Introduction Model Formulation Results Summary Mechanism of Fast Air Heating and Infrasound Generation by Sprites Caitano L. da Silva and Victor P. Pasko Communications and Space Sciences Laboratory Department of Electrical Engineering Pennsylvania State University 2014 USNC–URSI Meeting Boulder, Colorado 8 – 11 January 2014 C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 1 / 17
Introduction Model Formulation Results Summary Outline 1 Introduction 2 Model Formulation 3 Results 4 Summary C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 2 / 17
Introduction Model Formulation Results Summary Correlation of Sprite Optical Observations With Infrasound Recordings Farges et al. [GRL, 32, L01813, 2005] C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 3 / 17
Introduction Model Formulation Results Summary Outline 1 Introduction 2 Model Formulation 3 Results 4 Summary C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 4 / 17
Introduction Model Formulation Results Summary Schematics of Sprite Core 75 70 65 60 55 50 45 40 35 30 25 ~100 km 20 15 10 5 0 h (km) Ground Core Simulated Region Individual Streamers C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 5 / 17
Introduction Model Formulation Results Summary Electrical Currents Flowing Through Sprite Cores Sprites currents are inferred from ELF electromagnetic radiation recordings [Cummer et al., GRL, 25, 8, 1998] and from a moving-capacitor plate model [Pasko et al., GRL, 25, 18, 1998]. Icore = Isprite /Ncore ; Ncore = 10. Ip = 200–2000 A; τp and ∆τI are of the order of a few ms. Initial conditions at 70 km altitude: ne (t = 0) = ne,a e−r2/r2 c ; ne,a = 106 cm−3; rc = 20 m. C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 6 / 17
Introduction Model Formulation Results Summary Coupling of Gas Dynamics and Chemistry in The Sprite Plasma 1 Dynamics of neutral gas. 2 Comprehensive plasma chemistry. 3 Energy exchange between charged and neutral particles. 4 Delayed vibrational energy relaxation of nitrogen molecules. [da Silva and Pasko, JGR, 118, 2013, doi:10.1002/2013JD020618] C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 7 / 17
Introduction Model Formulation Results Summary Outline 1 Introduction 2 Model Formulation 3 Results 4 Summary C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 9 / 17
Introduction Model Formulation Results Summary Role of Electron Detachment 104 105 106 107 108 109 (a) 10−5 10−4 10−3 10−2 10−1 100 10−5 10−4 10−3 10−2 10−1 100 10−2 100 102 104 (b) The electron density increase in the sprite core is resulting from the accumulation of O− ions followed by electron detachment (νdet ), in agreement with conclusions of more detailed kinetic models [Gordillo-Vazquez and Luque, GRL, 37, L16809, 2010]. C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 11 / 17
Introduction Model Formulation Results Summary Optical Emissions 0 1 2 3 4 5 6 7 8 9 10 −200 −100 0 100 200 6 8 10 10 10 10 Intensity (R) 1PN2 Optical Emission The sustained luminosity associated to rapid current growth is in agreement with the mechanism for the sprite streamer luminous trail proposed by Liu [GRL, 37, L04102, 2010]. C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 12 / 17
Introduction Model Formulation Results Summary Effects of Different Current Waveform Parameters 0 500 1000 1500 2000 0 0.005 0.01 0.015 0.02 0.025 0 5 10 15 20 0 0.005 0.01 0.015 0.02 0.025 (a) (b) 15 MJ 92 MJ 330 MJ 390 MJ 900 MJ 137 MJ 4 ms 440 MJ 4.7 GJ 4 ms 40 ms 40 ms 4 ms 2000 A ∆pobs is directly proportional to Ip and weakly depends on τp and ∆τI . ∆pobs is a better tracer of Ip rather than total energy deposited (Ncore Lz W ). C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 14 / 17
Introduction Model Formulation Results Summary Outline 1 Introduction 2 Model Formulation 3 Results 4 Summary C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 15 / 17
Introduction Model Formulation Results Summary Summary 1 Air heating or, more precisely, fast air heating, is the physical mechanism responsible for infrasonic acoustic wave radiation from sprites. Typical in situ pressure perturbation amplitudes are ∼10−3–10−2 Pa. 2 Only a fraction of ∼16 % of the total energy deposited by sprites is used for air heating. 3 Pressure perturbations amplitudes of 0.01 Pa, registered on the ground in association with sprites, can only be produced by exceptionally strong currents in sprite cores, exceeding 2 kA. 4 Measured infrasound amplitudes are directly proportional to peak currents flowing through sprite cores, and are not necessarily uniquely related to the total energy deposited in sprites. C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 16 / 17
Introduction Model Formulation Results Summary Thank you for your attention! Acknowledgments This research was supported by NSF AGS-0734083 and AGS-1332199 grants to Penn State University. Reference da Silva, C. L., and V. P. Pasko (2014), Infrasonic acoustic waves generated by fast air heating in sprite cores, Geophys. Res. Lett., Submitted for Publication, doi:10.1002/2013GL059164. Contact C. L. da Silva and V. P. Pasko, Communications and Space Sciences Laboratory, Department of Electrical Engineering, Pennsylvania State University, 227 EE East, University Park, PA 16802-2706, USA ([email protected]; [email protected]). C. L. da Silva and V. P. Pasko Sprite Infrasound Mechanism 17 / 17
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