Transcript Document

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
CGRO locations usually over thunderstorms
Only 75 events observed over 9 years
Short ~1 ms spike
in gamma-rays!
Compton Gammaray Observatory
2
Mesospheric Phenomena
 TGFs associated with lightning
 Observed by gamma-ray detector on spacecraft
3
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??
4
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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Avalanche runaway
Electric
field
greater
than
threshold
Air
molecule
Air
molecule
Air
molecule
e-
“seed” electron
6
Avalanche runaway
Electric
field
greater
than
threshold
Air
molecule
Air
molecule
Air
molecule
Acceleration and collision
e-
“seed” electron
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Avalanche runaway
Electric
field
greater
than
threshold
Air
molecule
Air
molecule
e-
e-
Air
molecule
More electrons freed
8
Avalanche runaway
Electric
field
greater
than
threshold
Air
molecule
Air
molecule
e-
e-
Air
molecule
Acceleration and
collision again
More electrons freed
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Avalanche runaway
e-
e-
Electric
field
greater
than
threshold
e-
eAir
molecule
Air
molecule
Avalanche
continues
Air
molecule
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Bremsstrahlung
Emitted
gamma-ray
Air
molecule
e-
 Relativistic (i.e. close to
speed of light) electron
collides with air
molecule
 Electron may lose
energy, gamma-ray is
emitted
e11
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
Air
molecule
Emitted
gamma-rays
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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-
e13
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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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
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TGF from BATSE and VLF
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TGF from RHESSI and VLF
TGF-Lightning Timing
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Many types of sferic waveform
Streak of
smaller,
shorter
sferics
Single
strong,
longer
sferic
Multi-burst TGFs
RHESSI
CGRO/BATSE
Lightning characteristics
+ ++ + + +
+ + + ++
+
+ + +
+
+
+
- - - - - - -
-
+ ++ + + +
+ ++ ++ +
+
+ +
-
+
-
Return stroke
peak current
(i.e., kA)
+ ++ + + +
+ +
Total charge moment
(I.e., C•km)
+ +
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Peak Current
+ ++ + + +
+ + + ++
+
+ + +
+
+
+
- - - - - - - - -
 Peak current is
proportional to VLF
peak for a given
propagation path
Return stroke
peak current
(i.e., kA)
+ ++ + + +
+ +
VLF
Peak
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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]
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Regional division
Most TGFs (94%) of TGFs have associated sferics
detectable at Palmer Station, Antarctica
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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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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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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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