Mathematical model provides bolt of understanding for lightning-produced X-rays

Date:
March 31, 2023
Source:
Penn State
Summary:
In the early 2000s, scientists observed lightning discharge
producing X- rays comprising high energy photons -- the same
type used for medical imaging. Researchers could recreate this
phenomenon in the lab, but they could not fully explain how and
why lightning produced X-rays. Now, two decades later, a team has
discovered a new physical mechanism explaining naturally occurring
X-rays associated with lightning activity in the Earth's atmosphere.


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FULL STORY ==========================================================================
In the early 2000s, scientists observed lightning discharge producing
X-rays comprising high energy photons -- the same type used for medical imaging.

Researchers could recreate this phenomenon in the lab, but they could not
fully explain how and why lightning produced X-rays. Now, two decades
later, a Penn State-led team has discovered a new physical mechanism
explaining naturally occurring X-rays associated with lightning activity
in the Earth's atmosphere.


==========================================================================
They published their results on March 30 in Geophysical Research Letters.

The team's finding could also shed light on another phenomenon: the
small shock sometimes felt when touching a metal doorknob. Called spark discharge, it occurs when a voltage difference is created between a body
and a conductor. In a series of lab experiments in the 1960s, scientists discovered that spark discharges produce X-rays -- just as lightning
does. More than 60 years later, scientists are still conducting lab
experiments to better understand the mechanism underpinning this process.

Lightning consists in part of relativistic electrons, which emit
spectacular high-energy bursts of X-rays with tens of mega electron-volt energies called terrestrial gamma-ray flashes (TGFs). Researchers have
created simulations and models to explain the TGF observations, but there
is a mismatch between simulated and actual sizes, according to lead author Victor Pasko, Penn State professor of electrical engineering. Pasko and
his team mathematically modeled the TGF phenomenon to better understand
how it can occur in observed compact space.

"The simulations are all very big -- usually several kilometers across --
and the community has difficulty reconciling this right now with actual observations, because when lightning propagates, it's very compact,"
Pasko said, explaining that lightning's space channel is typically
several centimeters in scale, with electric discharge activity producing
X-rays expanding around tips of these channels up to 100 meters in extreme cases. "Why is that source so compact? It's been a puzzle until now. Since we're working with very small volumes, it may also have implications
for the lab experiments with spark discharges underway since the 1960s."
Pasko said that they developed the explanation for how an electric field amplifies the number of electrons, driving the phenomenon. The electrons scatter on individual atoms, which constitute the air, as they experience acceleration. As the electrons move, most of them go forward as they
gain energy and multiply, similar to a snow avalanche, allowing them to
produce more electrons. As the electrons avalanche, they produce X-rays,
which launch the photons backward and produce new electrons.

"From there, the question we wanted to answer mathematically was, 'What
is the electric field you need to apply in order to just replicate this,
to launch just enough X-rays backwards to allow amplification of these
select electrons?'" Pasko said.

The mathematical modeling established a threshold for the electric
field, according to Pasko, which confirmed the feedback mechanism that amplifies the electron avalanches when X-rays emitted by the electrons
travel backward and generate new electrons.

"The model results agree with the observational and experimental evidence indicating that TGFs originate from relatively compact regions of space
with spatial extent on the order of 10 to 100 meters," Pasko said.

In addition to describing high-energy phenomena related to lightning,
Pasko said the work may eventually help to design new X-ray sources. The researchers said they plan to examine the mechanism using different
materials and gases, as well as different applications of their findings.

The other authors on the paper are Reza Janalizadeh, a postdoctoral
scholar in the Penn State Department of Electrical Engineering; Sebastien Celestin of the University of Orleans in Orleans, France; Anne Bourdon,
of Ecole Polytechnique in Palaiseau, France; and Jaroslav Jansky of the University of Defense in Brno, Czechia.

The National Science Foundation funded this work.

* RELATED_TOPICS
o Matter_&_Energy
# Energy_Technology # Spintronics # Electricity # Physics
o Earth_&_Climate
# Storms # Severe_Weather # Energy_and_the_Environment #
Environmental_Science
* RELATED_TERMS
o X-ray o Radiography o Gamma_ray o Nuclear_fission o
Subatomic_particle o Photoelectric_effect o Thunderstorm
o Electricity

========================================================================== Story Source: Materials provided by Penn_State. Original written by
Sarah Small. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Victor P. Pasko, Sebastien Celestin, Anne Bourdon, Reza Janalizadeh,
Jaroslav Jansky. Conditions for Inception of Relativistic Runaway
Discharges in Air. Geophysical Research Letters, 2023; 50 (7)
DOI: 10.1029/2022GL102710 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/03/230331131501.htm

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