WEBVTT FILE 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.