1 00:00:00,000 --> 00:00:02,780 The night sky is never truly dark. 2 00:00:02,800 --> 00:00:06,940 If you removed light pollution, the moon, stars, and galaxies 3 00:00:06,960 --> 00:00:09,960 there would still be a very faint colorful glow. 4 00:00:09,980 --> 00:00:11,280 That’s airglow. 5 00:00:11,300 --> 00:00:15,300 With cameras, you can photograph it only on the darkest of nights. 6 00:00:15,320 --> 00:00:19,710 It’s about one tenth as bright as the combined light of all the stars. 7 00:00:19,730 --> 00:00:23,500 From above, it forms a luminous bubble encapsulating Earth. 8 00:00:23,520 --> 00:00:26,490 Appearing right at the interface to space, 9 00:00:26,510 --> 00:00:30,460 airglow holds clues to how our atmosphere affects weather in space 10 00:00:30,480 --> 00:00:33,610 and how space weather affects humans on Earth. 11 00:00:33,630 --> 00:00:38,420 The bands of light span from 50 to 400 miles above Earth’s surface. 12 00:00:38,440 --> 00:00:41,900 In the uppermost layer of the atmosphere is the ionosphere. 13 00:00:41,920 --> 00:00:45,790 This is where our GPS signals and astronauts travel. 14 00:00:45,810 --> 00:00:50,640 What makes this region complicated is that it’s constantly changing. 15 00:00:50,660 --> 00:00:55,870 It reacts to both energy emanating from the Sun and weather near Earth’s surface. 16 00:00:55,890 --> 00:01:02,570 And as the ionosphere fluctuates, so can conditions in near-Earth space, where the Space Station lies. 17 00:01:02,590 --> 00:01:06,990 But spotting changes in the ionosphere is a lot like trying to watch the wind 18 00:01:07,010 --> 00:01:11,050 - you need a marker of some kind to see the invisible particles move past. 19 00:01:11,070 --> 00:01:13,880 And for that, we have airglow. 20 00:01:13,900 --> 00:01:19,960 These colorful lights reflect changes in the ionosphere, and this is due to the way it's formed. 21 00:01:19,980 --> 00:01:26,280 Our atmosphere consists mainly of nitrogen and oxygen and small traces of other molecules. 22 00:01:26,300 --> 00:01:30,520 When these molecules reach the upper atmosphere they’re at the mercy of the Sun. 23 00:01:30,540 --> 00:01:34,180 Ultraviolet radiation from sunlight excites them -- 24 00:01:34,200 --> 00:01:38,630 they become energized and need to release that extra energy in some way. 25 00:01:38,650 --> 00:01:44,240 Atoms that remain energized long enough can emit that extra energy through light. 26 00:01:44,260 --> 00:01:48,920 In the lower atmosphere we don’t see as much light -- the atmosphere there is dense. 27 00:01:48,940 --> 00:01:53,190 So when an atom becomes energized, there's a high chance it will bump into another atom 28 00:01:53,210 --> 00:01:57,260 and lose energy in that collision instead of emitting light. 29 00:01:57,280 --> 00:02:00,180 But as you travel farther up, the atmosphere thins out. 30 00:02:00,200 --> 00:02:04,120 And like a game of dodgeball, the longer atoms stay untouched, 31 00:02:04,140 --> 00:02:08,510 the more time they have to emit a bright, colorful photon for us to see. 32 00:02:08,530 --> 00:02:12,020 That’s why airglow is only seen in the upper atmosphere. 33 00:02:12,040 --> 00:02:16,310 But it can get even more complicated. Some collisions can produce light too. 34 00:02:16,330 --> 00:02:19,690 On the nightside of Earth, green light is the brightest 35 00:02:19,710 --> 00:02:25,190 and occurs when oxygen atoms become excited through collisions with oxygen molecules. 36 00:02:25,210 --> 00:02:34,040 A variety of other complex reactions create red, and blue light, as well as UV and infrared light that are invisible to the human eye. 37 00:02:34,060 --> 00:02:40,570 Each type of airglow contains information about the composition, temperature, and density of the upper atmosphere 38 00:02:40,590 --> 00:02:44,910 — all of which are key factors that can change dramatically and rapidly. 39 00:02:44,930 --> 00:02:48,180 So airglow turns out to be a fantastic proxy 40 00:02:48,200 --> 00:02:51,800 illustrating not only how particles move through the ionosphere, 41 00:02:51,820 --> 00:02:55,050 but what kinds of particles even exist there, 42 00:02:55,070 --> 00:03:00,230 which is key information for helping us tease out how space and Earth's weather interconnect. 43 00:03:00,250 --> 00:03:14,455 And that’s a great reason for NASA to study this beautiful phenomenon.