Terrestrial gamma-ray flashes

Prepared by Morris CohenStanford University, Stanford, CA

IHY Workshop on Advancing VLF through the Global AWESOME

Network

2

CGRO designed to study cosmic gamma-raysA number of “surprise” events coming from EarthCGRO locations usually over thunderstormsOnly 75 events observed over 9 years

Short ~1 ms spike in gamma-rays!

Compton Gamma-ray Observatory

Discovery of TGFs by CGRO

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

e-

Electric field

greater than

threshold Air molecule

Air molecule Air

molecule

“seed” electron

7

Avalanche runaway

e-

Electric field

greater than

threshold Air molecule

Air molecule Air

molecule

“seed” electron

Acceleration and collision

8

Avalanche runaway

Electric field

greater than

threshold Air molecule

Air molecule Air

molecule

e-e-

More electrons freed

9

Avalanche runaway

Electric field

greater than

threshold Air molecule

Air molecule Air

molecule

e-e-

More electrons freed

Acceleration and collision again

10

Avalanche runaway

Electric field

greater than

threshold Air molecule

Air molecule Air

molecule

e- e-

e- e-

Avalanche continues

11

Bremsstrahlung

Air molecule

e-

Relativistic (i.e. close to speed of light) electron collides with air molecule

Electron may lose energy, gamma-ray is emitted

e-

Emitted gamma-ray

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

Air molecule

e- e-

Earth’s magnetic field

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

Peak-VLF ELF 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

TGF from BATSE and VLF

TGF from RHESSI and VLF

18

TGF-Lightning Timing

Many types of sferic waveform

Streak of smaller, shorter sferics

Single strong, longer sferic

Multi-burst TGFs

CGRO/BATSERHESSI

21

Lightning characteristics

+ + ++ +

++ ++

++ ++ + +

+++

- -- - -- -- ----

-

+ +++++ +

+

Return stroke peak current (i.e., kA)

+ + ++ +

+ ++

++ ++ + +

++

----

-

+ +

Total charge moment (I.e., C•km)

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

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., C•km)

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?