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sky brightness and solar maximum

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#1 whwang

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Posted 25 January 2023 - 03:08 PM

Hi,

I wonder if anyone else is feeling (or measuring, such as with SQM) that the sky is getting brighter and brighter in recent few years.

Many of astro fans in Taiwan including me have such a feeling. We all attribute this to the increasing light pollution. This is a reasonable guess given the population density in Taiwan. However, now I am in the desert of Western Australia, where light pollution is supposed to be minimum, and I still feel the sky is brighter than what I saw here four years ago. Previously, I could easily see the “spirals” of the large magellanic cloud with naked eyes. This time here I couldn’t. Other than the central bar of the LMC, I can only see a big fuzzy light ball, because of the brighter sky background. This brighter sky can’t be easily explained with light pollution given that I am in the desert. On the other hand, my wide-field pictures shows very intense airglow at low elevation, so the same must also exists around zenith.

The airglow is caused by the solar radiation or solar wind, and this correlates with the solar cycle. So if this is the correct explanation, we do not only need to get away from light pollution for deep-sky imaging, we also need to avoid the solar maximum, if we want the best imaging efficiency for faint nebulas. Now life gets even tougher.

How do you think?

Cheers,
Wei-Hao
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#2 freestar8n

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Posted 25 January 2023 - 06:46 PM

Hi Wei-Hao.  By coincidence I was recently at a dark site in Victoria, Australia and was thinking the same thing.  The sky was very dark and the coal-sack was quite visible - but it was not "inky-black" as I recall at other locations in the past.

 

But about 15 years ago I was at a very dark site in New Mexico and felt the same thing.  Years before that I was in Organ Pipe Natl. Monument in AZ and I recall it was very dark, except for somewhat blazing zodiacal light and gegenschein.  I find that gegenschein is a good indicator of sky darkness because the background needs to be black for it to stand out.

 

Another oddly dark site for me was Cherry Springs in PA.  I was very impressed by how clear and dark the sky was - despite being not that far from other towns.  I gather  it is much different now, but this was also around 15 years ago I think.

 

Unfortunately I don't have SQM readings for these experiences - and it's hard to separate out visual impressions from the actual brightness.  But I would describe it as a faint milky glow rather than inky-black.

 

It may be simple sky glow driven by solar wind or something - and I guess a spectrum would confirm.  Or timelapse video of the full sky might show ripples or something that would indicate it's not sky glow.

 

I think it is at least partly possible false perception and false memory - but if it doesn't really look black from an isolated location in Western Australia - that doesn't sound right.

 

Frank



#3 whwang

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Posted 26 January 2023 - 08:51 AM

Hi Frank,

Thank you for sharing your experience.

As for ripples on the sky, those can also be airglow, I believe. I have seen quite some nice fisheye photos showing green parallel bands across the sky. I also saw them once in New Mexico some 15 years ago. They move slowly, and they are definitely not clouds. I read somewhere that it’s called gravity wave (but not the kind of gravity wave caused by black hole merging etc). It could be some periodic density variation propagating in the upper atmosphere. This brings the ripple pattern to the airglow.

#4 freestar8n

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Posted 26 January 2023 - 05:11 PM

Hi Frank,

Thank you for sharing your experience.

As for ripples on the sky, those can also be airglow, I believe. I have seen quite some nice fisheye photos showing green parallel bands across the sky. I also saw them once in New Mexico some 15 years ago. They move slowly, and they are definitely not clouds. I read somewhere that it’s called gravity wave (but not the kind of gravity wave caused by black hole merging etc). It could be some periodic density variation propagating in the upper atmosphere. This brings the ripple pattern to the airglow.

Hi - yes I was trying to say that timelapse might give an indication it is a natural upper atmosphere phenomenon rather than a form of anthropogenic light pollution.

 

Here's a short ESO write up that suggests airglow is increasing recently:

 

https://www.eso.org/...no163-40-42.pdf

 

Frank



#5 calypsob

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Posted 26 January 2023 - 06:46 PM

The easiest way to inspect for airglow is with a wideangle lens, and a DSLR. In my experience, it is not present every single night. I like you noticed, however it does dramatically reduce the SQM measurements that you would expect. I was in Wyoming in August 2022 and drove around Teton and yellowstone national park.  It was during a new moon, and I took measurements around midnight to 1am when it is typically darkest. One night I measured a magnitude 21.7 but the rest of my measurements were 21.5. This sky was very clear and each night but I noticed in my wide angle images that there was a ton of red and green airglow illuminating the sky. I would suspect that the skies where I was should have been in the range of 21.8 to 22 but I think the air glow was causing my readings to be lower. 21.5 is what I typically find here in Virginia when I go to a location that is about 30 miles from the city. The terrain here is a lot different and often there are mountains that block cities.
 

on a sidenote, I hope that light pollution actually decreases in the future as populations increase. The government seems to spend a awful lot of money Researching climate change yet our cities still seem to leave lights on all night long. I understand that you can’t just turn off a light with a ballast and turn it back on but with the LEDs I am seeing everywhere there is no reason that leds can’t be on a motion sensor and turn off when no one is around. There is no reason to leave an entire empty parking lot illuminated all night, or five lane interstate with no cars on it.


Edited by calypsob, 26 January 2023 - 06:47 PM.

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