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Lunar and Planetary Science XLVIII (2017)
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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
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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.
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