# A Graduate Level Course on DNB and NCC

Is there any post on this blog that doesn’t have to do with scaling the DNB or NCC?

I was going to title this post “Revisiting ‘Revisiting “Revisiting Scaling on the Solstice”‘”, but that would just be ridiculous. Besides the fact that we just passed an equinox (and are months away from a solstice), this post is more of a follow-up to our very first post.

If that was an introduction, this is a graduate level course. Well, maybe not. It won’t take a whole semester to read through this, unless you are a really slow reader. But, since Near Constant Contrast (NCC) imagery is coming to AWIPS in the very near future, now is a good time to prepare for what’s coming.

We start off with some good news: NCC imagery is coming to AWIPS! NCC imagery provides an alternative to ERF-Dynamic Scaling, CSPP Adaptive Scaling and whatever this is:

Example VIIRS DNB image displayed in AWIPS using the Histogram Equalization method. Courtesy Eric Stevens (UAF/GINA).

(I know that the above image uses the “Histogram Equalization” algorithm that was developed for CSPP originally. I was just being dramatic.) NCC imagery is an operational product, not some fly-by-night operation from CIRA or CIMSS (who both do great work, by the way).

Now the bad news: NCC imagery as viewed in AWIPS might not be the best thing since sliced bread. It may not solve all of our problems. To understand why, you have to know the inner workings of AWIPS (which I don’t) and the inner workings of the NCC EDR product (which I do).

Here’s a first look at the NCC product as displayed in AWIPS:

Example NCC image (12:39 UTC 19 August 2015) displayed in AWIPS II. Image courtesy John Paquette (NOAA).

Notice the bright area of clouds northwest of Alaska that suddenly become black. Also notice the background surface just looks black, except for the Greenland Ice Sheet. These are examples of two different (but related) issues: the first being that AWIPS is blacking out areas where the image is saturated (the maximum value on the scaling bounds is too low), the second being that, at the low end of the scale, the image detail is lost (either the minimum value on the scaling bounds is too high, or AWIPS uses too few shades of gray in the display, which means you lose sensitivity to small changes in value).

If you read the first post on this blog (that I linked to), or you read my previous posts about ERF-Dynamic Scaling, you know that the primary problem is this: the VIIRS Day/Night Band is sensitive to radiance values that span 8 orders-of-magnitude from full sunlight down to the levels of light observed during a new moon at night. Most image display software, of which AWIPS is an example, are capable of displaying only 256 colors (or 96 colors if you use N-AWIPS). How do you present an 8-order-of-magnitude range of values in 256 colors without losing information?

Near Constant Constrast imagery does this by modeling the sun and moon to convert the Day/Night Band radiance values into a “pseudo-albedo”. Albedo (aka reflectance) is simply the percentage of incoming solar (and lunar) radiation that is reflected back to the satellite, so you end up with a decimal number between 0 and 1. That’s easy enough to display, but we’re not done. What happens when there is no moonlight at night because the moon is below the horizon (or it’s a new moon)? What happens when there is a vivid aurora, or bright city lights, or gas flares or fires? These light sources can all be several orders of magnitude brighter than the moon, especially when there is no moonlight. There are lots of light sources at night that the DNB detects that aren’t reflecting light – they’re emitting it. That’s why the NCC doesn’t provide a true albedo value.

When VIIRS first launched into space, the NCC algorithm assumed that pseudo-albedo values over the range from 0 to 5 would be sufficient to cover all these light sources, but that turned out to be incorrect.  If you weren’t within a few days of a full moon, images contained fill values (no valid data) because these myriad light sources fell outside the allowed range of 0 to 5. It took a lot of work by the VIIRS Imagery Team to fix this and get the NCC algorithm to where it stands now. And where it stands now is that pseudo-albedo values are allowed to vary over the range from -10 to 1000. (The “-10” accounts for the occasional negative radiance in the DNB data and the “1000” allows for light sources up to three orders of magnitude brighter than the moon.) Now, the images don’t saturate or get blanked out by fill values at night away from a full moon. But, the range from -10 to 1000 still presents a challenge for those who want to display images properly.

