||X-Ray & Gamma-Ray Emission From Lightning
||Rocket Triggered Lightning
||Thunderstorm Energetic Radiation Array (TERA)
||X-Ray Emission From Laboratory Sparks
||Terrestrial Gamma-Ray Flashes
||Runaway Breakdown Theory
|The Mysteries of Lightning
||by Dr. Joseph Dwyer
|•A garden variety spark?
|Consider the conventional spark, the kind you get when you touch a doorknob after walking across a carpet. When you traverse the carpet, your shoes rub off
electrons and you accumulate an electrical charge, which produces an electric field
between you and other objects in the room. For small electric fields, air is a
good insulator and electrical current cannot flow in any appreciable amount.
However, as your finger approaches the knob, the electric field becomes locally
enhanced. If it reaches a critical value of about 3 million volts per meter,
called the breakdown field, then a discharge occurs; the air becomes a conductor
and current bridges the gap, i.e. you get a spark. Because in some ways lightning
resembles a big spark, it seems natural to assume that something similar must be
going on inside thunderstorms. Unfortunately, there is a problem with this simple
picture: Decades of balloon, aircraft and rocket measurements made directly inside
the clouds rarely find fields above about 150,000 volts per meter, much too low to
cause air to breakdown like it does when we touch a doorknob.
Scientists have been trying to solve this conundrum of what causes lightning for
many years, and, until recently, mainly had two explanations: First, perhaps large
electric fields really do exist inside thunderstorms but only in relatively small
volumes that are unlikely to be measured. Although such a scenario cannot be ruled
out observationally, it is not all-together satisfying, since we are replacing one
problem with another: how do you get big electric fields in small volumes? The
second explanation comes from laboratory experiments that have shown that the
electric field needed to produce a discharge is reduced substantially when
raindrops or ice particles are present in the air--as they are inside
thunderstorms. Unfortunately, while the addition of rain or ice does alleviate
some of the discrepancy, thunderstorm electric fields still appear to be too low by
perhaps 50% to be explained entirely by this mechanism.
In addition to the process that initiates lightning, scientists are also uncertain
about how lightning actually propagates many kilometers through the air. It is
known that the mechanism involves the formation of a leader, a hot channel that can
breakdown the air and transport charge over long distances. However, exactly how
this all occurs is somewhat mysterious and previous efforts to model these processes
have not been entirely successful.
|•Stranger than we thought?
|In 1992, a new idea emerged that has shown promise for explaining what is happening inside thunderstorms and how lightning is occurring. A. V. Gurevich of the P. N.
Lebedev Institute of Physics in Moscow, G. M. Milikh of the University of Maryland
and R. Roussel-Dupre of Los Alamos National Laboratory proposed a new and unusual
kind of discharge that involves very fast electrons moving under the influence of
electric fields. It turns out that in sufficiently strong fields electrons can
gain very large amounts of energy, and the electrons are said to run away.
Actually, the idea of runaway electrons is not new and dates back to Nobel Laureate
C. T. R. Wilson, who in 1925 suggested that under some circumstances electrons can
run away in the atmosphere. However, an early attempt by A. V. Gurevich in 1961 to
use runaway electrons to model a discharge was widely considered inapplicable to
the atmosphere, since the electric field required for such a discharge is at least
a factor of 10 larger than the conventional breakdown field. In contrast, the new
theory of runaway breakdown uses an avalanche of relativistic electrons and only
requires an electric field 1/10 as large as that needed for a conventional
breakdown in dry air. At thunderstorm altitudes, the runaway breakdown threshold
is about 150,000 volts per meter-precisely in the range of values measured inside
the clouds. Indeed, it is probably not a coincidence that the maximum observed
electric field inside thunderclouds and the threshold for runaway breakdown are
about the same value, since runaway breakdown would efficiently discharge the field
if it were to rise much higher.
