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
June, 2008 Corsica 2008 2
Features from BATSE measurements
Low energy fall-off and time delays
June, 2008 Corsica 2008 3
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
June, 2008 Corsica 2008 4
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
June, 2008 Corsica 2008 5
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
June, 2008 Corsica 2008 6
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%
June, 2008 Corsica 2008 7
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
June, 2008 Corsica 2008 8
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
June, 2008 Corsica 2008 9
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°
June, 2008 Corsica 2008 10
Spectrograms - Beamed
Four parameters to determinethe production altitude1) Low energy cut-off2) Peak intensity3) Compton bump4) High energy cut-off
June, 2008 Corsica 2008 11
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
June, 2008 Corsica 2008 12
Altitude histograms
Most TGF produced ≤ 20 km
Significant portion is produced at 30 - 40
km altitude
June, 2008 Corsica 2008 13
Softening – beamed distribution The spectrum gets softer as the zenith angle increase
.
Indicates that BATSE are measuring outside the production cone.
June, 2008 Corsica 2008 14
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.
June, 2008 Corsica 2008 15
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.
June, 2008 Corsica 2008 16
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).
June, 2008 Corsica 2008 17
Monte Carlo input and features
- Attenuation coefficients
- Density profile
- Compton scattering
- Parameters
- Spectral distribution
- Altitude profiles
June, 2008 Corsica 2008 18
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
June, 2008 Corsica 2008 19
Altitude and initial spectral index
June, 2008 Corsica 2008 20
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