APOD: Sunspot at Sunset (2014 Jan 08)

[APOD Robot]

Sunspot at Sunset

Explanation: Sunsets may be the most watched celestial event, but lately sunsets have even offered something extra. A sunspot so large it was visible to the naked eye is captured in Swiss skies in this sunset scene from January 5, crossing left to right near the center of a solar disk dimmed and distorted by Earth's dense atomosphere. Detailed views reveal a large solar active region composed of sunspots, some larger than planet Earth itself. Cataloged as active region AR 1944, on January 7 it produced a substantial solar flare and a coronal mass ejection (CME) forecast to reach Earth. The CME could trigger geomagnetic storms and aurora on January 9.

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[Ann]

The sunspot is nice, but isn't there a blue flash there, too?

Ann

[Beyond]

Somehow i didn't even notice the blue, until you mentioned it, Ann. I guess 'flash' wouldn't apply to the red bottom?

[Nitpicker]

Click to view full size image

Animated gif of the sunset on Jan 5 from my backyard in Brisbane, seen through smoky bushfire haze.

[Ann]

Somehow i didn't even notice the blue, until you mentioned it, Ann. I guess 'flash' wouldn't apply to the red bottom?

Nope. Red bottoms don't count.

Ann

[Nitpicker]

I don't think I've ever noticed green or blue flash above the sunrise or sunset. Quite an intricate pattern. It is a surprisingly clear collection of sunspots in this APOD, too, despite all the mirage wobble going on. I think the lesser clarity in my comparison gif, from ~7 hours prior, is partly due to smoke haze and partly due to being slightly out of focus.

[DrWork]

The plural of "aurora" is either "auroras" or "aurorae", not "aurora".

[FloridaMike]

The approaching geomagnetic storm is predicted to be a “G3” in intensity. Does anyone have an idea on what the intensity would have to be for us to see auroras as far south as Florida?

[Psnarf]

KP=11 might be seen in Florida.

[auroractivity, auroroactivitation: from the Latin aurora usually translated as "dawn"]

http://www.swpc.noaa.gov/rt_plots/pro_3d.html

From http://spaceweather.com
This ongoing radiation storm ranks S2 on NOAA storm scales. It is rich in "hard" protons with more than 100 MeV of energy, which accounts for the snowiness of the SOHO coronagraph images. According to NOAA, "passengers and crew in high-flying aircraft at high latitudes may be exposed to elevated radiation risk" during such a storm.
The source of all this activity is AR1944, the largest sunspot in decades.

Solar wind:
speed: 309.2 km/sec
density: 6.8 protons/cm^3
I don't recall ever seeing the density of the solar wind close to nine protons/cm^3.

[Chris Peterson]

The sunspot is nice, but isn't there a blue flash there, too?

I'm not sure it could be called a flash. Any image of the Sun near the horizon (or stars, for that matter) will show the effects of dispersion, with a red fringe at the bottom and a blue fringe at the top. That effect is stable, lasting for many minutes.

[Ray-Optics2]

I'd call it a green flash (although where people draw their line between blue and green is fluid.) I've seen scores of them - they're very common from Southern California beaches. They are NOT stable over time, usually lasting no more than a second.

[Chris Peterson]

I'd call it a green flash (although where people draw their line between blue and green is fluid.) I've seen scores of them - they're very common from Southern California beaches. They are NOT stable over time, usually lasting no more than a second.

The green flash is certainly not stable. But the red and blue fringes seen above and below any body near the horizon are certainly stable (and virtually always present, under all conditions).

No green flash is evident in this image.

[geckzilla]

This page at AtOptics discusses the difference between green rim and green flash.

[CPatriot]

It takes light 8 minutes to reach earth from the sun.? what would be the speed of the energy released by these events.? Occurred on the 7th effects on earth on the 9th.?

[Ron-Astro Pharmacist]

It takes light 8 minutes to reach earth from the sun.? what would be the speed of the energy released by these events.? Occurred on the 7th effects on earth on the 9th.?

Lots of information at this site.

http://www.spaceweather.com/

[Rothkko]

este flash duró unos pocos segundos, y parece tan verde como azul
this flash lasted a few seconds, and it looks as green as blue

http://asterisk.apod.com/viewtopic.php? ... 50#p217555

[FloridaMike]

Click to view full size image

Animated gif of the sunset on Jan 5 from my backyard in Brisbane, seen through smoky bushfire haze.

