June, 2008Corsica 20081 TGF production altitude and time delays of the terrestrial gamma flashes – revisiting the BATSE spectra Nikolai Østgaard, Thomas

June, 2008 Corsica 2008 1

TGF production altitude and time delays of the terrestrial gamma flashes

– revisiting the BATSE spectra

Nikolai Østgaard, Thomas Gjesteland and Johan Stadsnes

University of Bergen , Norway

Paul H. Connell

University of Valencia, Spain

Brant Carlson

STAR lab, Electric Engineering, Stanford University, USA


Features from BATSE measurements

Low energy fall-off and time delays


The Monte Carlo simulations

Included- Photoelectric absorption- Compton scattering and

energy degradation - Pair production- Annihilation of positrons –

peak at 511 keV- Bremsstrahlung by electrons

from pair production


The Monte Carlo simulations

• Input to the simulations– Production altitude

– Energy distribution

– Angular distribution

• For each photon:p(E, z, β, α , s)

• Energy spectra

• Light Curvest = s / c


Building libraries

• 4 libraries:

1) Beamed distribution, Discrete altitude

2) Beamed distribution, Distributed altitude

3) Cone distribution (20°), Discrete altitude

4) Cone distribution (20°), Distributed altitude

• 6 different power-law energy spectra: E, = 1.0, 1.1, .. 1.5 • 7 different altitudes: 10, 15, 20, 30, 40 50 and 60 km

• Sample at 7 different 10° escaping angle-intervals, 0°-9°… 60°-69°

4 x 6 x 7 x 7 = 1176 spectra


Comparison with GEANT

15 km +3%

30 km -2%

40 km +1%

1 million photons

E-1 energy spectra

Beamed

Discrete altitude

Same shape and total photons within 2-3%


25-50 keV

50-100 keV

100-300 keV

>300 keV

1)Low energy cut-off2)Peak around 50 keV3)Bumb at 0.7-1.0 MeV4)High energy cut-off

Modelled spectra


Model characteristics

1) High energy cut-off moves to lower energies as escaping angles increase

2) Low energy cut-off moves to lower energies as TGFs are produced higher

3) Peak moves to lower energies

4) Bump around 0.7-1 MeV or flattening at 500 keV for TGFs produced deep. Disappears higher up

Time delays1) 100 micros delay below 20 km2) Above 40 km – no time delays3) At 0-9 deg – no time delays


Source: Isotropic within 20°

High energy cut-off only appears for escaping angles larger than the initial cone

No time delay are found when measuring inside the production cone but clearly seen

from zenith 40° -49°


Spectrograms - Beamed

Four parameters to determinethe production altitude1) Low energy cut-off2) Peak intensity3) Compton bump4) High energy cut-off


Optimizing procedure4 libraries + BATSE Energy Response Matrix

2 steps Determine a zenith angle range within 5° from BATSE measurements

Optimize the spectral index and production altitude.

χ2 divided by degrees of freedom < 1.5

Only one possible zenith angle

21 of 25 BATSE spectra meet both these criteria


Altitude histograms

Most TGF produced ≤ 20 km

Significant portion is produced at 30 - 40

km altitude


Softening – beamed distribution The spectrum gets softer as the zenith angle increase

.

Indicates that BATSE are measuring outside the production cone.


Conclusions

From the model spectra:

4 features that vary for different production altitudes

(1) A low energy cut-off (2) The intensity peak (3) A bump in the spectra at below 1 MeV(4) A high energy cut-off

Time delay:(1) Time delays can be explained as a Compton effect.


Conclusions

BATSE measurements an MC code:

(1) Half or more of the BATSE TGFs are produced at low altitudes, <20 km.

(2) A significant portion of the BATSE TGFs are produced at higher altitudes, 30 km to 40 km.

(3) For the TGFs produced at <20 km (and some at 30 km) altitudes the dispersion signatures can be explained as a Compton effect.

(4) The softening of the BATSE spectra for increasing zenith angles and the time dispersions both indicate that the initial TGF distribution is beamed.

Østgaard, N., T. Gjesteland, J. Stadsnes, P. H. Connell, and B. Carlson (2008), Production altitude and time delays of the terrestrial gamma flashes: Revisiting the Burst and Transient Source Experiment

spectra, J. Geophys. Res., 113, A02307, doi:10.1029/2007JA012618.


BATSE and deadtime

RHESSI and BATSE input spectra down to ISS and what ASIM would see.

BATSE on average: 46 counts

RHESSI on average: 105 counts

Indication of dead time effect a factor of 2 (or larger).


Monte Carlo input and features

- Attenuation coefficients

- Density profile

- Compton scattering

- Parameters

- Spectral distribution

- Altitude profiles


Building a library

1) 4 different spatial source distribution:1) Beamed – from discrete altitude

2) Isotropic within 20 deg – from discrete altitude

3) Beamed from distributed altitudes

4) Isotropic within 20 deg – from distributed altitudes

2) 6 different power laws: 1.0, 1.1 .... 1.5

3) 7 different altitudes, 10 km – 60 km

4) Sample at 7 different escaping angle intervals 0-9deg.....60-69deg

4 x 6 x 7 x 7 = 1176 spectra


Altitude and initial spectral index


Modelled spectra

Low energy cut-off

Peak around 50 keV

Bumb at 0.7-1.0 MeV

High energy cut-off

1/E - Beamed vertically – discrete altitude 15 km

Documents

June, 2008Corsica 20081 TGF production altitude and time delays of the terrestrial gamma flashes – revisiting the BATSE spectra Nikolai Østgaard, Thomas