Polar Opposites

Posted on August 15, 2019 by Curtis_Seaman

As we all know, the furthest south you can travel is to the South Pole – the Geographic South Pole, not the Magnetic South Pole or the Geomagnetic South Pole. When you get there, try to face east if you can. (This is easier to do at the “Ceremonial South Pole” than it is at the actual South Pole.)

The furthest south you can get by boat is an island off the coast* of Antarctica, called Ross Island. (*The term “coast” is used loosely here, since Ross Island is usually connected to Antarctica by the Ross Ice Shelf.) At the southern tip of Ross Island is the largest “city” in Antarctica: McMurdo Station. McMurdo is the port-of-entry for most visitors to Antarctica. It is also home to a ground station that receives data from NOAA-20 (and many other satellites). So, if you love the lower latency that comes with NOAA-20 VIIRS data, you have McMurdo Station to thank. (S-NPP data is only downlinked at Svalbard – once per orbit – while NOAA-20 is downlinked at both Svalbard and McMurdo.) This is the location of today’s resolved mystery.

The mystery began with the development of a new website for viewing global VIIRS imagery: Polar SLIDER*. (*Shameless self-promotion: I helped develop that website.) If you click on that link, choose “Southern Hemisphere” from the Sector menu to view Antarctica. With every product, you can zoom in anywhere on the globe* to view the full resolution data. (*Claim is void near the Equator.) Under the Product menu, you can choose between all 22 VIIRS channels (16 M-bands, 5 I-bands, and the Day/Night Band), or from a list of imagery products and cloud products. (And we are always working to add new products.) Since it’s perpetual night down there right now, you’ll notice that the visible and near-IR bands don’t give you much information – except the Day/Night Band, of course, which can provide images like this:

NOAA-20 VIIRS DNB image (14:25 UTC, 14 August 2019)

Ross Island is in the center of that image. That bright light at the southern tip of Ross Island is McMurdo Station. The second bright light south of that is the “airport“. Here’s an annotated image with the map plotted on it:

NOAA-20 VIIRS DNB image of Ross Island and surroundings (14:25 UTC, 14 August 2019)

As always, click on an image to see it in full resolution. Now that we have our bearings, let’s look at the high resolution mid-wave IR band (I-4/3.74 µm):

NOAA-20 VIIRS channel I-4 (14:25 UTC, 14 August 2019)

See that white dot in the middle of Ross Island? What is that? (Hint: it’s not part of the map.)

To make some sense of this, look at the color table plotted on the bottom of the image. White pixels on this scale (not counting the map) are +100°C (+373 K). In contrast, the dark turquoise color surrounding it is in the -25°C to -30°C range (243-248 K). What could be over 100°C in Antarctica in the winter? Did something catch on fire?

It turns out, it is a semi-permanent feature according to this animation collected from Polar SLIDER. (You have to click on the animation to see it play.)

Animation of VIIRS channel I-4 images (13 August 2019)

Looking at Day/Night Band images over the same time period, it also shows up as a bright spot:

Animation of VIIRS DNB images (13 August 2019)

Maybe it’s a nuclear reactor that powers all of McMurdo Station? (Nope. There was a nuclear power plant, but that was de-commissioned in 1972.) Maybe the fact that this bright (in the DNB), hot spot (mid-IR) is on top of a mountain has something to do with it? (Bingo!)

Ross Island is made up of volcanoes, the most prominent of which are Mt. Erebus and Mt. Terror (named for the ships on the original expedition that discovered them). Mt. Terror (the one on the right) is inactive. Mt. Erebus, on the other (left) hand, is the southernmost active volcano in the world. And, what’s relevant here is the fact that it is home to one of only five known lava lakes in the world. So, molten-hot liquid rock exists in an ice-covered environment where temperatures regularly dip down to -50°C or -60°C. And, it’s right next to the largest settlement in Antarctica. Sleep tight. (Since we’re less than a week away from the first sunrise of the spring, get your sleep while you can down there!)

