Astronomical News

Light Phenomena over the ESO Observatories II: Red Sprites

Petr Horálek1 Lars Lindberg Christensen1 József Bór2 Martin Setvák 3 Petr Horálek/ESO

1 ESO 2 Research Centre of Astronomy and Earth Sciences, GGI, Hungarian Acad­ emy of Sciences, Budapest, Hungary 3 Czech Hydrometeorological Institute, Prague, Czech Republic

A rare atmospheric phenomenon, known as red sprites, was observed and captured on camera from the La Silla Observatory. This event sig- nalled the first time that these extremely short-lived flashes of red light, origi­ Figure 1. (Above) Red sprites caught on extremely nating in the Earth’s upper atmosphere, deep digital­ camera images from the La Silla Obser­ vatory on 20 January 2015. The time of each expo­ were photographed from a major astro- sure is indicated. nomical observatory. Further images of red sprites from the La Silla Paranal Figure 2. This image Observatory sites are presented and from the EUMETSAT’s ­Meteosat-10 (Meteosat the nature of red sprites is discussed. Second Generation, MSG-3) satellite was taken on 20 January On 20 January 2015, ESO Photo Ambas­ 2015 at 7:30 UTC. The storm cell above sador Petr Horálek was photographing which the sprites were the Milky Way at the La Silla Observatory, observed (Figure 1) is when a series of short-lived flashes of marked. red light appeared above the horizon and caught his eye. Using a digital camera, adapted for better near-infrared perfor­ EUMETSAT; Martin Setvák/CHMI; Petr Horálek/ESO mance by removing the infrared blocking filter, a series of features known as red sprites was successfully photographed (see Figure 1). Red sprites have not been extensively imaged, and the image in ­Figure 1 marked the first time that they had been captured from a major astro­ A week later at Paranal, just a few hours and is located about 550 kilometres from nomical observatory. before daybreak on 27 January 2015, the Paranal Observatory. the first author once again spotted the Subsequent investigation revealed the unmistakable tendrils of red sprites, and The storms were so strong that another origin of the sprites to be a cluster of this time photographed several over display of their activity was observed and massive thunderstorms over northern an almost two-hour period (see Figure 3). documented. High in the atmosphere, Argentina on the eastern foothills of the From the camera’s perspective, they gravity waves generated by these storms Andes. Figure 2 shows the Meteosat-10 appeared to come from the direction of (Siefring et al., 2010) formed ripples in the satellite image of this thunderstorm; the rising Galactic Bulge of the Milky Way. greenish layer of airglow about four hours the storm’s activity reached its maximum In reality they originated from another before the red sprites appeared (Figure 5). at the time this image was recorded. huge complex of storms over the Andes The red sprites were photographed from at approximately the same location in La Silla around the same time. The core Argentina, this time about 620 kilometres The first observations of red sprites of the thunderstorm is marked in Figure 2 away. Figure 4 shows the Meteosat-10 and was located about 560 kilometres image of this storm. The most spectacular The existence of transient luminous from La Silla. red sprites were photographed from events, or TLEs, was predicted by the ­Paranal from 8:30 to 9:10 UTC. The core Scottish physicist Charles Wilson (1869– of the thunderstorm is marked in Figure 4 1959) in 1920. He was the inventor of the

The Messenger 163 – March 2016 43 Astronomical News Horálek P. et al., Light Phenomena over the ESO Observatories I: Red Sprites P. Horálek/ESO P. EUMETSAT; Martin Setvák/CHMI; Petr Horálek/ESO

Figure 3. The unmistakable tendrils of multiple red auroral cameras in Minnesota, USA (Franz Figure 4. Image taken by the Meteosat-10 satellite sprites imaged by ESO Photo Ambassador Petr et al., 1990), and interest has increased on 27 January 2015 at 8:45 UTC as the thunder­ Horálek from ESO’s Paranal Observatory on storm’s activity decreased. The weakening storm 27 January 2015. Two exposures were combined, following imaging from the NASA Space cell, above which the sprites in Figure 3 were with the upper sprite occurring at 7:46:43 UTC Shuttle and the International Space Sta­ observed, is indicated. and the lower at 8:07:25 UTC. In the foreground is tion (ISS); see Jehl et al. (2013) and Yair one of the 1.8 metre VLT Auxiliary Telescopes. et al. (2013). Astronauts on the ISS have a particularly good vantage point and have storms — another consequence of tropo­ Wilson cloud chamber for the detection recorded red sprites with digital cameras spheric lightning activity (Fishman et al., of elementary particles, and he was (Figure 6). The appearance of red sprites 1994). awarded the Nobel Prize in 1927 for his shortly follows­ the occurrence of the cor­ development of a method of visualising responding lightning stroke; both kinds electrical particle tracks in condensed of flashes are often captured on the same What causes red sprites to appear? vapours. Unfortunately, Wilson did not image that is taken from space. live to witness the experimental confirma­ In thunderstorms, most cloud-to-ground tion of his theory. A few years after the discovery of red discharges are called negative lightning, sprites, the NASA Compton Gamma as they transfer negative charge to The interest in TLEs has grown since Ray Observatory managed­ to observe the ground. However, up to 5% of all dis­ 1989, when they were first detected with gamma bursts ­originating above thunder­ charges are positive cloud-to-ground Petr Horálek/ESO

Figure 5. Gravity waves in the airglow above Paranal, just a few hours before the red sprites shown in Figure­ 3 were photographed over the distant storm.