Great work. I'd love to do something like that. Is the reddened tree shadow the most distant of three trees between the camera and the sun? The red effect is unexpected.

[Anthony Barreiro]

... A sunspot so large it was visible to the naked eye ... .

I wouldn't encourage people to try to see sunspots with their naked eyes. Even when the Sun is low to the horizon and the visible light is significantly attenuated, there's still a lot of dangerous infrared radiation. It would be safer to say something like, "visible without magnification through a safe solar filter."

[Chris Peterson]

... A sunspot so large it was visible to the naked eye ... .

I wouldn't encourage people to try to see sunspots with their naked eyes. Even when the Sun is low to the horizon and the visible light is significantly attenuated, there's still a lot of dangerous infrared radiation. It would be safer to say something like, "visible without magnification through a safe solar filter."

This is a bit of a myth. In fact, the cornea and lens pass nothing longer than 1200 nm. Except for a transmission peak around 1100 nm, nearly the entire IR range shorter than 1200 nm is highly attenuated. And even at sunset, the IR component of sunlight has an irradiance less than half of that of the peak visible light.

The reality is, if the Sun is attenuated enough in brightness by the atmosphere that you can look at it comfortably, it is going to be very difficult to damage your eyes. There simply isn't enough UV or IR making it to the back of the eye. You actually have to stare at the full, unattenuated Sun for quite a while (and quite painfully) to damage the retina if you aren't using any magnification.

[Ron-Astro Pharmacist]

... A sunspot so large it was visible to the naked eye ... .

I wouldn't encourage people to try to see sunspots with their naked eyes. Even when the Sun is low to the horizon and the visible light is significantly attenuated, there's still a lot of dangerous infrared radiation. It would be safer to say something like, "visible without magnification through a safe solar filter."

I was about to ask a dumb question such as, “Why is the landscape to dark?” when it dawned on me that this photo must have been taken through a solar filter. Not exactly naked-eye but I got that point. It might have more accurate to say "was visible but taken with magnification through a safe solar filter." That was probably obvious to most.

[Chris Peterson]

I was about to ask a dumb question such as, “Why is the landscape to dark?” when it dawned on me that this photo must have been taken through a solar filter. Not exactly naked-eye but I got that point. It might have more accurate to say "was visible but taken with magnification through a safe solar filter." That was probably obvious to most.

Nope. I don't think this image was made through any filters. Solar filters attenuate the light so much that you'd see nothing but the Sun- no landscape, no clouds. And at this time of day, you'd barely even see the Sun itself.

The foreground and sky are dark simply because of the limited dynamic range of the camera. Getting the proper exposure for the Sun meant that the rest of the scene was underexposed. This is pretty much how any ordinary, unfiltered camera will show the setting Sun.

[Nitpicker]

<animated gif>
Animated gif of the sunset on Jan 5 from my backyard in Brisbane, seen through smoky bushfire haze.

Great work. I'd love to do something like that. Is the reddened tree shadow the most distant of three trees between the camera and the sun? The red effect is unexpected.

Thanks Mike. The gum tree in the foreground is less than 100m from the camera. The gum trees on the ridgeline behind which the Sun sets, are about 1km away. The reddening of the tips of these more distant trees, is merely the effect of some of the sunlight passing through the thinner parts of the foliage.

The shots were taken with no filters whatsoever (apart from the smoke, but even that may not have been necessary, it just made it easier for the exposures to capture both the sunspots and the trees) with the sun about 6&deg; above horizontal.

I'm not sure it could be called a flash. Any image of the Sun near the horizon (or stars, for that matter) will show the effects of dispersion, with a red fringe at the bottom and a blue fringe at the top. That effect is stable, lasting for many minutes.

I'd say I've never noticed a green flash since I've lived most of my life on the east coast of a continent, or nowhere near the ocean. Green flashes seem most common over the ocean. I've only ever actively observed a handful of sunsets over the ocean in other parts of the world, and (prone to a nice sleep in) only a handful of sunrises anywhere.

