Terrestrial gamma-ray flash

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Artist's impression: Gamma-ray flash and related phenomena.

Terrestrial bursts ( English terrestrial gamma-ray flash , TGF ) are bursts of high energy electromagnetic radiation ( gamma radiation ) in the atmosphere, in contrast to other gamma-ray bursts . TGFs were recorded with a duration of 0.2 to 3.5  ms and energies of up to 20  MeV . It is believed that they are caused by electric fields at the top of thunderclouds .

discovery

Terrestrial gamma-ray bursts were first discovered in 1994 by the Compton Gamma Ray Observatory's BATSE ( Burst and Transient Source Experiment ) , a NASA space probe.

Another study at Stanford University in 1996 was able to assign a TGF to an individual lightning strike that occurred simultaneously with the TGF within a few milliseconds. BATSE was only able to register a small number of TGF events in nine years because it was originally designed for research into extraterrestrial gamma-ray bursts of longer duration.

The newer RHESSI satellite has observed TGFs with much higher energies than those registered by BATSE. In addition, new observations show that around fifty TGFs occur every day, more than previously thought, but only a very small fraction of the total thunderstorm lightning that occurs on earth (3-4 million lightning bolts on average per day). However, the number can be much higher if the gamma-ray flashes are emitted in a narrow cone of radiation and are therefore difficult to see, or if a large number of TGFs arise at low altitudes so that the gamma-rays are absorbed by the atmosphere before they reach the satellite.

Emergence

According to the prevailing assumption, TGFs arise from the fact that electrons at relativistic speeds (speeds close to the speed of light ) hit atomic nuclei in the air and thereby release energy in the form of bremsstrahlung . Sometimes this also releases more electrons with relativistic energies from the atoms, so that an avalanche of fast electrons forms, a phenomenon known as the "relativistic runaway breakdown ". A process in which both electrons and bremsstrahlung photons are released is electron-electron bremsstrahlung: This increases the number of high-energy electrons that can then subsequently generate high-energy photons. The electrons are likely to be accelerated by a strong electric field , but from here on there is considerable uncertainty. The discharge is presumably considerably intensified by positrons , which are generated by gamma quanta through pair formation . Due to their charge, they move in the opposite direction to the electrons and, when they collide with air molecules, release additional electrons, which in turn are accelerated again. A model that takes these positrons into account predicts the duration, intensity and energy spectrum of the gamma rays, which match observations from the satellites.

Some of the standard explanations are borrowed from other discharge phenomena associated with thunderstorm lightning, the goblins discovered a few years before the TGFs. For example, the field could be caused by charge separation in a thundercloud (DC field), as is often associated with the Kobold phenomena. Another explanation would be the electromagnetic pulse (EMP) associated with a lightning strike , which also often occurs with discharges in the high atmosphere. There is also some evidence that TGFs occur in the absence of lightning strikes, albeit in the vicinity of general lightning activity, such as blue jets . However, most TGFs were detected within a few milliseconds before or after a lightning event.

The DC field model requires a very large charge of the thundercloud at high altitudes (around 50–90 km where goblin phenomena form). In contrast to goblin apparitions, such large charges apparently cannot be associated with lightning bolts that generate TGFs. Therefore, the DC field model requires that the TGFs be generated at a lower altitude, at the top of the storm cloud (10-20 km), where stronger local fields can occur. This hypothesis is supported by two independent observations. First, the spectrum of radiation registered by RHESSI fits very well with the forecast of runaway breakdowns at an altitude of 15–20 km. Second, TGFs are highly concentrated around the equator and over the water compared to the totality of lightning . Storm clouds are higher near the equator. This means that the gamma radiation created by TGFs in the upper part of the cloud has a better chance of escaping through the atmosphere. The conclusion would then be that there are many TGFs, especially at higher latitudes, which cannot be seen from space because of the low altitude of their formation.

The EMP model requires less energy for the TGFs, since the gamma rays are generated in the high atmosphere, so that all the resulting gamma ray flashes can also be seen from space. This model has so far been insufficiently confirmed by observations. The requirements for an electromagnetic pulse with the required properties are quite strict.

With some likelihood, several mechanisms are also involved in the generation of the TGFs.

Possible triggering by fast particles

It has been suggested that TGFs are by-products of rays of highly relativistic particles that escape the atmosphere, propagate along magnetic field lines, and re-enter on the opposite hemisphere. In some cases, TGFs registered by both RHESSI and BATSE exhibit unusual patterns that appear to support this explanation. However, these cases contradict the majority of statistical data on TGF events, so that these types of TGFs probably represent only a fraction, if at all, of the total events.

On December 14, 2010, the Fermi satellite observed a TGF with the number TGF 091214 over the Egyptian Sahara , in the vicinity of which there were no thunderstorms. The associated thunderstorm event took place 4000 km away, in Zambia . The particles that triggered the TGF had moved along a magnetic field line. When examining the energy distribution, an accumulation at 511 keV was also discovered, which is viewed as a trace of electron-positron annihilations . This supports the assumption that antimatter can also form in earthly lightning bolts .

According to calculations, TGF can not only release positrons, but also fast neutrons and protons. Neutrons have already been measured in discharges, but so far (2016) there is no experimental confirmation for protons. These gamma-ray bursts can generate secondary particles such as electrons , positrons , neutrons and protons with energies of up to 50 MeV.

literature

  • CP Barrington-Leigh: Terrestrial Gamma-ray Flashes After CGRO: Prospects From HESSI . In: AGU Fall Meeting Abstracts . tape 31 , November 1, 2001, pp. 60 ( PDF [accessed December 30, 2010] conference poster).
  • US Inan, MB Cohen, RK Said, DM Smith, LI Lopez: Terrestrial gamma ray flashes and lightning discharges . In: Geophysical Research Letters . tape 33 , August 19, 2006, p. 5 pp ., Doi : 10.1029 / 2006GL027085 .
  • JR Dwyer, DM Smith, MA Uman, Z. Saleh, B. Grefenstette, B. Hazelton, HK Rassoul: Estimation of the fluence of high-energy electron bursts produced by thunderclouds and the resulting radiation doses received in aircraft . In: Journal of Geophysical Research . tape 115 , April 15, 2010, p. 10 pp ., Doi : 10.1029 / 2009JD012039 .