To show this, here are the same three VIIRS NCC images linearly scaled between the full range of values (-10 to 1000), the original range of values (0 to 5) and the ideal range of values (which was subjectively determined for this scene):

Example VIIRS NCC image (08:55 UTC 5 August 2015) linearly scaled between -10 and 1000.

Can you see the one pixel that shows up in the above scaling? (There is one pixel with a value over 900.)

Example VIIRS NCC image (08:55 UTC 5 August 2015) scaled between 0 and 5.

Now you can start to see some cloud features and the city lights, but this image still looks too dark.

Example VIIRS NCC image (08:55 UTC 5 August 2015) scaled between 0 and 1.5.

Now we’re talking!

The above images were taken when there was moonlight available. What happens when there is no moonlight?

Example VIIRS NCC image (12:57 UTC 26 July 2015) scaled from -10 to 1000.

Scaling over the full range of values means you only see the city lights of Honolulu and the islands drawn on the map.

Example VIIRS NCC image (12:57 UTC 26 July 2014) scaled from 0 to 1.

Scaling from 0 to 1 is better, but I would argue that it’s still too dark. Let’s stretch it further.

Example VIIRS NCC image (12:57 UTC 26 July 2015) scaled between 0 and 0.5.

This is about as good as you can do without the image becoming too noisy.

And, of course, the presence of the aurora gives yet another result:

Example VIIRS NCC image (11:32 UTC 22 January 2015) scaled from -10 to 1000.

Can you see the aurora over northern Alaska? Maybe just barely. Once again, scaling over the full range of values doesn’t work (just like it wouldn’t for the DNB radiance values). What about using the scale of 0 to 1.5? It worked before…

Example VIIRS NCC image (11:32 UTC 22 January 2015) scaled from 0 to 1.5.

GAHH! I’m blinded! Although, you can see the clouds over the Gulf of Alaska pretty easily as well as ice leads in the Arctic Ocean. But, the aurora is too bright and you can’t see any details over most of Alaska.

It turns out, in order to prevent the aurora from saturating this scene, the image needs to be scaled over a range of 0 to 21:

Example VIIRS NCC image (11:32 UTC 22 January 2015) scaled from 0 to 21.

But, notice that you lose the detail of the cloud field over the Gulf of Alaska and the ice over the Arctic Ocean. This is a difficult case to scale correctly. More on that later.

So, we’ve seen that the optimum scaling bounds vary from scene to scene. The 0 to 1.5 scale seems to work for daytime and full moon scenes. New moon scenes require a scale more like 0 to 0.5 (or thereabouts) to be able to detect clouds, snow and ice. And the occasional scene requires a totally different scale altogether. Wouldn’t it be great if there were some way to automate this, so we wouldn’t have to keep fussing with the scaling on every image?

I’m here to say, “there might be.” And, it’s called “Auto Contrast.” The idea is to do what Photoshop and other image editing software do when they “automatically” improve the contrast in the image. The idea is to take the NCC image data, scaled over a range from 0 to 2, for example, bump up the maximum value bound of the scaling with the same kind of adjustment the ERF-Dynamic Scaling uses to prevent saturation in auroras, then apply something similar to Photoshop’s Auto Contrast algorithm to create the ideal scene contrast. Here’s what Auto Contrast does for the three cases above:

Example VIIRS NCC image (08:55 UTC 5 August 2015) scaled with Auto Contrast.

Example VIIRS NCC image (12:57 UTC 26 July 2015) scaled with Auto Contrast.

Example VIIRS NCC image (11:32 UTC 22 January 2015) scaled with Auto Contrast.

For the first two cases, Auto Contrast is very similar to the subjectively determined “ideal scaling”. For the aurora case, we can see that Auto Contrast is a compromise between “not allowing the aurora to saturate” and “allowing the aurora to saturate half of the image.” The aurora does saturate a portion of the scene, but you can still see ice on the Arctic Ocean and clouds over the Gulf of Alaska when you look closely.

Of course, there are a few caveats:

1) Auto Contrast has not been fully tested. These results are promising enough that I wanted to share it right away, but it might not produce ideal results in all cases. We are continuing to investigate this.