Prismatic dispersion of light does make more sense in this APOD (thanks Chris). I've never noticed it in any of my sunset or sunrise shots, but that's not that many. However, every single crepuscular colour shot of Mercury and Venus I've ever taken, shows it to some extent. And I can often see it if I bother to observe stars low on the horizon, too. Initially I thought this was an aberration in my optics, but it is really an aberration of the sky (which some people seem to like). Anyway, it just made me revisit the theory on this topic, which immediately confused me, as my initial reckoning put red on the top and blue on the bottom. It was not until I read the last section -- "A Seeming Discrepancy Explained" -- in the following link, that it suddenly made sense:
http://www.astropix.com/HTML/L_STORY/At ... ersion.HTM

[Nitpicker]

I wouldn't encourage people to try to see sunspots with their naked eyes. Even when the Sun is low to the horizon and the visible light is significantly attenuated, there's still a lot of dangerous infrared radiation. It would be safer to say something like, "visible without magnification through a safe solar filter."

This is a bit of a myth. In fact, the cornea and lens pass nothing longer than 1200 nm. Except for a transmission peak around 1100 nm, nearly the entire IR range shorter than 1200 nm is highly attenuated. And even at sunset, the IR component of sunlight has an irradiance less than half of that of the peak visible light.

The reality is, if the Sun is attenuated enough in brightness by the atmosphere that you can look at it comfortably, it is going to be very difficult to damage your eyes. There simply isn't enough UV or IR making it to the back of the eye. You actually have to stare at the full, unattenuated Sun for quite a while (and quite painfully) to damage the retina if you aren't using any magnification.

It is always a good idea to exercise caution. I was just reading in the news about a whole bunch of North Americans burning themselves with boiling water yesterday, after throwing it (poorly) into the freezing air and trying to make snow, after watching a broadcast of someone else doing it (safely). There's nothing like a good example!

The few minutes after sunrises and before sunsets are typically quite safe for your unfiltered camera to photograph the Sun, although I recommend not looking through the view finder, if possible, but using the live view screen instead. You can normally glimpse directly at the Sun at these times without doing any permanent damage to your naked eyes, but I'd still recommend you don't do it too much, or too often. And glimpse, rather than stare.

With the Sun higher in the sky, it is a different story altogether. If I happen to look anywhere near the Sun in the middle of the day, my body reacts almost involuntarily by throwing up an occulting hand. It would likely damage an unfiltered camera, too.

[Chris Peterson]

It is always a good idea to exercise caution.

Certainly. Nevertheless, it is somewhere between difficult and impossible to damage your eyes looking at the Sun if it's attenuated below the level of causing discomfort, unless you magnify it.

The few minutes after sunrises and before sunsets are typically quite safe for your unfiltered camera to photograph the Sun, although I recommend not looking through the view finder, if possible, but using the live view screen instead. You can normally glimpse directly at the Sun at these times without doing any permanent damage to your naked eyes, but I'd still recommend you don't do it too much, or too often. And glimpse, rather than stare.

With the Sun higher in the sky, it is a different story altogether. If I happen to look anywhere near the Sun in the middle of the day, my body reacts almost involuntarily by throwing up an occulting hand. It would likely damage an unfiltered camera, too.

It also requires some effort to damage your eyes looking at the full daytime Sun. That's because the blood supply on your retina is very efficient at carrying away heat. So for most people, it requires quite a few seconds of staring at the Sun (which isn't easy) to cause permanent retinal damage or phototoxic retinopathy. Still, there are a variety of otherwise fairly mild pathologies that can amplify that effect. Nobody in their right mind would suggest staring at the Sun when it's high in the sky.

I wouldn't worry about a camera. If it isn't mounted on a tripod, it isn't going to point to the Sun long enough to cause sensor damage. My allsky cameras (which are fixed mount, of course) run all day, with their lenses open, and the slowly moving Sun tracing a focal line across their sensors. Some of those cameras have been operating more than ten years, and still no damage.

[Nitpicker]

This is from the safety pages of my DSLR's manual:

nanny_state.JPG (34 KiB) Viewed 5448 times

I think the second warning is a pretty good warning, but the first one may be over the top, even when the sun is high and even if the camera is on a fixed tripod. I wonder if the risk of damage to the camera is greater with the shutter closed, or the mirror down, or even to the perimeter of the sensor? As for damaging the sensor itself, I imagine a wider image might be worse, in terms of concentrating the direct photons from the Sun onto fewer pixels. But I really don't know. I keep thinking about when I was a kid and setting fire to paper with a magnifying glass.