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Lunar and Planetary Science XLVIII (2017) 2847.pdf GAMMA-RAY EMISSION IN PLANETARY ATMOSPHERES DUE TO RELATIVISTIC RUNAWAY ELECTRON AVALANCHES. E. S. Cramer1, J. R. Dwyer2, and M. Bagheri2 1Center for Space Plasma and Aeronomic Research (CSPAR), The University of Alabama in Huntsville, 320 Sparkman Dr NW. Huntsville, AL 35805. [email protected], 2Space Science Center (EOS) and Department of Physics, The University of New Hampshire, 105 Main St. Durham, NH 03824. Introduction: On Earth, relativistic electrons can accelerate inside thunderstorms due to large scale electric fields, producing x-ray and gamma ray radiation as they propagate [1]. Figure 1 shows the effective drag force acting on an electron as it moves through Earth’s atmosphere at STP [2]. The solid curve is due to inelastic collisions with air molecules and the dashed curve includes the effects of bremsstrahlung emission. In-situ measurements have found maximum electric fields near the breakeven field (Eb), which suggests that this process is common inside thunderstorms [3]. For a specific value of the electric field, > Eb, runaway electrons exist between energies εth and εmax. Above the critical value of the electric field (Ec), all thermal electrons can run away. does not require external seeds as the process becomes self-sustaining. The relativistic feedback process can enhance the particle flux up to 1013 and discharge the electric field in less than 0.1 ms [7]. The resulting x-ray and gamma ray emission can escape the thunderstorm region and propagate up through the atmosphere and into space. In 1994, the Burst and Transient Source Experiment (BATSE) onboard NASA’s Compton Gamma-ray Observatory (CGRO), recorded a very bright gamma ray flash (multi-MeV) emanating from the earth [8]. The duration of these Terrestrial Gamma-ray Flashes (TGFs) are on the order of milliseconds. Currently, the Gamma-ray Burst Monitor (GBM), part of NASA’s Fermi satellite, has detected more than 3,000 TGFs since its launch in 2009. GBM provides very good spectral resolution, containing 12 NaI and 2 BGO scintillators that detect photon energies from 8 keV to 40 MeV [9]. With TGF data readily available, one of the goals of this research is to model the observations so that the physics of RREA generation inside thunderstorms can be understood. Monte Carlo Simulations: In order to study RREA development and resulting photon propagation through the atmosphere, a particle simulation is used to model Fig.1: The drag force acting upon an electron as a function of kinetic energy for Earth’s atmosphere at STP [2]. Initially, these electrons can be seeded by an external source such as cosmic rays or radioactive decay [4]. If electron-electron scattering is included, then runaway electrons will avalanche multiply sporadically for every seed electron that enters the acceleration region [5]. These runaway electrons exponentially increase to form a Relativistic Runaway Electron Avalanche (RREA). Additionally, x-rays and gamma rays will pair produce, creating positrons that will travel backwards in the acceleration region and create new avalanches at the injection point [6]. At this point, RREA Fig.2: The Runaway Electron Avalanche Model (REAM). One seed electron is injected at the bottom of the electric field region (between the dotted lines) and accelerated, producing an avalanche of electrons [10]. the acceleration region inside the thundercloud. Figure 2 shows the simulation domain in which one electron Lunar and Planetary Science XLVIII (2017) avalanche is developed [10]. This Monte Carlo code includes all the major relevant physics for the interaction and propagation of photons and energetic electrons in air. Ionization cross sections, atomic excitation, and Møller scattering are included. Elastic scattering is fully modeled using a shielded Coulomb potential, and the code includes bremsstrahlung x-rays and gamma rays. For the propagation of photons, photoelectric absorption, Compton scattering, and pair production are also fully modeled. Secondary electrons produced by RREA photons are also modeled and bremsstrahlung from these electrons are included as the radiation propagates