2) Sometimes, the image has poor contrast that Auto Contrast can’t fix. For example, a new moon case over land where there are lots of city lights or a vivid aurora. Non-city areas will be more like the Hawaii case, where clouds have pseudo-albedo value between 0 and 0.5, and the city lights or aurora will have pseudo-albedo values well over 100. If you stretch the scaling enough to see the clouds, you’ll be blinded by the city lights. If you scale it to the city lights, you won’t see the clouds or snow or ice.

3) Individual users may not care that the aurora saturated half the image in the third example because they can see the clouds and ice just fine. Auto Contrast makes the clouds and ice darker and harder to see. This is example of how “ideal contrast” not only varies scene to scene, but also from one user application to another. Pretty pictures are not always the same thing as usable images.

4) Demonstrating the utility of “Auto Contrast” is not the same thing as getting the algorithm up and running within AWIPS. (Or, sending files to AWIPS that have optimized contrast.) The JPSS Imagery Team is working with the developers of the AWIPS NCC product to improve how it is displayed, but it will likely take some time.

While it’s not clear how the NCC images are currently scaled in AWIPS, they almost certainly use a fixed scale. However, the examples shown here make it clear that the scaling needs to adjust from scene to scene – even if Auto Contrast is not the ultimate solution. So while we work to figure this out, if the NCC imagery looks sub-optimal in your AWIPS system, you know why.

One final thought: the Auto Contrast algorithm is designed to work with any image, not just NCC images. It’s possible that DNB images created with ERF-Dynamic Scaling may be improved with Auto Contrast as well. But, that’s a topic for another blog post about image scaling for the future. I may yet title a post “Revisiting ‘Revisiting “Revisiting Scaling on the Solstice”‘”.

# Revisiting “Revisiting Scaling on the Solstice”

Imagine that you are an operational forecaster. (Some of you reading this don’t need to imagine it, because you are operational forecasters.) You’ve been bouncing off the walls from excitement because of all the great information the VIIRS Day/Night Band (DNB) provides. “This is so great! Visible imagery at night! It helps in so many ways,” you say to yourself or to anyone within earshot. What’s more: you read this blog and, in particular, you’ve read this blog post and/or this paper. “All our problems have been solved! We can use the DNB for any combination of sunlight and moonlight! I am so happy!” Then you come across an image like this:

VIIRS DNB image created using “erf-dynamic scaling” (15:14 UTC 21 January 2015)

If you’re short tempered, you’re thinking, “@&*!@#&#!!!” If you have better control of your emotions, you’re thinking, “Me-oh-my! Whatever happened here?” Welcome to the third installment of the seemingly-never-ending series on how difficult it is to display the highly variable DNB radiance values in an automated way.

In the previous installment, which I will keep linking to until you click on it and re-read it, I outlined a great new way to scale the radiance values as a function of solar and lunar zenith angles that I call the “erf-dynamic scaling” (EDS) algorithm because it is based on the Gaussian error function (erf). This algorithm uses smooth, continuous functions to account for the 8 orders-of-magnitude variability in DNB data that occurs between day and night, and which was demonstrated to beat many previous attempts at image scaling. Unfortunately, that algorithm produced the image you see above.

So, is my algorithm a failure?

Well, if you’re going to jump right to “failure” based on this, you need to calm down and back off the hyperbole. Do you feel like a failure every time you make a mistake? Besides, mistakes are opportunities for learning.

My demonstration of the quality of the EDS method was based on images taken near the summer solstice. Now, we’re a month after winter solstice. And you know what happens in the winter that doesn’t happen in the summer? The aurora! (Actually, the aurora is present just as much in the summer, but you can’t see it because the sun is still shining.) Now that the nights are so long and dark, the aurora is easily visible.

My EDS method accounts for sunlight and moonlight. It doesn’t account for auroras and they can be several orders of magnitude brighter than the moonlight – especially near new moon when there is no moonlight. And guess when the image above was taken relative to the lunar cycle.

Now, I knew auroras would mess up my scaling algorithm (“Oh, sure you did!”), but I underestimated their occurrence. As a “Lower-48er,” I’ve seen the aurora once in my life. But, at high latitudes (*cough* Alaska *cough*) they happen almost every night in the winter. They’re not always visible due to clouds, but you can’t call them a “rare occurrence”.