In normal discharges, all the electrons have low energies and travel slowly, so at
most the emitted electromagnetic spectrum extends into the ultra-violet range. In
runaway breakdown, on the other hand, the electrons travel very close to the speed
of light, and, consequently, produce large quantities of x-rays as they collide
with air molecules. Therefore, one way to test for runaway breakdown is to search
Because of Wilson's work, scientists have attempted to observe x-rays from
thunderstorms and lightning since the 1930's. Unfortunately, such measurements are
very challenging to make and so until recently have produced mostly ambiguous
results. One difficulty is that x-rays don't travel very far through the
atmosphere and are usually absorbed within a few hundred meters of the source.
Another difficulty is that thunderstorms are electromagnetically very noisy
environments, and lightning, in particular, emits large amounts of radio frequency
noise. This noise produces the familiar crackle on an AM radio-and that can be
from kilometers away. Now imagine trying to make sensitive electrical measurements
with lightning less than 100 meters away. It is, therefore, important to be able
to distinguish real electrical signals produced by x-rays from spurious ones
produced by the radio frequency emission. For this reason, many of the early
measurements were not readily accepted.
The situation got more interesting in the 1980's when G. K. Parks, M. McCarthy and
collaborators at the University of Washington made aircraft observations inside
thunderstorms, and later K. B. Eack (now at New Mexico Tech) and collaborators made
a series of balloon soundings through thunderclouds. These observations provided
tantalizing hints that occasionally large bursts of x-rays are produced by
thunderstorms. The source of these x-rays could not be deduced, but it seemed to
be associated with the enhanced electric fields inside the cloud. Interestingly
the x-ray emission sometimes began right before lightning and stopped once the
lightning occurred, perhaps because lightning shorted out the electric fields
needed to produce runaway breakdown. Besides runaway breakdown, there are no other
mechanisms known to produce such large quantities of x-rays in our atmosphere.
Even during the bright return stroke of lightning, when the channel can reach
30,000 °C (54,000 °F), virtually no x-rays are produced by this high temperature.
|•Superman, put on your sunglasses!
|Our view of lightning took a radical turn in 2001, when C. B. Moore and
collaborators at New Mexico Tech reported observing strong x-ray emission from
several natural lightning strikes on top of a tall mountain. Unlike the earlier
aircraft and balloon observations, these x-rays seemed to be produced by the
lightning itself and not by the large-scale electric fields inside the
thundercloud. Furthermore, the emission seemed to be associated with the lightning
stepped-leader process, which was something entirely new.
All right, here is where I enter the picture: As a physicist I have always been
interested in how x-rays and gamma-rays are produced. While such energetic
radiation is commonly created both at distant astrophysical sites and in our solar
system, usually this only occurs in the vacuum of space where energetic particles
can travel unimpeded. Consequently, I became fascinated by the idea that the same
kind of x-rays produced in places like solar flares could also be made by
thunderstorms and lightning. Because I live in Florida, which has lots of
thunderstorms, I decided to look for myself to see if these supposed x-rays really
existed. In 2002, with funding from the National Science Foundation, my group (H.
K. Rassoul, M. Al-Dayeh, L. Caraway, B. Wright and A. Chrest) at the Florida
Institute of Technology, in collaboration with M. A. Uman and V. A. Rakov at the
University of Florida, began a systematic campaign to search for x-ray emission
from lightning, thereby clarifying the role of runaway breakdown in our atmosphere.
To reduce the problems of spurious electromagnetic signals we placed sensitive
x-ray detectors inside heavy aluminum boxes, designed to keep out moisture, light
and radio frequency noise. In order to get close enough to lightning to measure
x-rays, we set up our instruments at the International Center for Lightning
Research and Testing (ICLRT) at Camp Blanding, FL. The ICLRT is a facility run by
the University of Florida, equipped to measure, among other things, the electric
and magnetic fields, and optical emission associated with lightning. Moreover, the
facility is capable of artificially triggering lightning from natural thunderstorms
using small rockets.
At the ICLRT, when a thunderstorm is overhead and the electric field on the ground
reaches several thousand volts per meter, a small rocket is launched that uncoils a
spool of thin Kevlar coated copper wire, one end of which remains attached to the
ground. As the rocket rises, the vertical grounded wire enhances the electric
field at the rocket's tip, resulting in an upward propagating leader that
eventually snakes its way up into the thundercloud. Electrical current into the
leader quickly vaporizes the wire, and the resultant lightning usually strikes the
rocket launcher. The advantage of using triggered lightning over natural lightning
is that with triggered lightning the exact time and place of the lightning strike
is experimentally controlled. Furthermore, the experiment can be repeated over and
over with dozens of triggered lightning flashes produced at the ICLRT each summer.