Protected: Clouds at the Edge of Space

Posted on September 19, 2018 by Curtis_Seaman

Steve and the Color Purple

Posted on April 24, 2017 by Curtis_Seaman

It’s not often that a new discovery takes place that baffles the minds of lifelong scientists. This is a story about one that seems to have gone viral over the last few days. The abbreviated version (summarized from this article, this article, and this article, and many others like it) is as follows:

A group of dedicated aurora photographers noted a particular type of aurora that was different from what we normally think of. Instead of a rapidly changing curtain of light glowing green or red, it is a single arc of light, “purple” in color, with less apparent motion than a normal aurora. It doesn’t appear to move with the Earth’s magnetic field. The picture that accompanies every article about it is this one:

Photograph credited to Dave Markel Photography

The early guess was that it’s an example of a “proton arc” – a type of aurora caused by high energy protons rather than electrons. (Do a Google Image Search for “proton arc” and you’ll see many other examples.) However, the plot thickened when an expert on the aurora, Prof. Eric Donovan at the University of Calgary, debunked that guess based on the fact that proton arcs are not visible to the human eye. This was backed up by a graduate student at the University of Alaska-Fairbanks. Not knowing what else to call it, the dedicated aurora photographers named it Steve. No joke. (It comes from the animated movie, Over the Hedge.) The name has caught on, and now the internet is full of photographic examples of “Steve”. Here’s a time lapse video.

The Aurorasaurus Project has compiled a list of things we know about Steve. Our expert aurora professor matched up a known time and location of a Steve photograph with an overpass of the European Space Agency’s Swarm satellites and found this out:

“As the satellite flew straight though Steve, data from the electric field instrument showed very clear changes. The temperature 300 kilometres (185 miles) above Earth’s surface jumped by 3,000°C (5,400 degrees Fahrenheit) and the data revealed a 25 kilometre (15.5 mile) wide ribbon of gas flowing westwards at about 6 km/s (3.7 miles per second) compared to a speed of about 10 m/s (32.8 feet per second) either side of the ribbon.”

So, while we don’t exactly know what causes “Steve”, we do know that it is relatively common. (Do that Google Image Search for “proton arc” again for proof.) And we know it’s not a proton arc. Of course, the question that is relevant to us on this blog is: Can the VIIRS Day/Night Band see Steve?

There was a significant geomagnetic storm 22-23 April 2017 that may provide the answer. One of the Alberta Aurora Chasers (our dedicated group of aurora photographers) took this picture and, in the comments, noted the location (Lake Minnewanka, Alberta) and approximate time (“maybe 12:30” AM on the 22nd). Compare that with the nearest Day/Night Band image:

VIIRS Day/Night Band image (08:12 UTC 22 April 2017)

I put a gold star on there to indicate the location of Lake Minnewanka. Don’t see it? Here’s a close-up:

VIIRS Day/Night Band image above zoomed-in on Lake Minnewanka. The gold star indicates the location of the lake.

Unfortunately, Lake Minnewanka is outside the VIIRS swath. But, Aurorasaurus says Steve is often hundreds or thousands of miles long, and oriented east-west, so it should extend into the VIIRS swath. Now, this VIIRS image was taken at about 2:15 AM local time, almost two hours after the photograph was taken. Aurorasaurus also says Steve is visible on the order of minutes, “up to 20 minutes or more”. So, maybe Steve disappeared in the time between the two images. I certainly don’t see any straight or smooth arc of light near the star that resembles Steve. Although, just north of Calgary (the closest city within the VIIRS swath to Lake Minnewanka) there is faint evidence of aurora light, and it is on the equator-ward side of the aurora, which is consistent with previous observations.

The streaks of light visible near Calgary (and general streakiness across the whole aurora) are due to the way the VIIRS instrument scans the scene and the high-temporal variability of the aurora, which we’ve discussed before. But, as I mentioned, these streaks don’t extend for hundreds or thousands of miles.

Maybe, VIIRS had better luck on the next overpass (~3:55 AM local time):

VIIRS Day/Night Band image (09:53 UTC 22 April 2017)

Again, nothing jumps out to say, “Aha! That’s Steve!” So, was Steve there and VIIRS failed to see it? Or, was Steve not there at the time of the VIIRS overpass? The answer to that depends in part on the definition of “purple”.

Is Steve really “purple” as people describe? Or, is it violet? Wikipedia actually has a good section on this (at least, until someone edits it). There’s also the page discussing the “Line of Purples“. The problem stems from the fact that violet is a color similar to purple, but is physically very different. Violet is the name given to a specific wavelength range of light, specifically the visible portion of the spectrum less than 450 nm. Purple is a combination of blue and red wavelengths – blue being wavelengths between ~450 nm and ~495 nm and red being anything visible above ~620 nm. Violet and purple look similar to us because the cone cells in our eyes have a similar response to both colors. However, in the RGB color space of the computer you’re viewing this on, and in the color cameras used to take pictures of Steve, violet is impossible to duplicate. This is because violet is not a combination of red, green and blue – it’s its own wavelength. The red, green and blue light emitting diodes (or phosphors on a plasma screen) don’t emit violet wavelengths. Your camera stores the information it collects in RGB color space, too, and has to approximate violet the same way your computer does – by making it a bluer shade of purple. Depending on the camera, the detectors used may not even be sensitive to violet light.

