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Terrestrial gamma-ray flashes

Terrestrial gamma-ray flashes. Prepared by Morris Cohen Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network. Discovery of TGFs by CGRO. CGRO designed to study cosmic gamma-rays A number of “surprise” events coming from Earth

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Terrestrial gamma-ray flashes

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  1. Terrestrial gamma-ray flashes Prepared by Morris Cohen Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network

  2. Discovery of TGFs by CGRO CGRO designed to study cosmic gamma-rays A number of “surprise” events coming from Earth CGRO locations usually over thunderstorms Only 75 events observed over 9 years Short ~1 ms spike in gamma-rays! Compton Gamma-ray Observatory

  3. Mesospheric Phenomena • TGFs associated with lightning • Observed by gamma-ray detector on spacecraft

  4. TGF Physical Origins • For energetic electrons, frictional losses in air is a function of energy • Frictional losses reach local minimum near 1 MeV • What if an electric field exists that is larger than this frictional force??

  5. Step 1: Avalanche Runaway • TGFs formed via four step process • Acceleration of electrons due to high electric fields from lightning • More electrons freed due to collisions with air molecules • Relativistic electrons collide with molecules, release gamma radiation • Gamma-rays escape atmosphere

  6. Avalanche runaway Air molecule Electric field greater than threshold Air molecule Air molecule e- “seed” electron

  7. Avalanche runaway Air molecule Electric field greater than threshold Air molecule Air molecule Acceleration and collision e- “seed” electron

  8. Avalanche runaway Air molecule Electric field greater than threshold Air molecule e- e- More electrons freed Air molecule

  9. Avalanche runaway Air molecule Electric field greater than threshold Air molecule Acceleration and collision again e- e- Air molecule More electrons freed

  10. Avalanche runaway e- e- e- e- Avalanche continues Air molecule Electric field greater than threshold Air molecule Air molecule

  11. Bremsstrahlung Emitted gamma-ray e- • Relativistic (i.e. close to speed of light) electron collides with air molecule • Electron may lose energy, gamma-ray is emitted Air molecule e-

  12. Escaping the atmosphere: Gamma-rays • Gamma-rays may be emitted in many directions • Gamma-rays continue straight but may also be absorbed by molecules • Gamma-rays generated high enough (where air is thin) escape atmosphere more easily Emitted gamma-rays Air molecule

  13. Escaping the atmosphere: Electrons • Do sufficient number of electrons escape? • Lower in atmosphere: Collisions too frequent, electrons cannot escape • Higher at atmosphere: Electrons confined to motion around magnetic field Earth’s magnetic field Air molecule e- e-

  14. Mechanisms Figure courtesy Brant Carlson • Lightning generates high electric fields in several different ways • Which process is responsible for accelerating electrons to 0.999c?

  15. Studying TGFs with ELF/VLF • Lightning also generates sferic, detectable at long distances • Sferic is also “fingerprint” of lightning • Properties of received sferic may help learn about mechanism of generating TGFs Peak-VLF ELF tail

  16. TGF from BATSE and VLF

  17. TGF from RHESSI and VLF

  18. TGF-Lightning Timing

  19. Many types of sferic waveform Streak of smaller, shorter sferics Single strong, longer sferic

  20. Multi-burst TGFs RHESSI CGRO/BATSE

  21. Lightning characteristics + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - Return stroke peak current (i.e., kA) Total charge moment (I.e., C•km) + + + + + + + + + +

  22. Peak Current + + + + • Peak current is proportional to VLF peak for a given propagation path + + + + + + + + + + + + + + - - - - - - - - - - - - - Return stroke peak current (i.e., kA) + + + + VLF Peak + + + +

  23. Total Charge Moment + + • Total ELF energy is proportional to total charge transfer • ELF energy attenuates more in Earth-ionosphere waveguide + + + + + + + + + + + + + + - - - - - Total charge moment (I.e., C•km) + ELF Energy + Reising [1998]

  24. Regional division Most TGFs (94%) of TGFs have associated sferics detectable at Palmer Station, Antarctica

  25. TGF-associated sferics • TGF-associated sferics have high VLF energy • High peak current lightning • No high ELF energy • Low peak current, unlike sferics that generate sprites

  26. TGFs – Research done to date • TGFs occur within a few ms of lightning • Tend to be higher peak current • Characteristics of lightning do not resemble those of sprites • TGFs likely sourced 15-20 km altitude (above thunderclouds)

  27. TGFs – Open questions • Do electrons escape atmosphere along field lines, precipitate at conjugate region? • What lightning processes are chiefly responsible for TGFs? • How often do TGFs occur? • Do cosmic ray fluxes affect TGFs?

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