Web links

Commons : Terrestrial gamma-ray bursts  - collection of images, videos and audio files

Individual evidence

  1. ↑ GJ Fishman, PN Bhat, R. Mallozzi, JM Horack, T. Koshut, C. Kouveliotou, GN Pendleton, CA Meegan, RB Wilson, WS Paciesas, SJ Goodman, HJ Christian: Discovery of Intense Gamma-Ray Flashes of Atmospheric Origin . In: Science . tape 264 , no. 5163 , 1994, pp. 1313-1316 , doi : 10.1126 / science.264.5163.1313 .
  2. ↑ David M. Smith, Liliana I. Lopez, RP Lin, Christopher P. Barrington-Leigh: Terrestrial gamma-ray flashes observed up to 20 MeV . In: Science . tape 307 , no. 5712 , 2005, p. 1085-1088 , doi : 10.1126 / science.1107466 .
  3. ↑ Koehn, C., Ebert, U .: Angular distribution of Bremsstrahl photons and of positrons for calculations of terrestrial gamma-ray flashes and positron beams, Atmos. Res. (2014), vol. 135-136, pp. 432-465 ( preprint online )
  4. ↑ AV Gurevich , GM Milikh, R. Roussel-Dupre: Runaway electron mechanism of air breakdown and preconditioning during a thunderstorm . In: Physics Letters A . tape  165 , no. 5-6 , May 1, 1992, pp. 463-468 , doi : 10.1016 / 0375-9601 (92) 90348-P .
  5. ↑ C. Koehn and U. Ebert: The importance of electron-electron Bremsstrahlung for terrestrial gamma-ray flashes, electron beams and electron-positron beams J. Phys. D .: Appl. Phys. as Fast Track Communication (2014), vol. 47, 252001 ( abstract )
  6. ↑ Joseph R. Dwyer, David M. Smith, Gamma-ray bursts from the clouds. In: Spektrum.de. Spektrum der Wissenschaft Verlagsgesellschaft mbH, December 13, 2012, accessed on April 11, 2013 .
  7. ↑ Inan et al. 1996
  8. ↑ a b Steven A. Cummer, Yuhu Zhai, Wenyi Hu, David M. Smith, Liliana I. Lopez, Mark A. Stanley: Measurements and implications of the relationship between lightning and terrestrial gamma ray flashes . In: Geophysical Research Letters . tape 32 , March 30, 2005, p. 5 pp ., Doi : 10.1029 / 2005GL022778 .
  9. ↑ a b US Inan, NG Lehtinen: Production of terrestrial gamma-ray flashes by an electromagnetic pulse from a lightning return stroke . In: Geophysical Research Letters . tape 32 , September 15, 2005, p. 5 pp ., Doi : 10.1029 / 2005GL023702 .
  10. ↑ MB Cohen, US Inan, G. Fishman: Terrestrial gamma ray flashes observed aboard the Compton Gamma Ray Observatory / Burst and Transient Source Experiment and ELF / VLF radio atmospherics . In: Journal of Geophysical Research . tape 111 , November 21, 2006, p. 11 pp ., Doi : 10.1029 / 2005JD006987 .
  11. ↑ JR Dwyer, DM Smith: A comparison between Monte Carlo simulations of runaway breakdown and terrestrial gamma-ray flash observations . In: Geophysical Research Letters . tape 32 , October 19, 2005, p. 4 pp ., Doi : 10.1029 / 2005GL023848 .
  12. ↑ E. Williams, R. Boldi, J. Bór, G. Sátori, C. Price, E. Greenberg, Y. Takahashi, K. Yamamoto, Y. Matsudo, Y. Hobara, M. Hayakawa, T. Chronis, E. Anagnostou, DM Smith, L. Lopez: Lightning flashes conducive to the production and escape of gamma radiation to space . In: Journal of Geophysical Research . tape 111 , July 29, 2006, p. 7 pp ., Doi : 10.1029 / 2005JD006447 .
  13. ↑ Jan Hattenbach: Lightning, thunder, antiparticles. In: Spectrum of Science. Edition 06/2011. Spektrumverlag, Heidelberg. ISSN  0170-2971
  14. ↑ Köhn, C., Ebert, U .: Calculation of beams of positrons, neutrons and protons associated with terrestrial gamma-ray flashes. J. Geophys. Res. Atmos. (2015), vol. 23, doi: 10.1002 / 2014JD022229
  15. ↑ a b C. Köhn, G. Diniz, Muhsin Harakeh: Production mechanisms of leptons, photons, and hadrons and their possible feedback close to lightning leaders . In: J. Geophys. Res. Atmos. . 122, 2017. doi : 10.1002 / 2016JD025445 .
  16. ↑ Agafonov, AV, AV Bagulya, OD Dalkarov, MA Negodaev, AV Oginov, AS Rusetskiy, VA Ryabov, and KV Shpakov (2013), Observation of neutron bursts produced by laboratory high-voltage atmospheric discharge, Phys. Rev. Lett., 111, 115003
  17. ↑ Köhn, C., Ebert, U .: Calculation of beams of positrons, neutrons and protons associated with terrestrial gamma-ray flashes. J. Geophys. Res. Atmos. (2015), vol. 23, doi: 10.1002 / 2014JD022229