-Sunward, not what is seen in the Gegenschein, when the observer is between the light source and the scattering particles.

And henk (henk, thank you for your clear explanation. I could not explain what day it is in Dutch!)
You invoke anisotropy. Why should flat platelike particles be orientated with their flat faces sunward? Any irregularity, and light pressure or the Sun's wind would turn them edge on!

John

[bystander]

Maybe similar to a planetary ring?

More a "solar ring".

The gegenschein is sunlight back-scattered off small interplanetary dust particles. These dust particles are millimeter sized splinters from asteroids and orbit in the ecliptic plane of the planets.

[Arramon]

Are these particles orbitting between Mars and Jupiter or are they more dispersed throughout the ecliptic plane of the system?

Do the particles come from that belt of asteroids beyond Mars or from randomly orbitting asteroids that weave in and out of the inner/outer solar system?

[bystander]

Are these particles orbitting between Mars and Jupiter or are they more dispersed throughout the ecliptic plane of the system?

Do the particles come from that belt of asteroids beyond Mars or from randomly orbitting asteroids that weave in and out of the inner/outer solar system?

My guess is that they are dispersed througout the ecliptic. I suspect the source is multiple: asteroid collisions, cometary dust, and possibly just left overs from the Sun's proto-planetary disk. The particles forming the gegenschein, I suspect, are in relatively near Earth orbit.

[NoelC]

Doesn't full phase illumination serve to explain a majority of the Gegenshein effect? Particles directly opposite the sun are fully illuminated. We see those at other angles partially in shadow.

I was just looking at some Apollo 15 photos last night, and the down-sun images showed a significant brightening around the shadow of the astronaut/camera.

Same thing happens when you look out a plane window, and I've seen this of course in photography. It's just somehting one comes to expect, and one learns not to shoot directly down-sun.

-Noel

[neufer]

neufer, you invoke Mie scattering, and show an illustration of enhanced scattering distal to the light source. I've confirmd that I am not mistaken as that is shown in other articles on Mie scattering.
That would produce a darker area to anti-Sunward, not what is seen in the Gegenschein, when the observer is between the light source and the scattering particles.

Mie scattering (like Rayleigh scattering) is TWICE as strong in anti-Sunward compared with the linearly polarized light that is scattered in the perpendicular direction:


With SPHERICAL water droplet glories & heiligenschein the backscatter effect is even more dramatic:

http://www.atoptics.co.uk/droplets/gloab.htm

[JohnD]

neufer,
Your diagram, like the previous one, shows that unlike Rayleigh scattering that throws as much light back towards the illuminator as forwards away from it, Mie scattering scatters light far more behind the object than backwards.

This site: http://omlc.ogi.edu/cgi-bin/mie_angles. ... ensity=0.1 shows that a little light is reflected straight back at the source, with further, stronger lobes of reflection at about 150 degrees. Is this the mechanism?

If so, then why do we not see the gegenshein as a ring, like a parahelion? The explanation for that is also flat objects, ice crystals, but this time transparent. When they are aligned, the parahelion condenses into sun dogs, when they are random, the ring dominates.

The above site includes similar, but more detailed diagrams, calculated from data on particle size etc, that you may be more familar with than I. If you have some idea of the relevant data, it would be interesting to read your conclusions using that calculator.
I don't understand the references to 'Normal', 'Perpendicular' and 'Parallel'. Polarisation??

John

[neufer]

neufer,
Your diagram, like the previous one, shows that unlike Rayleigh scattering that throws as much light back towards the illuminator as forwards away from it, Mie scattering scatters light far more behind the object than backwards.

The strong forward scattering prevents us from observing the sun's corona simply by blocking out the photosphere.

This site: http://omlc.ogi.edu/cgi-bin/mie_angles. ... ensity=0.1 shows that a little light is reflected straight back at the source, with further, stronger lobes of reflection at about 150 degrees. Is this the mechanism?

If so, then why do we not see the gegenshein as a ring, like a parahelion? The explanation for that is also flat objects, ice crystals, but this time transparent. When they are aligned, the parahelion condenses into sun dogs, when they are random, the ring dominates.

With water droplets & ice crystals you are talking about well defined prisms and/or lens that will produce both rainbow scattering and very strong backscatter glories.

Gegenshein from randomly shaped dust particles is quite different.

The above site includes similar, but more detailed diagrams, calculated from data on particle size etc, that you may be more familar with than I. If you have some idea of the relevant data, it would be interesting to read your conclusions using that calculator.
I don't understand the references to 'Normal', 'Perpendicular' and 'Parallel'. Polarisation?? John

Bees rely on the strong polarization of blue/UV skylight 90º from the sun to orient themselves. You can detect this polarization yourself by looking at skylight 90º from the sun with polarized sunglasses which you rotate in front of your face. Only one polarization of light can be scattered from this 90º angle.