44 The Messenger 163 – March 2016 ISS/NASA Luis Calçada/ESO

Figure 6. Red sprite photographed from the Interna­ Figure 7. The family of TLE phenomena is depicted. tional Space Station above Southeast Asia in April Adapted from an illustration by Frankie Lucena. 2012. lightning that transfer positive charge has been speculated that similar phe­ Red sprites are not yet completely under­ from the thundercloud to the ground. Up nomena could occur on other planets in stood and any new image showing them to ten times more energetic than negative the Solar System (Yair et al., 2009). is valuable to atmospheric scientists lightning, positive lightning seems to be studying these phenomena. Now ESO what makes the Earth’s atmosphere pro­ Red sprites are the most frequently photo­ has been able to contribute to this intrigu­ duce red sprites and some of the other graphed type of upper-atmospheric light­ ing puzzle of the Earth’s atmosphere. TLE phenomena, commonly known as ning phenomena on Earth. In photo­ upper-atmospheric lightning (­Boccippio graphs, they appear rather dim and the et al., 1995). exact time and location of their appear­ Acknowledgements ance in the sky is unpredictable. They The contribution of József Bór was supported by Red sprites are a manifestation of com­ show up for only a fraction of a second, the National Research, Development and Innovation plex high-altitude electrical discharge making them difficult to document and Office, Hungary (NKFIH), K115836, and by the processes (Pasko et al., 2012; Siingh et study. However with a wide-aperture János Bolyai Research Scholarship of the Hungarian al., 2012). They typically show up over lens and a clear view to the horizon in the ­Academy of Sciences. We are grateful to Bob ­Fosbury for comments. large and powerful thunderstorms that direction of a powerful thunderstorm, are strong enough to trigger gravity together with low light pollution, they can waves in the upper atmosphere. These be imaged with commercial cameras. References unusual flashes are formed at altitudes Boccippio, D. J. et al. 1995, Science, 269, 1088 up to 90 kilometres and get their distinct The La Silla and Paranal Observarory Bór, J. 2013, J. Atmospheric & Solar-Terrestrial red hue from the excited nitrogen mole­ sites provide ideal conditions for photo­ Physics, 92, 151 cules in the atmosphere. They show up graphing TLE phenomena. The obser­ Fishman, G. J. et al. 1994, Science, 264, 1313 as figures composed of channels, beads vatories are located at more than 2000 Franz, R. C., Nemzek, R. J. & Winckler, J. R. 1990, Science, 249, 4964, 48 and puffs (Bór, 2013). Red sprites are metres above sea level and the dry and Jehl, A., Farges, T. & Blanc, E. 2013, J. Geophysical caused by electrical discharge fronts that transparent air allows the phenomena Research: Space Phys., 116, 454 move rapidly in the high-altitude electric to be observed at great distances and Marshall, R. A. & Inan, U. S. 2007, Geophysical field generated by tropospheric lightning1. very low above the horizon. During mid- Research Letters, 34, L05806 Pasko, V. P., Yair, Y. & Kuo, C.-L. 2012, Space Sci. summer (January–February), South Rev., 168, 475 There are many different species of America is rich in strong thunderstorm Siefring, C. L. et al. 2010, J. Geophysical Research, TLEs (see Figure 7). The nomenclature systems, especially above Brazil and 115, A00E57 of TLE phenomena is usually based on Argentina. Due to the extremely low levels Siingh, D. et al. 2012, Space Sci. Rev., 169, 73 Yair, Y. et al. 2009, J. Geophysical Research, 114, their shape and colour in the sky: blue of light pollution above the ESO sites and E09002 jets, which occur at heights of 15–45 kilo­ the unimpeded view to the east, there Yair, Y. et al. 2013, J. Atmospheric & Solar-Terrestrial metres above the ground; red sprites is a good chance of capturing such phe­ Physics, 102, 140 at heights 50–85 kilometres; and elves at nomena with digital cameras, despite around 90–100 kilometres altitude (Pasko the very large distances to the thunder­ Links et al., 2012; Marshall & Inan, 2007). The storms. Recording the same red sprites vertically longest TLE phenomenon is the from La Silla and Paranal simultaneously 1 The BBC documentary Exploring the Edge of gigantic jet, which starts from the cloud would enable the determination of their Space features spectacular footage of red sprites: http://www.youtube.com/watch?v=lqeqWOQQe2Y tops and ends around 90 kilometres height and size by triangulation. above the ground (Siingh et al., 2012). It

The Messenger 163 – March 2016 45