So, what does this mean for VIIRS? The Day/Night Band is not sensitive to radiation at wavelengths shorter than ~500 nm, which includes blue and violet. But, it is sensitive to red and beyond – up to ~900 nm. So, if Steve really is purple, the Day/Night Band will only be sensitive to the red component of it. (It would be more faint, but VIIRS would likely be sensitive to it, given that it is sensitive to airglow, which is much more faint than the aurora.) If Steve is really violet, than the Day/Night Band won’t see it at all.

So, can the Day/Night Band detect Steve? I can’t answer that based on this information. We will have to wait for another dedicated aurora photographer to take a picture of Steve at a time and place when VIIRS is directly overhead. Feel cheated by that? Just enjoy the images of the aurora above. And, here are a few more from this event:

VIIRS Day/Night Band image (11:34 UTC 22 April 2017)

VIIRS Day/Night Band image (07:53 UTC 23 April 2017)

VIIRS Day/Night Band image (09:34 UTC 23 April 2017)

Don’t forget to click on them to see the full resolution!

UPDATE (13 October 2017): Over the years, I have looked at a number of Day/Night Band images of the aurora. During that time, I’ve noticed some “auroras” that appear to be very “Steve”-like. One example is shown in the image below from 17 January 2015.

VIIRS Day/Night Band image (13:09 UTC 17 January 2015)

The question is: is this an example of Steve? Or, just a less active aurora?

Of course, being over a remote part of northern Alaska, it’s unlikely anyone got a photograph to prove it was Steve. We’ll still have to wait for the perfect alignment of Steve, Steve-hunters and VIIRS to know if the Day/Night Band can (or cannot) detect them.

December Fluff

Posted on December 19, 2016 by Curtis_Seaman

By now, you probably know the drill: a little bit of discussion about a particular subject, throw in a few pop culture references, maybe a video or two, then get to the good stuff – high quality VIIRS imagery. Then, maybe add some follow-up discussion to emphasize how VIIRS can be used to detect, monitor, or improve our understanding of the subject in question. Not today.

You see, VIIRS is constantly taking high quality images of the Earth (except during orbital maneuvers or rare glitches). There isn’t enough time in a day to show them all, or go into a detailed discussion as to their relevance. And, nobody likes to read that much anyway. So, as we busily prepare for the upcoming holidays, we’re going to skip the in-depth discussion and get right to the good stuff.

Here then is a sample of interesting images taken by VIIRS over the years that weren’t featured on their own dedicated blog posts. Keep in mind that they represent the variety of topics that VIIRS can shed some light on. Many of these images represent topics that have already been discussed in great detail in previous posts on this blog. Others haven’t. It is important to keep in mind… See, I’m starting to write too much, which I said I wasn’t going to do. I’ll shut up now.

Without further ado, here’s a VIIRS Natural Color image showing a lake-effect snow event that produced a significant amount of the fluffy, white stuff back in November 2014:

VIIRS Natural Color RGB composite of channels M-5, M-7 and M-10 (18:20 UTC 18 November 2014)

As always, click on the image to bring up the full resolution version. Did you notice all the cloud streets? How about the fact that the most vigorous cloud streets have a cyan color, indicating that they are topped with ice crystals? The whitish clouds are topped with liquid water and… Oops. I’m starting to discuss things in too much detail, which I wasn’t going to do today. Let’s move on.