However, forward & backward scattered light contain BOTH polarizations and therefore is TWICE as strong because of it.

The same holds for polarized zodiacal light and unpolarized gegenshein.

[Qev]

Bees rely on the strong polarization of blue/UV skylight 90º from the sun to orient themselves. You can detect this polarization yourself by looking at skylight 90º from the sun with polarized sunglasses which you rotate in front of your face. Only one polarization of light can be scattered from this 90º angle.

Actually, in clear, low-humidity conditions, you can see the polarization of the sky without any optical aid at all (at least, most people can). Simply look at a point on the blue sky -- 90º from the sun is best as you said, preferably well away from the horizon -- and slowly roll your head left and right (ie. tip your head to the left, then to the right) repeatedly while staring at that point. Dead center in your vision you should begin to see what appears to be a faint after-image, a yellowish dumbbell shape, crossed perpendicularly with a fainter deep blue dumbbell shape (although some people see the blue one more clearly).

What's also interesting is that you can also see how the light is polarized, as the plane of polarization is perpendicular to the axis of the yellow dumbbell image. You can even detect if the light is circularly polarized, and which handedness it is!

If it's cloudy or humid or the Sun's gone down, and you happen to have an LCD monitor for your computer, you can also see the effect there. Set the screen to be a medium sky blue (either by creating an image file that's all one flat colour and viewing it fullscreen, or setting your desktop to a flat colour, or something), and do the same thing you would looking at the sky. LCD monitors produce polarized light.

This effect is called Haidinger's Brush, and is one of those weird little 'secret senses' that people don't realize they have.

Okay, I'm waaaay off topic here, so I'll shush.

[JohnD]

neufer,
Your diagram, like the previous one, shows that unlike Rayleigh scattering that throws as much light back towards the illuminator as forwards away from it, Mie scattering scatters light far more behind the object than backwards.

The strong forward scattering prevents us from observing the sun's corona simply by blocking out the photosphere.
The Gegenshein is seen looking away from the Sun. Relevance of corona?? J.

This site: http://omlc.ogi.edu/cgi-bin/mie_angles. ... ensity=0.1 shows that a little light is reflected straight back at the source, with further, stronger lobes of reflection at about 150 degrees. Is this the mechanism?

If so, then why do we not see the gegenshein as a ring, like a parahelion? The explanation for that is also flat objects, ice crystals, but this time transparent. When they are aligned, the parahelion condenses into sun dogs, when they are random, the ring dominates.

With water droplets & ice crystals you are talking about well defined prisms and/or lens that will produce both rainbow scattering and very strong backscatter glories.
Gegenshein from randomly shaped dust particles is quite different.
No doubt, but please explian further. Let's not get hung up on ice/dust. The site I found (below) shows lobes of back scattering, analogous to the angles through which light is refracted/reflected. I don't confuse that with scatter. J.

The above site includes similar, but more detailed diagrams, calculated from data on particle size etc, that you may be more familar with than I. If you have some idea of the relevant data, it would be interesting to read your conclusions using that calculator.
I don't understand the references to 'Normal', 'Perpendicular' and 'Parallel'. Polarisation?? John

Bees rely on the strong polarization of blue/UV skylight 90º from the sun to orient themselves. You can detect this polarization yourself by looking at skylight 90º from the sun with polarized sunglasses which you rotate in front of your face. Only one polarization of light can be scattered from this 90º angle.
However, forward & backward scattered light contain BOTH polarizations and therefore is TWICE as strong because of it.
The same holds for polarized zodiacal light and unpolarized gegenshein.

See my comments inserted above, in italics.

Thanks! Ok, scatter induces polarisation, as can refraction/reflection. "Learn something every day" (Aristotle) and I have learnt something!
But, unlike bees we don't see polarised light except as Quv explains, so that doesn't explain the Gegenshein. And quote, "forward & backward scattered light contain BOTH polarizations and therefore is TWICE as strong because of it" Surely not! No more light energy can come out of a scattering event than went in.
Please have another go at explaining the Gegenshein to me.
Or approve this one, "Randomly shaped, randomly orientated dust scatters light. The scattering straight back towards the light source is slightly more than to the side, so that an observer between the light source and the dust can see a glow when looking directly away from the light source."

