APOD: Night-Shining Clouds (2007 Jul 05)

FieryIce · Post by **FieryIce** » Mon Jan 07, 2008 2:31 pm

June 11, 2007 image of NLC's from the AIM satellite posted at Earth Observatory latest images.
It is impossible to take a straight down picture and have the centre blacked out.

Chris Peterson · Post by **Chris Peterson** » Mon Jan 07, 2008 2:50 pm

FieryIce wrote:June 11, 2007 image of NLC's from the AIM satellite posted at Earth Observatory latest images.
It is impossible to take a straight down picture and have the centre blacked out.

The image wasn't taken straight down over the pole. The AIM spacecraft is in a 97.8° inclination orbit, so it never passes over the poles. Swaths of data are collected by the instruments and then mathematically projected onto a sphere centered on the pole. You can see these data bands in the image.

This imaging technique is common, and is used frequently for presenting data from many spacecraft, both around Earth and other planets.

FieryIce · Post by **FieryIce** » Tue Jan 08, 2008 1:56 pm

Saturn's North Pole Hot Spot and Hexagon

They can take a picture of a planetary pole without the blacked out area, so there is no excuse for the data block out.

Chris Peterson · Post by **Chris Peterson** » Tue Jan 08, 2008 3:25 pm

FieryIce wrote:They can take a picture of a planetary pole without the blacked out area, so there is no excuse for the data block out.

Only if the camera can actually image the pole at some point in its orbit. As I noted before, the satellite is in a 97.8° inclination orbit. That means it never gets closer than about 900 km from the poles. Its camera has a 800 km field of view on the latitudinal axis. That leaves an approximately 400-500 km radius zone around the poles that cannot be imaged. You can see this zone very clearly, and the individual data stripes, in this image.

The fact that a different satellite, around a different planet, in a different polar orbit, using different imaging instruments can capture the pole demonstrates nothing about AIM's capabilities.

FieryIce · Post by **FieryIce** » Tue Jan 08, 2008 3:41 pm

You can side step the issue, go ahead but the facts remains.

Chris Peterson · Post by **Chris Peterson** » Tue Jan 08, 2008 3:49 pm

FieryIce wrote:You can side step the issue, go ahead but the facts remains.

Please quote who and what you are responding to. What issue exactly is being sidestepped?

iamlucky13 · Post by **iamlucky13** » Tue Jan 08, 2008 8:10 pm

FieryIce wrote:You can side step the issue, go ahead but the facts remains.

Err...so are you claiming that NASA is hiding the imagery? If they had something to hide, wouldn't it be more convincing to photoshop fake data over the poles than to simply black it out. I think Chris explained the technical reason data was not taken directly over the pole pretty well. If you were to challenge that explanation or take issue with the fact that the satellite orbits at 97 degrees inclination instead of a true polar orbit I think we'd have something to discuss, but as is, I don't get what you're hinting at.

By the way, Cassini is in a very elliptical orbit that carries it hundreds of thousands of kilometers away from Saturn on a regular basis, and uses gravitational assist manuevers from the moons to occassionally swing around the poles. This gives it a wider field of view relative to the planet's size and more complete coverage.

zbvhs · Post by **zbvhs** » Fri Feb 01, 2008 10:47 am

One night about 2 AM while motoring across North Dakota, we saw what appeared to be rays of sunlight radiating upward from the northern horizon. Would those have been noctilucent clouds of some sort?

zbvhs · Post by **zbvhs** » Fri Feb 01, 2008 10:52 am

I should have added that this occurred in June or July and didn't appear to be aurorae.

auroradude · Post by **auroradude** » Fri Feb 01, 2008 3:39 pm

While growing up in Montana, I had abserved the aurora on many occasions and noctilucent clouds on a few. Now, living in Alaska I have observed both many, many times.
Noctilucent clouds have always appeared as cirrus-like or in wave patterns but never ray-like. It sounds more like you saw the rays from an aurora.
2:00 a.m. would be typical for auroral activity. Rays can be very tall features appearing above a horizon without the main body of the aurora. They might move rapidly or not much at all. They can appear/disappear quickly or gradually.

zbvhs · Post by **zbvhs** » Fri Feb 01, 2008 7:46 pm

Consensus of opinion in the car was that we were seeing sunlight coming over the Pole reflected upward by the polar icecap. Could be we had one of those rare nights when skies were clear all the way north to the pole.

I grew up out there also and don't recall seeing anything similar as a kid. What we called Northern Lights was usually a diffuse, kind of flickering glow on the northern horizon. Sometimes they appeared as high-altitude streamers overhead that came and went.

zbvhs · Post by **zbvhs** » Fri Feb 01, 2008 7:54 pm

Y'know, I hope the APOD guys aren't deterred by grumps who complain about non-astronomical subjects in APOD. I see something interesting every time I look up. People need to look up more.

Chris Peterson · Post by **Chris Peterson** » Fri Feb 01, 2008 8:17 pm

zbvhs wrote:Consensus of opinion in the car was that we were seeing sunlight coming over the Pole reflected upward by the polar icecap. Could be we had one of those rare nights when skies were clear all the way north to the pole.

Quite impossible. Because of the curvature of the Earth, about the farthest you can see atmosphere is 700 miles. That is, at 700 miles, you are seeing the very top of the atmosphere on the horizon. You were what, something like 3000 miles south of the North Pole?

Realistically, from the surface of the Earth, you are basically living in the center of a shallow cylinder several hundred miles in radius. You can't see any atmospheric phenomena outside that cylinder- clouds, meteors, aurora, etc.

Auroral activity is only one possibility. If the rays you saw were above the solar point, you may have seen the Zodiacal light (possibly fragmented by otherwise invisible clouds). Depending on how far north you were, you may even have been seeing twilight (again, possibly fragmented by clouds- essentially, twilight crepuscular rays). From Bismarck, on July 1 at 2 AM, the Sun is directly north and only 20° below the horizon- just about the beginning of astronomical twilight.

zbvhs · Post by **zbvhs** » Sat Feb 02, 2008 3:31 am

What we saw was definitely not auroral. The rays or streamers of light were radiating upward from the horizon in a divergent pattern. Aurorae come straight down from above and don't converge. Another possibility we considered was lights from a city north of us. Problem there is that city lights usually produce a diffuse glow on the horizon. Remains a mystery.