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Terrestrial gamma-ray flashes Prepared by Morris Cohen Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network
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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
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3 Mesospheric Phenomena TGFs associated with lightning Observed by gamma-ray detector on spacecraft
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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??
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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
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6 Avalanche runaway e-e- Electric field greater than threshold Air molecule “seed” electron
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7 Avalanche runaway e-e- Electric field greater than threshold Air molecule “seed” electron Acceleration and collision
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8 Avalanche runaway Electric field greater than threshold Air molecule e-e- e-e- More electrons freed
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9 Avalanche runaway Electric field greater than threshold Air molecule e-e- e-e- More electrons freed Acceleration and collision again
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10 Avalanche runaway Electric field greater than threshold Air molecule e-e- e-e- e-e- e-e- Avalanche continues
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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
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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
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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
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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?
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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
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16 TGF from BATSE and VLF
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TGF from RHESSI and VLF
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18 TGF-Lightning Timing
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Many types of sferic waveform Streak of smaller, shorter sferics Single strong, longer sferic
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Multi-burst TGFs CGRO/BATSE RHESSI
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21 Lightning characteristics + + + + + + + + + + + + + + + + + + - - - - - - - - - - -- - + + + + + + + + Return stroke peak current (i.e., kA) + + + + + + + + + + + + + + ++ - - - - - + + Total charge moment (I.e., Ckm)
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22 Peak Current + + + + + + + + + + + + + + + + + + - - - - - - - - - - -- - + + + + + + + + Return stroke peak current (i.e., kA) Peak current is proportional to VLF peak for a given propagation path VLF Peak
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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]
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24 Regional division Most TGFs (94%) of TGFs have associated sferics detectable at Palmer Station, Antarctica
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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
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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)
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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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