If that's true, then an astronaut looking away from the Sun should see a brighter Gegenschein than we do, as the light being back scattered towards them by dust will be closer to the optimum angle than we can see, with the whole Earth as our 'helmet'. Has this been observed?
John

[henk21cm]

You invoke anisotropy. Why should flat platelike particles be orientated with their flat faces sunward? Any irregularity, and light pressure or the Sun's wind would turn them edge on!

Nature invokes anisotropy, not i! Whereas larger particles are usually ellipsoids, particles smaller than 10 μm of soil and dust -as far as i know them- are usually plates, much less rounded than the larger particles. I think of them as similar in shape as glossy magazines. The reason for this (on earth) is the process of cristallisation, which is in favour for a flat layered structure. As far as i can read in my answer i did not mention alignment towards the sun. The phenomena with incident electro magnetic radiation (EMR) are not similar to light reflected by a mirror, so such an ideally aligned situation is not needed. The mathematical treatment of the relective and dispersive results of incident EMR is rather complicated and invokde -even for the most simple geometry of a particle- Bessel functions, which makes by back 'itch'. The problem was in the focus of the attention of the 19th centuries phycisists. Art Neuendorfer posted some references to Mie and Rayleigh scattering. To complete the spectrum, i'll add Raman scattering.

1. Raman scattering is a process inwhich EMR is captured by the tiniest particles (e.g. molecules) and where EMR is emitted slightly later, in a different frequency. So a molecule might capture a blue photon and emitt a green photon, usually at a lower frequency. There is no preference for direction.
2. Rayleigh scattering occurs when the fraction between diameter and wavelength is much smaller than 1. The intensity of the scattered EMR is strongly (quartic) dependent on its wavelength. As a result the intensity of blue light (400 nm) is 10 times stronger than that of the scattered red (700 nm) light. There is hardly a preference for direction. An example are tiny (airborne) particles in our atmosphere, which colour the sky blueish.
3. Mie scattering occurs when particles are (substantially) larger than the wavelength of the incident EMR. There is hardly any dependence on wavelength. When sunlight is scattered by haze of tiny droplets in a cumulo nimbus cloud, the scattered light is white. There is some preference for a direction. The diagrams i see in Arts answers, resemble a simple Yagi antenna, a dipole with with one reflective element.

Raman scattering is the only item in this list which alters the wavelength. The change in frequency is continuous, virtually any wavelength can be emitted. In early 'tunable' laser systems Raman scattering was used to change the wavelength of the laser. Unfortunatedly the coherence of the light is lost. (There is no such thing as a free meal).

Rayleigh scattering produces partly polarised light. This phenomenen is known for at least a thousand years. In Denmark, Roskilde (pronouce roskeald) in the 1960 four large Viking ships were excavated in an artifical island in the bay (fjord). These ships were sunk a thousand years ago, in order to block the path for roaming Vikings which regularly plundered the city. On board of these ships two small stones were found, natural cristals, which polarise light. During nautical and civilian dusk the captains of the ships could investigate the polarisation of the sky to navigate. These crystals were placed in a brass (copper?) mounting in the orientation which made North easily to be found.

John, you are right that the solar wind will blow small anisotropic particles in the direction of least friction: "edge on". For the scattered light (Rayleigh, Mie or Raman) that is of no importance.

On the web there are several sources for the calculation the intensity of the scattered light. On this website a list of sources is compiled. The most promissing to me seemed to be the Ph. D of Sylvain Lecler, who has assembled an archive file of Matlab code for the calculation of Mie scattering on multiple particles, including a graphical user interface and graphical presentation. Unfortunatedly a limitation of his sources is that the maximum distance between the particles is 30 wave lengths, which is for the interplanetary space fully 'overcrowded' . A minor disadvantage is that the entire program is in French, so it will not be so accessible for you. (I presume that you don't spreak French, maybe that is a prejudice, sri).

Most probably, Mie scattering is for Gegenschein the dominant phenomenon, regarding the size of the particles and observed colour of the scattered light. Where i do differ in opinion from Art Neufer is his approach of a phased array of antennae. Such a configuration would -it is true- explain the sharp area where the Gegenschein could be seen. By non linear phenomena, as found in a Synthesis Radio Telescope (SRT), e.g. Westerbork, or Jocelyn Bell arrays), the side lobes can be dimmed. However interplanetary particles are not carefully aligned as in a SRT, which leaves the resulting angular plot of the Mie intensity virtually unchanged, compared with the intensity diagram of just one particle.

Maybe Arts explanation of phased arrays is right if we combine light of those particles which are in the right position for non linear effects, whereas combination of light of those particles not in the right position, leads to a much lower intensity. It is up to you Art, to adress this point (if you like to do so)!