The production and propagation of energetic particles and LF/VLF radio waves associated with terrestrial gamma-ray flashes

Transcription

1 The production and propagation of energetic particles and LF/VLF radio waves associated with terrestrial gamma-ray flashes Joseph R. Dwyer Space Science Center (EOS) and Department of Physics, University of New Hampshire Courtesy NASA

2 Terrestrial Gamma-ray Flashes (TGFs) (Fishman et al. 1994) CGRO/BATSE TGF 106 Fermi/GBM TGF

3 Terrestrial Gamma-ray Flashes (TGFs) CGRO/BATSE multi-pulsed TGFs TGFs are sub-millisecond bursts of gamma rays that originate from thunderstorms from altitudes around 12 km. The TGF energy spectrum extends up to at least 40 MeV. Thunderstorms probably create at least several thousand TGFs per day around the planet. Each TGF produces at least high-energy electrons inside the storm. TGFs are associated with some of the largest peak currents produced by thunderstorms, sometimes reaching 500,000 amps. These large currents generate VLF/LF radio pulses, aka sferics, that are measured large distances from the storms.

4 Terrestrial Electron Beams (TEBs) As the TGF gamma-rays (red) propagate to space they create electrons and positrons (blue) that also propagate to space, following the geomagnetic field line. Runaway electrons

5 Terrestrial Electron Beam observed by Fermi/GBM Figure courtesy Michael Briggs

6 Runaway Electron Physics

7 25 MeV electron moving through air at 1 atm

8 25 MeV electron moving through air at 1 atm in a 3 kv/cm electric field

9 25 MeV electron moving through air at 1 atm in a 3 kv/cm electric field Relativistic Runaway Electron Avalanche (RREA) Gurevich et al. (1992)

10 Runaway electrons Wilson (1925) Energy loss and gain experienced by an electron in air From Dwyer (2004)

11 Relativistic Runaway Electron Avalanches (RREAs) Gurevich et al. (1992) The avalanche length,, is the e-folding length. Gamma-rays From Dwyer et al. (2012) Electric field High-energy Runaway Electrons

12 Positron production by RREAs

13 Relativistic feedback discharge (RFD) aka Dark Lightning due to x-rays and positrons. The central avalanche is due to the injection of a single, 1 MeV seed electron. All the other avalanches are produced by x- ray and positron feedback. The top panel is for times, t < 0.5 s. The middle panel is for t < 2 s, and the bottom panel is for t < 10 s. From Dwyer (2007)

14 Relativistic Feedback Discharge (RFD) Model 3-D PIC code runaway electron propagation (Monte Carlo) electric field calculation (SOR) X-ray & gamma-ray emission and prop. (Monte Carlo) Ionization in cells low-energy electron and ion drift Thunderstorm and lightning currents and charge densities plus lightning leader runaway electron production positron production (Monte Carlo) generate seed runaway electrons (Monte Carlo) runaway positron propagation (Monte Carlo)

15 Simulation results showing a RFD inside a thundercloud, including Earth s magnetic field

16 CGRO/BATSE TGFs Model result

17 Current moment versus time RFDs may produce some of the largest current pulses in our atmosphere

18 Modeling radio emissions from TGFs Gamma-rays Fermi/GBM data Runaway electrons LF radio data I mom B rad J dv sin 4 c o 3 R o I t mom From Dwyer and Cummer (2013)

19 Current AFOSR project Use REAM Monte Carlo and RFD PIC codes to model TGFs and TEBs, connecting gamma-ray, particle, radio and optical observations. Determine how TGFs are made inside thunderstorms. Quantify their high-energy, radio and optical emissions. Calculate the effects of their LF/VLF emissions.

20 There have been large disagreements in the predicted number of runaway electrons in the TGF source region Comparing different Monte Carlo codes initial number of gamma rays (E > 1 MeV) GEANT3 SWORD/GEANT4 REAM polar angle (degrees) Dwyer et al. 2017

21 TGF fluence at 500 km versus angle relative to zenith found from REAM Monte Carlo simulations Dwyer et al. 2017

22 Source current produced by TGFs for a 0.1 cm -2 fluence at the spacecraft found from REAM Monte Carlo simulations Dwyer et al. 2017

23 Modeling elves produced by TGFs Simulations show that the EMPs generated by TGFs are sometimes large enough to generate elves in the lower ionosphere. (N. Liu, J. Dwyer and S. Cummer, Elves associated with terrestrial gamma ray flashes, JGR-Space, 2017 in preparation)

24 An elve produced by a TGF: overhead view

25 An elve produced by a TGF: view from ground

26 Summary Terrestrial gamma-ray flashes (TGFs) and their accompanying terrestrial electron beams (TEBs) remain poorly understood. In recent years, there have been substantial improvements in the gamma-ray and radio observations of TGFs and TEBs. We are working on models of both the gamma-ray and radio emissions to test their production mechanisms and to understand their effects on the geospace environment.

27 Thank you

28 Papers resulting from this work so far: Dwyer, J., et al. Characterizing the source properties of terrestrial gamma-ray flashes, JGR-Space, 2017 (submitted) N. Liu, J. Dwyer and S. Cummer, Elves associated with terrestrial gamma ray flashes, JGR-Space, 2017 (in preparation) Thanks to AFOSR for supporting this research.

29 Courtesy NASA