through the atmosphere. In this work, we summarize some results from the simulation including spectra, angular distributions and avalanche properties that are dependent on the electric and magnetic field [11,12]. Lightning and RREA Production on Other Planets: Observations of lightning on Jupiter have opened up the discussion for electrical discharges on other planets in our solar system. Indeed, the cause of lightning on Jupiter is said to be associated with water and ice clouds that are similar to those that are found on Earth [13]. Although no visible evidence has been detected on Saturn, it is suggested that the HF radio emissions come from lightning discharges [14]. Since it is plausible that lightning discharges occur on these planetary bodies, the REAM code was used to study RREA in a helium rich atmosphere [15]. It was concluded that properties of electron avalanches in Jovian atmospheres are significantly different than those on Earth. In fact, the minimum electric field needed to produce runaway electrons (as discussed above) is about 7.9 times smaller than the value found on Earth [15]. It has also been suggested that lightning exists on Venus [16,17,18]. A study done with data from the Pioneer Venus Orbiter Gamma-ray Burst Detector (OGBD), attempted to model the propagation of gamma rays in the Venusian atmosphere [19]. In that work a particle transport code developed at Los Alamos National Laboratory, was used to simulate gamma rays in the atmosphere of Venus. The same code was used to transport gamma-rays and neutrons for the CO2-rich Mars atmosphere [20]. The conclusion, however, was that the Pioneer data was too coarsely-binned in time for possible gamma-ray flashes to be captured above the background. More recently, the REAM code was used to simulate RREAs on Venus [21]. In this work, the electron avalanche length and spectra were determined and compared to those found on Earth. It was also determined that the gamma-ray fluences are similar to the ones measured from orbiting spacecraft detecting TGFs on Earth. If this is true, then gamma-ray 2847.pdf bursts emanating from the Venusian atmosphere should also be detected [21]. Future Work: The goal of this poster presentation is to show some modeling results of RREA on Earth and the comparison made to other planetary bodies. It is hope that these results will encourage future missions to look for these types of signals in planetary atmospheres. The most recent observation of charging on the Moons poles [22] suggests that RREAs may not all be uncommon in our solar system and should be investigated in connection with electrical discharges and/or lightning. Acknowledgements: This material is based partly upon work supported by the National Science Foundation under grant 1524533. References: [1] Dwyer, J.R. (2007) Phys. Plasmas, 14, 042901. [2] Dwyer, J.R. (2004), Geophys. Res. Lett, 31, L12102. [3] Marshall, T.C., McCarthy, M.P., and Rust, W.D. (1995), JGR, 100, 7097-7103. [4] Wilson, C.T.R (1925) Math. Proceedings of the Cambridge Philosophical Society, 22, 534-538. [5] Gurevich, A.V., Milikh, G.M., and Roussel-Dupre, R. (1992), Phys. Lett. A, 165, 463-468. [6] Dwyer, J.R. (2003), Geophys. Res. Lett, 30. [7] Dwyer, J.R. (2012), JGR, 117. [8] Fishman, G.J. (1994), Science, 264, 1313. [9] Briggs, M.S., et al. (2010), JGR, 115, A07323. [10] Dwyer, J.R., and Uman, M.A. (2014), Physics Reports, 534, 147-241. [11] Cramer, E.S., et al. (2014), JGR, 119, 7794-7823. [12] Cramer, E.S. Dwyer, J.R., and Rassoul, H.K. (2016), JGR, 121. [13] Desch, S.J., Borucki, W.J., and Bar-Nunn, A. (2002), Rep. Prog. Phys., 65, 955-997. [14] Fischer, G., et al. (2006), Icarus, 184, 135-152. [15] Dwyer, J.R., et al. (2006), Geophys. Res. Lett, 33, L22813. [16] Scarf, F.L., and Russell, C.T. (1983), Geophys. Res. Lett, 10, 1192-1195. [17] Singh, R.N., and Russell, C.T (1986), Geophys. Res. Lett, 13, 1051-1054. [18] Hansell, S.A., Wells, W.K, and Hunten, D.M. (1995), Icarus, 117, 345-351. [19] Lorentz, R.D., and Lawrence, D.J. (2015), Planetary and Space Science, 109, 129-134. [20] Prettyman, T.H., et al. (2004), JGR, 109. [21] Bagheri., M., and Dwyer, J.R. (2016), JGR, 121, 90209029. [22] Jordan, A.P., et al. (2017), Icarus, 283, 352358.