Midsommernatten giver elverlys over La Silla. Det sjældne fænomen Sprites fanget med kameraet.
Det danske 1,54m teleskop på La Silla Observatoriet danner forgrund for vidt forskellige himmellys
Der er mange lysfænomener på en skyfri himmel over ESOs La Silla observatorium - både sjældne og smukke. Der er rød og grøn airglow på en baggrund af stjernehimlens egne smukke lys. Her iblandt er både den store og den lille Magellanske Sky, lige til højre for midten, og der er det rosenrøde skær fra områder med stjernedannelse i Mælkevejen. Oven i det hele er der så også en grøn stribe fra et stjerneskud lige til venstre for Mælkevejen, og på de store udgaver af billedet er der flere stjerneskud at finde - gå selv på jagt. Skråt op mod venstre lige til venstre for Mælkevejen ser man zodiakallyset - et sikkert tegn på, at morgenen ikke er så fjern. Kuppelbygningen i forgrunden huser det danske 1,54m teleskop. De tre gulgrønne områder i horisonten til højre for masten er lysskæret fra byer mere end 100 km væk. Uanset om det er generende eller smukke fænomener, er de dog nok så velkendte. Men der er også noget på færde, som er meget tættere på, og meget sjældnere.
De seks små billeder under det store viser en serie med det meget sjældne atmosfæriske fænomen sprites. Nogle få timer før morgengryet dukkede et kraftigt tordenvejr op den nat, og selvom det holdt sig under horisonten i det fjerne, bevirkede det, at disse flygtige lysskær kunne ses og fotograferes.
Sprites har deres navn efter Shakespeares drilagtige alfer, Puck i "En Skærsommernatsdrøm" og Ariel i "Stormen". De himmelske sprites forårsages af uregelmæssigheder i ionosphæren i højder omkring 80 kilometer over tordenskyer. Typisk ses de som bundter af rødorange glimt. De udløses af positive sky-til-jord-lyn, som er sjældnere og kraftigere end deres negative modparter. Det er de fordi positive lyn udsendes fra skyernes øverste dele, som er længere fra jordoverfladen. Sprites er meget kortvarige. De udbreder sig lynhurtigt (!) nedad, og på vej ned skabes disse røde fangarme i korte øjeblikke.
De viste sprites var synlige i en periode på 40 minutter, og de var sandsynligvis over 500 kilometer væk. Dette satellitbillede viser den pågældende storm over Argentina. Det er normalt ekstremt svært af fange disse lysfænomener, med mindre man har specielle ultralysfølsomme højhastighedskameraer. For ESOs fotoambassadør Petr Horálek var det dog blot stor tålmodighed, som til sidst gav disse fantastiske billeder.
Petr tog også et andet smukt spritebillede på La Silla, og få dage senere var han på Paranal, hvor han fangede dette formidable skud fra VLTplatformen. Den storm kan ses på satellitbilledet her.
Links:
- Red Sprites fra La Silla Observatoriet
- Sprites fra Paranal Observatoriet
- Flere sprites fra La Silla Observatoriet
P. Horálek/ESO
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