From the perspective of DNB imagery, auroras can get in the way. Or, auroras can act as another illumination source to light up important surface features. Let’s look at the above image, with the data re-scaled by manually tweaking the settings in McIDAS-v:

VIIRS DNB image manually scaled (15:14 UTC 21 January 2015)

Of course, this image is rotated differently, but that’s not important. The important thing is that you can see now that it’s an aurora and you can see surface features underneath it. Cracks in the sea ice are visible! (And, remember, there is no moonlight here – just aurora and airglow.) Much better than the wall of white image, right? This proves that it’s a problem with my scaling and not with the DNB itself.

So, how do we get my scaling to work for this case? In theory, the answer is simple: bump up the max values until it’s no longer saturated. In practice, however, it’s not that simple. This was a broad, relatively diffuse aurora that was barely brighter than the max values. Some auroras are much more vivid (and much brighter than the max values), like this one:

VIIRS DNB image with modified “erf-dynamic scaling” (11:34 UTC 22 January 2015)

If you increase the max values until nothing is saturated, you’ll only be able to see the brightest pixels (which are usually city lights) and nothing else. And, don’t forget: we don’t want to increase the max values everywhere all the time, because the algorithm works as-is when the aurora isn’t present (or when the moonlight is brighter than the aurora).

Here’s the solution: calculate max and min values with the EDS method as before, but increase the max values by 10% at a time until only a certain percentage of the image is saturated. That’s what I’ve done in the last image above, where I’ve adjusted the max values until only 0.5% of the image is saturated. In case you’re wondering, here’s the same image without this additional correction:

VIIRS DNB image with unmodified “erf-dynamic scaling” (11:34 UTC 22 January 2015)

The correction makes it much better. What about for the first case I showed? Here’s the corrected version:

VIIRS DNB image with modified “erf-dynamic scaling” (15:14 UTC 21 January 2015)

Once again, much better than before. You can see the cracks in the sea ice now! (Maybe it’s not as good as the manual scaling but, because it’s automated, it takes less time to produce. )

Of course, this correction assumes that less than 0.5% of the image is city lights or wildfires or lightning. And, it might not work too good if the data spans all the way from bright sunlight to new-moon night beyond the aurora because it darkens the non-aurora parts of the scene (as can be seen in the images from 22 January 2015). But, the great thing is: if the scene is not saturated by the aurora (or some other large bright feature) no correction is applied, so you still get the same great EDS algorithm results you had before.

As a bonus to make up for the initial flaws in the EDS algorithm (and to get any short-tempered viewers to stop cursing), enjoy the images below of a week’s worth of auroras as seen by the DNB (with the newly modified scaling). Make sure you look for the “Full Resolution” link to the upper right of each image in the gallery to see the full resolution version:

# Glow-in-the-dark Water

Have you ever started looking for something, only to find something else that was more interesting than what you were originally looking for?

Back on 10 January 2014, there were widespread rumors of a significant aurora event that would be visible much further south than usual. It got a lot of people excited, even in our backyard here in Colorado. But did it happen?

If you’re curious, here is an explanation as to why the aurora forecasts were a bust. But, that’s not to say the aurora didn’t exist anywhere on the globe. The VIIRS Day/Night Band image below shows there was an aurora that made it as far south as Iceland.

VIIRS Day/Night Band image, taken 02:31 UTC 10 January 2014

What about on the next orbit? Was the aurora still there?

VIIRS Day/Night Band image, taken at 04:13 UTC 10 January 2014

If you squint, you can maybe see it over south-central Greenland. But, hold on a minute! What’s that in the upper-left corner? Why is the water so bright off the west coast of Greenland?

This is a nighttime scene, as evidenced by the city lights over Iceland, Ireland and the UK, although you might not think that by looking at only the left side of the image. And, let me assure you, the day/night terminator never appears at this angle at this time of day in January.

