Lyn

For alternative betydninger, se Lyn (flertydig). (Se også artikler, som begynder med Lyn)

Torden En kort optagelse af et tordenbrag'' Er der problemer med lyden? Se da eventuelt Hjælp:Ogg Vorbis eller "Media help" (engelsk)

Et lyn er en stor elektrisk udladning mellem sky og jord eller hav, eller mellem sky og sky. Et lyn er en stor gnist i jordens atmosfæres luft. Lyn løber gennem luften ved at en elektrisk spændingsforskel omdanner luftens molekyler til ioner. Derved dannes der en kanal som lynet kan løbe igennem. Mens lynet løber igennem kanalen omdannes molekylerne til plasma, og der er typisk omkring 15.000 °C og helt op til 30.000 °C i lynkanalen mens lynets elektricitet løber igennem, og det er faktisk op til 5 gange så høje temperaturer, som der findes på Solens overflade.^[1][2]

Selve lynkanalen er fra 2–15 cm tyk og holdes på det lille tværsnit grundet det elektromagnetiske knibningsfænomen. Hele lynforløbet af et typisk lyn varer i ca. 1/4 sekund og består af tre til fire hovedudladninger med ca. 0,04 sekunders tidsinterval. Det typiske lyn overfører kortvarigt en strøm på ca. 10.000 ampere og en elektrisk ladning på i alt ca. 25 coulomb. Strømstyrken varierer, og der er målt lyn med strømstyrke op til 200.000 ampere på det tidspunkt, hvor lynet springer.^[2]

For at lynet kan springe, skal der være en spændingsforskel på ca. 200-1000 volt per millimeter højde. Et typisk lyn kan således være baseret på en spændingsforskel på op til flere hundrede millioner volt. Man har i dag ingen forklaring hvordan lyn starter - og videnskaben kan endnu ikke forklare, hvordan den store ladning som afledes via lynet, blev dannet og hvor den var.^[3][1][4][5] Nogle lyn udsender store mængder gammastråling og/eller røntgenstråling.^{[6][7][8][9][10]} Nogle tordenskyers lyn (lyn i den øvre atmosfære) sender elektroner og overraskende nok positroner ud i rummet.^[11] Lyn i den øvre atmosfære laver også røde feer og mange andre fænomener. Lyn laver også radioaktive isotoper som fx kulstof-13, kulstof-14 og nitrogen-15.^[12][13]

Både lyn og mindre udladninger i jordens atmosfære danner stoffer, fx OH og HO2, som renser luften.^[14]

