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

2. 2 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 Discovery of TGFs by CGRO

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

4. 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. 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. 6 Avalanche runaway e-e- Electric field greater than threshold Air molecule “seed” electron

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

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

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

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

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

12. 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. 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 Air molecule e-e- e-e- Earth’s magnetic field

14. 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. 15 Studying TGFs with ELF/VLF Peak-VLFELF tail  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

16. 16 TGF from BATSE and VLF

17. TGF from RHESSI and VLF

18. 18 TGF-Lightning Timing

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

20. Multi-burst TGFs CGRO/BATSE RHESSI

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

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

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

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

25. 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. 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. 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?