Here’s another Natural Color RGB image using the high-resolution imagery bands showing a variety of cloud streets and wave clouds over the North Island of New Zealand:

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (02:55 UTC 3 September 2016)

Here’s a Natural Color RGB image showing a total solar eclipse over Scandinavia in 2015:

VIIRS Natural Color RGB composite of channels M-5, M-7 and M-10 (10:06 UTC 20 March 2015)

Here’s a VIIRS True Color image and split-window difference (M-15 – M-16) image showing volcanic ash from the eruption of the volcano Sangeang Api in Indonesia in May 2014:

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (06:20 UTC 31 May 2014)

VIIRS split-window difference (M-15 – M-16) image (06:20 UTC 31 May 2014)

Here’s a VIIRS True Color image showing algae and blowing dust over the northern end of the Caspian Sea (plus an almost-bone-dry Aral Sea):

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (09:00 UTC 18 May 2014)

Here is a high-resolution infrared (I-5) image showing a very strong temperature gradient in the Pacific Ocean, off the coast of Hokkaido (Japan):

VIIRS I-5 (11.45 um) image (03:45 UTC 12 December 2016)

The green-to-red transition just southeast of Hokkaido represents a sea surface temperature change of about 10 K (18 °F) over a distance of 3-5 pixels (1-2 km). This is in a location that the high-resolution Natural Color RGB shows to be ice- and cloud-free:

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (03:45 UTC 12 December 2016)

Here’s a high-resolution infrared (I-5) image showing hurricanes Madeline and Lester headed toward Hawaii from earlier this year:

VIIRS I-5 (11.45 um) image (22:55 UTC 30 August 2016)

Here are the Fire Temperature RGB (daytime) and Day/Night Band (nighttime) images of a massive collection of wildfires over central Siberia in September 2016:

VIIRS Fire Temperature RGB composite of channels M-10, M-11 and M-12 (05:20 UTC 18 September 2016)

VIIRS Day/Night Band image (19:11 UTC 18 September 2016)

Here is a 5-orbit composite of VIIRS Day/Night Band images showing the aurora borealis over Canada (August 2016):

Day/Night Band image composite of 5 consecutive VIIRS orbits (30 August 2016)

Here is a view of central Europe at night from the Day/Night Band:

VIIRS Day/Night Band image (01:20 UTC 21 September 2016)

And, finally, for no reason at all, here’s is a picture of Spain wearing a Santa hat (or sleeping cap) made out of clouds:

VIIRS Natural Color RGB composite of channels M-5, M-7 and M-10 (13:05 UTC 18 March 2014)

There you have it. A baker’s ten examples showing a small sample of what VIIRS can do. No doubt it will be taking more interesting images over the next two weeks, since it doesn’t stop working over the holidays – even if you and I do.

UHF/VHF

Posted on January 28, 2016 by Curtis_Seaman

Take a second to think about what would happen if Florida was hit by four hurricanes in one month.

Would the news media get talking heads from both sides to argue whether or not global warming is real by yelling at each other until they have to cut to a commercial? Would Jim Cantore lose his mind and say “I don’t need to keep standing out here in this stuff- I quit!”? Would we all lose our minds? Would our economy collapse? (1: yes. 2: every man has his breaking point. 3: maybe not “all”. 4: everybody panic! AHHH!)

It doesn’t have to just be Florida. It could be four tropical cyclones making landfall anywhere in the CONUS (and, maybe, Hawaii) in a 1-month period. The impact would be massive. But, what about Alaska?

Of course, Alaska doesn’t get “tropical cyclones” – it’s too far from the tropics. But, Alaska does get monster storms that are just as strong that may be the remnants of tropical cyclones that undergo “extratropical transition“. Or, they may be mid-latitude cyclones or “Polar lows” that undergo rapid cyclogenesis. When they are as strong as a hurricane, forecasters call them “hurricance force” (HF) lows. And guess what? Alaska has been hit by four HF lows in a 1-month period (12 December 2015 – 6 January 2016).

With very-many HF lows, some of which were ultra-strong, we might call them VHF or UHF lows. (Although, we must be careful not to confuse them with the old VHF and UHF TV channels, or the Weird Al movie.) In that case, let’s just refer to them as HF, shall we?

The first of these HF storms was a doozy – tying the record for lowest pressure ever in the North Pacific along with the remnants of Typhoon Nuri. Peak winds with system reached 122 mph (106 kt; 196 k hr^-1; 54 m s^-1) in Adak, which is equivalent to a Category 2 hurricane!

Since Alaska is far enough north, polar orbiting satellites like Suomi-NPP provide more than 2 overpasses per day. Here’s an animation from the VIIRS Day/Night Band, one of the instruments on Suomi-NPP:

Animation of VIIRS Day/Night Band images of the Aleutian Islands (12-14 December 2015).

