Terrestrial Gamma-Ray Flashes as Powerful Particle Accelerators

AGILE-TGF_Nat_11

SUBMITTED TO NATURE: December 22, 2009

Terrestrial Gamma-Ray Flashes and their possible impact on airplane environment

M. Tavani1,2,3,4, M. Marisaldi5, C. Labanti5, F. Fuschino5, P. Giommi20, S. Colafrancesco20, C. Pittori20, M. Trifoglio5, F. Gianotti5, A. Bulgarelli5, A. Argan1, A. Trois1, F. Verrecchia20, L. Salotti22, G. Barbiellini3,8,9, P. Caraveo7, P.W. Cattaneo10, A.W. Chen3,7, T. Contessi7, E. Costa1, F. D'Ammando1,2, E. Del Monte1, G. De Paris1, G. Di Cocco5, G. Di Persio1, I. Donnarumma1, Y. Evangelista1, M. Feroci1, A. Ferrari3,11, M. Galli12, A. Giuliani7, M. Giusti1,3, I. Lapshov13, F. Lazzarotto1, P. Lipari14,15, F. Longo8,9, S. Mereghetti7, E. Morelli5, A. Morselli4, L. Pacciani1, A. Pellizzoni17, F. Perotti7, G. Piano1,4, P. Picozza2,4, M. Pilia17,18, G. Pucella6, M. Prest18, M. Rapisarda6, A. Rappoldi10, E. Rossi5, A. Rubini1, S. Sabatini2,4, E. Scalise1, P. Soffitta1, E. Striani2,4, E. Vallazza9, S. Vercellone19, V. Vittorini1,3, A. Zambra3,7, D. Zanello14,15

1

1. INAF-IASF Roma, via del Fosso del Cavaliere 100, I-00133 Roma, Italy 2. Dipartimento di Fisica, Università Tor Vergata, via della Ricerca Scientifica 1,I-00133 Roma, Italy 3. Consorzio Interuniversitario Fisica Spaziale (CIFS), villa Gualino - v.le Settimio Severo 63, I-10133 Torino, Italy 4. INFN Roma Tor Vergata, via della Ricerca Scientifica 1, I-00133 Roma, Italy 5. INAF-IASF Bologna, via Gobetti 101, I-40129 Bologna, Italy 6. ENEA Frascati, via Enrico Fermi 45, I-00044 Frascati(RM), Italy 7. INAF-IASF Milano, via E. Bassini 15, I-20133 Milano, Italy 8. Dip. Fisica, Università di Trieste, via A. Valerio 2, I-34127 Trieste, Italy 9. INFN Trieste, Padriciano 99, I-34012 Trieste, Italy 10. INFN Pavia, via Bassi 6, I-27100 Pavia, Italy 11. Dipartimento di Fisica, Università di Torino, Torino, Italy 12. ENEA Bologna, via don Fiammelli 2, I-40128 Bologna, Italy 13. IKI, Moscow, Russia 14. INFN Roma 1, p.le Aldo Moro 2, I-00185 Roma, Italy 15. Dip. Fisica, Università La Sapienza,p.le Aldo Moro 2, I-00185 Roma, Italy 16. CNR, IMIP, Montelibretti (Rome), Italy 17. INAF Osservatorio Astron. Cagliari, Poggio dei Pini, I-09012 Capoterra, Italy 18. Dip. Fisica, Univ. Insubria, via Valleggio 11, I-22100, Como, Italy 19. INAF IASF Palermo, via La Malfa 153, I-90146 Palermo, Italy 20. ASI Science Data Center, ESRIN, I-00044 Frascati, Italy 21. Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy 22. Agenzia Spaziale Italiana, viale Liegi 26, Roma, Italy

2

Tropical thunderstorms can produce terrestrial gamma-ray flashes1 (TGFs), i.e., intense bursts of X-rays and gamma-rays associated with very powerful lightning. TGFs typically last a few milliseconds and produce impulsive radiation that can be detected by space satellites1,2,3,4 from tens of kiloelectronvolt up to tens of megaelectronvolt. TGFs are now established5 to occur deep in the atmosphere (~10-25 km above ground) near the upper regions of tropical thunderclouds. Here we present new TGF timing and spectral data based on the observations of the Italian Space Agency’s AGILE satellite. We determine that the TGF photon energies may reach 50 MeV and above, implying accelerating potential differences of order of hundreds of megavolts in the cloud-to-ground or intercloud discharges. These huge potentials can accelerate particles in large numbers5,6. Despite a perception of their rarity and unlikeliness to affect humans, TGF-thunderstorms may constitute a serious hazard to airplane equipment being hit by the very intense electromagnetic discharge and by the particle and photon irradiation. TGF-thunderstorms can be active for several hours8 in tropical regions, and a TGF continuous monitoring and alert system may contribute to maximize aircraft safety. Establishing a TGF-alert system with a joint use of high-energy and weather satellite data is doable with existing instruments. We discuss an operational example of how a TGF-alert system can be rapidly implemented.

3

Since their serendipitous discovery by the BATSE instrument on board of the Gamma-Ray Observatory in 1994 (ref. 1), TGFs have attracted considerable attention. They are intense and usually very brief (lasting a few millisecond with temporal structure) bursts of energy originating in tropical storms. Several models have been proposed since the realization of their terrestrial origin. Initial proposals associating TGFs with upper atmosphere phenomena (“sprites”) or other highaltitude (> 30 km) phenomena9,10 are now superseded by models placing TGFs in the range of 10-25 km above sea level4,6. A total of a thousand TGFs have been detected by low-Earth orbiting satellites equipped with instruments sensitive in the MeV energy range and with optimized millisecond timing (BATSE-GRO1, RHESSI2, AGILE3 and GBM-Fermi4). The typical TGF radiated energy2 is ETGF = 20-40 kJ, and the spectrum is well described2,3,8 by a power-law Bremsstrahlung model of relativistic electrons with a cutoff photon energy of Ec ~ 10 MeV. The TGF rate is estimated to be in the range of 100-1000 per day depending on intensity and model assumptions2. The geographical distribution is peaked in tropical regions8, indicating that thunderstorms influenced by the strong convective air currents typical of those regions can generate TGFs.

New substantial contributions to TGF studies are provided by the AGILE gamma-ray astrophysics mission11. The AGILE very compact instrument covers the energy range from several tens of keV to several GeV with excellent timing capabilities. In particular, the (non-imaging) Mini-Calorimeter detector12 (MCAL) can detect flashes of terrestrial and cosmic origin in the energy range 350 keV – 100 MeV. The on-board data acquisition is capable of recording and transmitting to the ground very fast transient events using a special sub-millisecond trigger logic13. Millisecond flash candidates are triggered on board, recorded, and transmitted to the ground for additional analysis. On-the-ground analysis includes event filtering from instrumental background and a determination of the event timing and spectral properties. Cosmic and terrestrial events can be distinguished by their spectral hardness. TGFs are usually detected with relatively large values (larger than 0.5) of the hardness ratio defined as HR= (counts with E>1.4MeV)/(counts with E