1. ^ ^{a b} maj 2013, dmi.dk: Lyn og torden Citat: "...Endnu i dag har videnskaben svært ved at give en entydig forklaring på, hvordan lyn opstår...Derved når temperaturen i selve lynet typisk op omkring 30.000oC, altså ca. fem gange varmere end Solens overflade..."
2. ^ ^{a b} Jim Lux: Lightning Facts Citat: "...A typical lightning flash lasts about a quarter of a second and consists of 3 or 4 individual discharges called strokes. Each stroke lasts a few ten thousandths of a second, although the visual appearance is longer. The "flicker" sometimes observed in lightning is due to seeing the actual strokes making up the flash...", backup
3. ^ 2003-11-06, ScienceDaily: Thunderstorm Research Shocks Conventional Theories; Florida Tech Physicist Throws Open Debate On Lightning's Cause Citat: "...scientists have searched inside thunderstorms for many years, looking for these large electric fields, only to come up empty handed..."Although everyone is familiar with lightning, we still don't know much about how it really works," said Dwyer..."
4. ^ May 7, 2013, physicsworld.com: New insights into what triggers lightning Citat: "...Although most people have witnessed a flash of lightning during a thunderstorm at some point in their lives, scientists still do not completely understand what triggers the discharge in the first place...Physicist and lightning expert Joseph Dwyer of the Florida Institute of Technology, who was not involved in the current research, says that the new model is “an interesting idea, but much more work is still needed, for example experiments to measure radio pulses and air showers at the same time”, which is something that Dwyer and his colleagues are currently working on themselves..."
5. ^ 21. okt 2012, ing.dk: Hvorfra kommer gnisten til et lyn? Citat: "...Den hidtil mest plausible teori for, hvordan lyn opstår, er blevet undersøgt i detaljer og fundet problematisk. Der ser ud til at være brug for kosmisk baggrundsstråling for at få et lyn til at starte...Kosmiske bombardementer og den resulterende lavine af højenergi-elektroner er simpelthen ikke nok til at starte et lyn. Hvis man derimod medregner effekten fra den langsomme baggrundsstråling, kan det til gengæld godt lykkes. Og det er noget nyt..."
6. ^ 31 January, 2003, BBC News: Lightning's X-ray zap Citat: "...Lightning does emit X-rays, scientists have confirmed..."
7. ^ 4. jan 2011, ing.dk: Tordenskyer kan give flypassagerer voldsom gammastråling Citat: "... Tordenskyer kan i visse tilfælde udsende meget intens, kortvarig gammastråling, og nu har en amerikansk forsker beregnet, at strålingen på mindre end et milisekund kan give flypassagerer samme dosis, som er maksimum for et helt liv...Piloter vil normalt gøre alt for at flyve uden om tordenskyer, så risikoen for at blive udsat for en sådan stråling er meget lille..."
8. ^ Florida Institute of Technology (2013, April 10). Dark lightning: Are airplane passengers exposed to radiation from intense bursts of gamma-rays from thunderclouds?. ScienceDaily Citat: "...According to their model, instead of creating normal lightning, thunderstorms can sometimes produce an exotic kind of electrical breakdown that involves high-energy electrons and their anti-matter equivalent called positrons. The interplay between the electrons and positrons causes an explosive growth in the number of these high-energy particles, emitting the observed terrestrial gamma ray flashes while rapidly discharging the thundercloud, sometimes even faster than normal lightning. Even though copious gamma-rays are emitted by this process, very little visible light is produced, creating a kind of electrical breakdown within the storms called "dark lightning."..."
9. ^ Jul 10, 2013, physicsworld.com: Japanese team sees gamma-ray pulse before lightning flash Citat: "...While all of these bursts are thought to be created when charged particles are accelerated by the huge electric fields that build up in a thundercloud, the exact mechanism – or mechanisms – that produce them remains a mystery...Unless it was an incredible coincidence that the burst ended a second before a flash of lightning occurred, it appears that the two events are linked...As the researchers used several detectors, they were also able to work out where the gamma rays were being produced, finding that photons with energies greater than 10 MeV were created in a region stretching across about 180 m in the thundercloud. This suggests that the gamma rays are produced in a relatively small section of the much larger cloud. Furthermore, the 800 ms delay between the end of the gamma-ray pulse and the lightning flash suggests that the lightning is initiated some distance away from the acceleration – although the process that connects the two is still unknown..."
10. ^ University of Utah. (2018, May 17). Breakthrough in understanding rare lightning-triggered gamma-rays. ScienceDaily Citat: "...The Telescope Array detected 10 bursts of downward terrestrial gamma-ray flashes (TGFs) between 2014 and 2016, more events than have been observed in rest of the world combined. They are the first to detect downward TGFs at the beginning of cloud-to-ground lightning, and to show where they originated inside thunderstorms..."The mechanism that produces the gamma rays has yet to be figured out," added Krehbiel...The researchers have many questions left unanswered. For example, not all lightning strikes create the [TGF] flashes. Is that because only one particular type of lightning initiation produces them? Are the scientists only seeing a subset of TGFs that happen to be large enough, or point in the right direction, to be detected?..."
11. ^ 11 January 2011, bbc.co.uk: Antimatter caught streaming from thunderstorms on Earth Citat: "...But results from the Fermi telescope show they also give out streams of electrons and their antimatter counterparts, positrons...These flashes are intense - for a thousandth of a second, they can produce as many charged particles from one flash as are passing through the entire Earth's atmosphere from all other processes..."It has some very important implications for our understanding of lightning itself. We don't really understand a lot of the detail about how lightning works..."
12. ^ 22. nov 2017, ing.dk: Radioaktive isotoper dannes ved lynnedslag Citat: "...Japanske forskere har observeret den første sikre dannelse af radioaktive isotoper ved lynnedslag og dermed opdaget en naturlig måde til dannelse af isotoper som kulstof-13, kulstof-14 og nitrogen-15..."
13. ^ November 24, 2017, scitechdaily.com: How Gamma-Rays from Lightning Produce Radioisotopes and Positrons Citat: "...In a collaborative study appearing in Nature, researchers from Japan describe how gamma rays from lightning react with the air to produce radioisotopes and even positrons — the antimatter equivalent of electrons. “We already knew that thunderclouds and lightning emit gamma rays, and hypothesized that they would react in some way with the nuclei of environmental elements in the atmosphere,” explains Teruaki Enoto from Kyoto University, who leads the project...When they analyzed the data, the scientists found three distinct gamma-ray bursts. The first was less than one millisecond in duration; the second was a gamma-ray afterglow that decayed over several dozens of milliseconds; and finally there was a prolonged emission lasting about one minute..."
14. ^ Penn State. (2021, April 29). Lightning and subvisible discharges produce molecules that clean the atmosphere. ScienceDaily Citat: "...Now, a team of atmospheric chemists and lightning scientists have found that lightning bolts and, surprisingly, subvisible discharges that cannot be seen by cameras or the naked eye produce extreme amounts of the hydroxyl radical -- OH -- and hydroperoxyl radical -- HO2..."