CIRA researchers have recently begun producing VIIRS imagery centered on Alaska on a quasi-operational basis. About a month ago, I noticed this image that also shows “glow-in-the-dark” water, and the mystery deepened:

VIIRS Day/Night Band image, taken 11:37 UTC 9 February 2014

And again, a few days ago, the Day/Night Band captured this image:

VIIRS Day/Night Band image, taken 12:35 UTC 10 March 2014

This time, there is a pretty vivid aurora but, you can also see bright water off the southern coast of Russia.  So, what’s with water that appears to be glowing in the dark?

Is it some kind of bio-luminescent phenomenon, like milky seas? Is it some kind of radioactivity that makes everything glow, like in The Simpsons? Or an alien-UFO conspiracy to control the world’s population?

Sorry to get your hopes up, “truthers,” but it’s a pretty mundane explanation. (Either that, or I’m a member of the Illuminati. MWAH HA HA!) Have you ever looked at a body of water and saw glare from the sun? Or seen glare off of snow and ice? We call that sunglint. It is related to the Bi-directional Reflectance Distribution Function (BRDF), the mathematical way we describe that incoming light on a surface reflects more at certain angles than others. But, it’s not only sunlight that causes glint. Moonlight does it, too. (What is moonlight, if not reflected sunlight?)

Notice that the images with the glowing water were taken roughly a month apart. That’s not just a coincidence. According to this website, each of those images was taken 2-3 days after the moon reached first quarter, when the moon was 75-80% full. Why is this important? Because the phase of the moon is related to when the moon rises and sets, and this determines where the moon is in the sky when VIIRS passes overhead.

From a day or two after last quarter to new moon to a day or two after first quarter, the moon is below the horizon when VIIRS passes overhead during the nighttime overpass. (It’s above the horizon on the daytime overpass, but you can’t tell because the sun is so bright.) From just after first quarter to full moon to just after last quarter, the opposite is true – the moon is up at night and down during the day. When you get to 2-3 days after first quarter, that’s when the moon is close to the western horizon when VIIRS passes over at night. That’s why the left sides of the above images are brighter than the right sides. And, that’s also when this form of moon glint occurs, just like in this clip.

It’s not aliens or UFOs or mysterious radioactivity. It’s the geometry between the satellite, the Earth and the moon and the preferential reflection of light off of a body of water. It’s repeatable and predictable. It’s science.

UPDATE (3/14/2014): “Glow-in-the-dark” water is not confined to high latitudes like Greenland and Alaska. It happens anywhere the angle between the satellite, the Earth’s surface and the moon is in the glint range. Steve Miller (CIRA) forwarded information about a case he looked at off the coast of Louisiana. Here’s one of his images with everything labelled:

VIIRS Day/Night Band image, taken 07:08 UTC 12 January 2014. Interesting features have been identified and labelled.

This case occurred when the moon was 90% full. The brightest water occurs where the surface is calm and the “glint angle” is less than 10°.  When the surface is not calm, waves scatter the light in different directions and only a portion of the light is reflected to the satellite. This makes the water appear not as bright. For glint angles between 0° and 30°, waves will scatter some of the light back to the satellite, and the water won’t appear dark. Calm water outside the 10° glint zone will appear dark, though, because the angle of the water surface isn’t right to reflect the moonlight back to the satellite. This is what you see along the coast of Texas. Outside of the 30° zone, waves aren’t at the proper angle to reflect light back to the satellite.

To demonstrate this, here’s a comparison with the same area on the next orbit along with the glint angles:

Comparison between DNB images and lunar glint angle for consecutive VIIRS overpasses on 12 January 2014.

On the next overpass, about 100 minutes later, all the water is outside the glint zone (the glint angles are all higher than 100°) and the water is dark everywhere, as expected.

# Santa Claus and the Olympic Flame

In the lead up to the 2014 Winter Olympics, the Olympic Torch was sent on a grueling journey across Russia and beyond – including a trip to the North Pole and to Outer Space. (Obviously, the torch won’t be lit when it is in space. You don’t want to burn up all the oxygen on the International Space Station – the astronauts need that to breathe. It also wouldn’t burn during the space walk, since there is no air out there.) An offshoot of the flame did make it to the North Pole, though, which is the first time that has ever happened. One could argue that it wasn’t really the true Olympic Flame, since the original flame burned out during a jog around the Kremlin in Moscow:

But, I’m sure the backup cigarette lighter is a valid substitute for having to jog all the way back to Athens, Greece to get the high priestess of the Temple of Hera to invoke the power of the sun to relight it. (We poke fun in good nature, knowing full well that it could happen to any of us – any of us lucky enough to carry the torch.)