It’s almost like a geostationary satellite! (Not quite, as I’ll show later.) This is the view you get with just 4 images per day. (The further north you go, the more passes you get. The Interior of Alaska gets 6-8 passes, while the North Pole itself gets all 15.) Seeing the system wrap up into a symmetric circulation would be a thing of beauty, if it weren’t so destructive. Keep in mind that places like Adak are remote enough as it is. When a storm like this comes along, they are completely isolated from the rest of Alaska!

Here’s the same animation for the high-resolution longwave infrared (IR) band (I-5, 11.5 µm):

Animation of VIIRS I-5 images of the Aleutian Islands (12-14 December 2015).

I’ve mentioned Himawari before on this blog. Well, Himawari’s field of view includes the Aleutian Islands. Would you like to see how this storm evolved with 10 minute temporal resolution? Of course you would.

Here is CIRA’s Himawari Geocolor product for this storm:

Here is a loop of the full disk RGB Airmass product applied to Himawari. Look for the storm moving northeast from Japan and then rapidly wrapping up near the edge of the Earth. This is an example of something you can’t do with VIIRS, because VIIRS does not have any detectors sensitive to the 6-7 µm water vapor absorption band, which is one of the components of the RGB Airmass product. The RGB Airmass and Geocolor products are very popular with forecasters, but they’re too complicated to go into here. You can read up on the RGB Airmass product here, or visit my collegue D. Bikos’ blog to find out more about this storm and these products.

You might be asking how we know what the central pressure was in this storm. After all, there aren’t many weather observation sites in this part of the world. The truth is that it was estimated (in the same way the remnants of Typhoon Nuri were estimated) using the methodology outlined in this paper. I’d recommend reading that paper, since it’s how places like the Ocean Prediction Center at the National Weather Service estimate mid-latitude storm intensity when there are no surface observations. I’ll be using their terminology for the rest of this discussion.

Less than 1 week after the first HF storm hit the Aleutians, a second one hit. Unfortunately, this storm underwent rapid intensification in the ~12 hour period where there were no VIIRS passes. Here’s what Storm #2 looked like in the longwave IR according to Himawari. And here’s what it looked like at full maturity according to VIIRS:

VIIRS DNB image (23:17 UTC 18 December 2015).

VIIRS I-5 image (23:17 UTC 18 December 2015).

Notice that this storm is much more elongated than the first one. Winds with this one were only in the 60-80 mph range, making it a weak Category 1 HF low.

Storm #3 hit southwest Alaska just before New Year’s, right at the same time the Midwest was flooding. This one brought 90 mph winds, making it a strong Category 1 HF low. This one is bit difficult to identify in the Day/Night Band. I mean, how many different swirls can you see in this image?

VIIRS DNB image (13:00 UTC 30 December 2015).

(NOTE: This was the only storm of the 4 to happen when there was moonlight available to the DNB, which is why the clouds appear so bright. The rest of the storms were illuminated by the sun during the short days and by airglow during the long nights.) The one to focus on is the one of the three big swirls closest to the center of the image (just above and right of center). It shows up a little better in the IR:

VIIRS I-5 image (13:00 UTC 30 December 2015).

The colder (brighter/colored) cloud tops are the clue that this is the strongest storm, since all three have similar brightness (reflectivity) in the Day/Night Band. If you look close, you’ll also notice that this storm was peaking in intensity (reaching mature stage) right as it was making landfall along the southwest coast of Alaska.

Storm #4 hit the Aleutians on 6-7 January 2016 (one week later), and was another symmetric/circular circulation. This storm brought winds of 94 mph (2 mph short of Category 2!) The Ocean Prediction Center made this animation of its development as seen by the Himawari RGB Airmass product. Or, if you prefer the Geocolor view, here’s Storm #4 reaching mature stage. But, this is a VIIRS blog. So, what did VIIRS see? The same storm at higher spatial resolution and lower temporal resolution:

Animation of VIIRS DNB images of the Aleutian Islands (6-7 January 2016).

Animation of VIIRS I-5 images of the Aleutian Islands (6-7 January 2016).

This storm elongated as it filled in and then retrograded to the west over Siberia. There aren’t many hurricanes that do that after heading northeast!

So, there you have it: 4 HF lows hitting Alaska in less than 1 month, with no reports of fatalities (that I could find) and only some structural damage. Think that would happen in Florida?

JPSS (SNPP and NOAA-20)

VIIRS Imagery and Visualization Team Blog

Tag Archives: day/night band

Polar Opposites

Protected: Clouds at the Edge of Space

Steve and the Color Purple

December Fluff

UHF/VHF