1. June, 2008Corsica 20081 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

2. June, 2008Corsica 20082 Features from BATSE measurements Low energy fall-off and time delays

3. June, 2008Corsica 20083 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

4. June, 2008Corsica 20084 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 Curves t = s / c

5. June, 2008Corsica 20085 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

6. June, 2008Corsica 20086 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%

7. June, 2008Corsica 20087 25-50 keV 50-100 keV 100-300 keV >300 keV 1)Low energy cut-off 2)Peak around 50 keV 3)Bumb at 0.7-1.0 MeV 4)High energy cut-off Modelled spectra

8. June, 2008Corsica 20088 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 delays 1)100 micros delay below 20 km 2)Above 40 km – no time delays 3)At 0-9 deg – no time delays

9. June, 2008Corsica 20089 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°

10. June, 2008Corsica 200810 Spectrograms - Beamed Four parameters to determine the production altitude 1)Low energy cut-off 2)Peak intensity 3)Compton bump 4)High energy cut-off

11. June, 2008Corsica 200811 Optimizing procedure 4 libraries + BATSE Energy Response Matrix 2 steps Determine a zenith angle range within 5° from BATSE measurements Optimize the spectral index and production altitude. χ 2 divided by degrees of freedom < 1.5 Only one possible zenith angle 21 of 25 BATSE spectra meet both these criteria

12. June, 2008Corsica 200812 Altitude histograms Most TGF produced ≤ 20 km Significant portion is produced at 30 - 40 km altitude

13. June, 2008Corsica 200813 Softening – beamed distribution The spectrum gets softer as the zenith angle increase. Indicates that BATSE are measuring outside the production cone.

14. June, 2008Corsica 200814 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.

15. June, 2008Corsica 200815 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.

16. June, 2008Corsica 200816 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).

17. June, 2008Corsica 200817 Monte Carlo input and features -Attenuation coefficients -Density profile -Compton scattering -Parameters -Spectral distribution -Altitude profiles

18. June, 2008Corsica 200818 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

19. June, 2008Corsica 200819 Altitude and initial spectral index

20. June, 2008Corsica 200820 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