Now, back to the Olympic Flame’s trip to the North Pole. Under the cover of clouds, the nuclear-powered icebreaker, 50 Years of Victory (50 лет Победы in Russian), carried the flame to the furthest north it could go. Once there, the torchbearers gave Santa Claus quite the light show. Check out the videos and photos of the trip – it was pretty impressive. Santa was grateful for the presentation. It was his last opportunity to take a break before finishing his Christmas preparations.

So, what does this have to do with a blog about a weather satellite? VIIRS saw the Olympic Flame and the Star Wars-like light show put on at the North Pole.

According to those news articles, the ship arrived at the North Pole on 19 October 2013. Below is an animation images from the Day/Night Band for every VIIRS overpass from 01:38 UTC on 19 October to 06:23 UTC on 20 October 2013.

Animation of VIIRS Day/Night Band images from 19-20 October 2013. The North Pole is located at the center of the image. Light from the ship carrying the 2014 Winter Olympic torch is visible.

The yellow dotted lines are latitude and longitude lines. The longitude lines converge on the North Pole. Initially, there is an opaque cloud layer that obscures the view of the 50 Years of Victory, but by the 08:23 UTC 19 October 2013 frame, the light from the ship is clearly visible. In the last two frames, the icebreaker can be seen heading back to Russia, which is off the top of the image. (Canada and the United States are below the bottom edge of the image.)

Keep in mind, since we are past the Autumnal Equinox, it is always night at the North Pole. That’s why we can see the ship’s lights. (It would be too bright to see the ship if it were daylight.) That also means that Santa has to finish making presents for everyone in the dark.

And, sorry kids. The Day/Night Band does not have high-enough resolution to be able to see Santa’s house. But, it does have high-enough resolution to see an icebreaker ship at work.

UPDATE/ASIDE: William Straka III (U. of Wisconsin/CIMSS) has done some investigating of ships at night in the Arctic using the Day/Night Band, and has shared these images (converted to a single animation):

Animation of selected VIIRS Day/Night Band images from 30 October to 2 November 2013. Images courtesy William Straka III (CIMSS).

This animation covers several days (30 October to 2 November 2013) where a couple of icebreaker ships are visible. Using the website sailwx.info, he was able to identify one of the ships as the icebreaker Taimyr (Таймыр). Here’s a plot of the ship’s track over this period:

Plot of the track of the Russian icebreaker Taimyr, 30 October to 4 November 2013. Image courtesy sailwx.info and William Straka III (CIMSS).

The other ship (or ships, since there seem to be two areas of light in some of the images) are unidentified. He was able to deduce the following:

One of the ships in “Group 1” is an icebreaker. (It has to be, because there is ice covering the ocean in this region.) That icebreaker cannot be the 50 Years of Victory (50 лет Победы), since it had returned to port following its trip to the North Pole. Tracking information from sailwx.info also shows that it was not the Vaygach (Вайгач). News reports show that the Rossiya (Россия) was retired from service in May 2013. The only other nuclear-powered icebreakers in the Russian fleet are the Yamal (Яма́л) and the Sovetskiy Soyuz (Советский Союз). (Of course, there is the possibility that the icebreaker isn’t nuclear-powered, which increases the number of possibilities.)

In case you’re interested, this scene takes place near the New Siberian Islands. I’m not sure what kind of services they have on the islands but, judging by the images above, they look like a good place to view the aurora!

# Auroras, Volcanoes and Bears, Oh My!

It’s amazing what you can see in a single image from the VIIRS Day/Night Band.

OK, so you can’t actually see any bears with VIIRS (even bears that have fattened up for the winter are less than 375 m across – and certainly less than the 742 m resolution of the Day/Night Band), but you can see auroras and volcanoes. And a lot more! Take a look at this image from the VIIRS Day/Night Band (DNB) taken at 12:19 UTC 7 October 2013:

VIIRS Day/Night Band image, taken 12:19 UTC 7 October 2013

What do you see? Make sure you click on it to see the full resolution image.

