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

June, 2008Corsica 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, 2008Corsica 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

June, 2008Corsica 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, different altitudes: 10, 15, 20, 30, 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, 2008Corsica 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, 2008Corsica keV keV keV >300 keV 1)Low energy cut-off 2)Peak around 50 keV 3)Bumb at MeV 4)High energy cut-off Modelled spectra

June, 2008Corsica 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 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

June, 2008Corsica 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, 2008Corsica 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

June, 2008Corsica 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

June, 2008Corsica Altitude histograms Most TGF produced ≤ 20 km Significant portion is produced at km altitude

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

June, 2008Corsica 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, 2008Corsica 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: /2007JA

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

June, 2008Corsica 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, )7 different altitudes, 10 km – 60 km 4)Sample at 7 different escaping angle intervals 0-9deg deg 4 x 6 x 7 x 7 = 1176 spectra

June, 2008Corsica Altitude and initial spectral index

June, 2008Corsica Modelled spectra Low energy cut-off Peak around 50 keV Bumb at MeV High energy cut- off 1/E - Beamed vertically – discrete altitude 15 km