Well, there’s a bright arc that stretches from Siberia, over the Brooks Range in northern Alaska and into the Yukon and Northwest Territories of Canada, and there are even brighter arcs north of that (between the Brooks Range and Barrow, even extending over the Arctic Ocean). Those are examples of the aurora borealis (a.k.a. Northern Lights).

As an aside, did you know that VIIRS can provide information about the speed of the aurora? I’ll wait if you want to read that. The aurora in this case is moving more in the along-track direction than the across-track direction, so the method for calculating the speed of the aurora won’t work so well with this aurora. Although, there does seem to be some apparent motion of the auroral element immediately off the coast from Prudhoe Bay. See if you can spot the scan-to-scan “shifts”.

Speaking of Prudhoe Bay, you can also see the tremendous amount of light given off by the oil and gas operations there, making the area look like the largest city in Alaska. (Compare the size of the lit-up area with that of Anchorage, which actually is the largest city in Alaska.) If you know your Alaska and Yukon geography, you should also be able to pick out Barrow, Fairbanks, Delta Junction, Whitehorse, Wasilla, Kodiak and Juneau.

Now, what is that big, circular city over the Alaska Peninsula? (It also looks bigger than Anchorage.) That is the volcano Veniaminof, which was erupting last time we looked at it (at the end of August), and appears to still be going strong as of 7 October 2013 (or it calmed down in the meantime before acting up again, as this article suggests). It’s not that the volcano is actually larger than the city of Anchorage, as it appears. What you are seeing is the light emitted by the liquid-phase rock erupting from the volcano. Rock that is hot enough to be liquid is hot enough to emit radiation in the visible and near-infrared wavelengths the Day/Night Band is sensitive to. This light is illuminating a circular area of the clouds surrounding Veniaminof, and enough of it is reflected or scattered to outer space that VIIRS is able to detect it.

Another question you might ask is, “What is that bright band across the image that parallels the scan lines and passes over both Anchorage and Juneau?” It almost looks like an aurora, but it is in too straight a line. It is another example of “stray light”. Now, if you read the previous post (and remember what it said) you might be confused, because I said that stray light was fixed and shouldn’t be a problem anymore. Well, the stray light correction is not perfect, especially in near-new moon images such as this. For example, say that the stray light correction introduces an error in the radiance on the order of 1 x 10-10 W cm-2. During a full moon, the radiance value observed for a clear background surface pixel would be on the order of 1 x 10-7 W cm-2, so this error is too small to notice (0.1 % error). During a new moon, the radiance values of a clear background surface are more like 5 x 10-10 W cm-2 (20% error, which is noticeable).

Another thing to consider is that the stray light correction requires post-processing, and is dependent on the moon’s phase and time of year. Since this image was near new moon in October (13% of full on the 7th), it uses data from the previous new moon (in September) to do the stray light correction, since we didn’t have a similar data from October to use. (Of course, we have that data now, but it wasn’t available at the time this image came off the satellite. It will be used on next year’s October data.) Slight differences in the solar-satellite geometry between September and October are the largest source of error in the stray light correction. Since the stray light correction began in late August 2013, once we get to September 2014 the stray light correction ought to perform much better (but it will still be more likely to show errors during a new moon than a full moon).

Speaking of this being near a new moon, how is it possible that you can see clouds and sea ice and snow? It’s true that the aurora does a good job illuminating the surface underneath, but the aurora doesn’t cover everywhere. What about over the ocean? There are no city lights, no erupting volcanoes, and not enough ships in the sea to light up the sky (like there are off the coast of Korea). Where is the light coming from?

It’s actually a phenomenon known as “airglow” (or “nightglow”, since it is easiest to detect at night). The shortened version is that molecules in the upper atmosphere interact with ultraviolet radiation and, as a result, emit photons. This happens around the clock. Airglow is enough to detect a Super Cyclone at night with no moon.  That means, even on the darkest nights, the Day/Night Band is capable of viewing clouds and ice and snow. You don’t even need the moon at all!