To be honest, when we first set up our instruments at the ICLRT, I wasn't really
expecting to measure any x-rays from lightning. The New Mexico Tech observations
were fascinating but at that point had not been independently verified, and so in
light of the long history of negative or ambiguous x-ray results, they needed to be
viewed with caution. I figured we would do a series of careful experiments and
most likely rule out lightning as a source of x-rays. For this reason, after we
made our first triggered lightning measurements, I didn't get around to looking at
our data for over a week. When I finally sat down with my graduate student, Maher
Al-Dayeh, and plotted the data from the x-ray detectors, I nearly fell off my
chair. To my surprise--and to the surprise of just about everyone else--we
discovered that not only does triggered lightning produce x-rays, it produces lots
and lots of x-rays just about every time. Indeed, the flashes were so intense in
x-rays that our instruments were temporarily blinded by the radiation.
Subsequent experiments over the next year showed that the x-ray emission is
produced by the lightning dart leader with possibly some contribution from the
beginning of the return stroke, and it is composed of x-rays with energies
extending to at least twice that found in a chest x-ray. Furthermore, the x-ray
emission was not produced continuously, but instead occurred in rapid bursts a
millionth of a second apart. Indeed, if we somehow had x-ray vision, like
Superman, lightning would look quite different than what we are used to: As the
lightning leader propagates downward, we would see a rapid series of bright flashes
descending from the clouds. The flashes would intensify as they approached the
ground, ending with a very intense burst at the instant the return stroke begins.
The optically bright return stroke that follows, on the other hand, would look
black in x-rays.
The mechanism for producing these x-rays is presently not known, but almost
certainly involves runaway breakdown. The measurements also suggest that the
electric fields produced by lightning leaders are much, much larger than what was
previously believed possible. The observed x-rays don't seem to have anything to
do with the vaporized wire used to trigger the lightning, since they all occur well
after the wire is gone. Indeed, since the initial discovery of x-rays from
triggered lightning, we have observed natural lightning strikes at the ICLRT as
well. These data also show beautiful x-ray emission from the stepped-leader phase,
confirming the earlier New Mexico Tech measurements.
The observations of x-rays from lightning illustrate that our current understanding
of how lightning works is far from complete, since none of the conventional models
of lightning can explain the x-ray emission. The observations also show that
runaway breakdown may be a common phenomenon is our atmosphere. Because air
molecules hinder the acceleration of fast electrons, runaway breakdown has a much
easier time at thunderstorm altitudes or above where the air is thinner, yet we see
evidence for runaway breakdown near the ground at sea level (most of the x-rays we
observe come from the bottom 50 meters or so of the lightning channel). In other
words, if it can happen there it can happen anywhere.
|•Meanwhile, back inside the thunderstorm
|So, what about lightning initiation inside thunderstorms? In the last few years,there have been substantial theoretical advances in understanding runaway breakdown
and promising models have emerged that involve the combination of high-energy
cosmic-ray air showers and runaway breakdown. If these models are valid, lightning
on Earth has a direct physical connection to exploding stars in other parts of our
galaxy. A fascinating piece of evidence further supporting runaway breakdown
inside thunderstorms came from our experiment at the ICLRT last summer. During the
last rocket launch of the season we fortuitously caught a huge burst of very
high-energy x-rays, more appropriately called gamma-rays, using three detectors
placed 650 m from the lightning channel. The energies of the individual gamma-ray
photons extended up to over 10 MeV, much larger than emitted by the dart leaders
and about 100 times larger than found in dental x-rays! Anyone who pictures
scientists as calm and reserved should have seen how we reacted when we saw that
gamma-ray flash. Instead of looking at data on a computer, one might have thought
we had just seen our favorite team score the winning touchdown at the super bowl.
Interestingly, using lightning channel current and electric field data, along with
properties of the gamma-rays we have inferred that the source of the emission was
likely many kilometers up in the thundercloud. We did not expect to see gamma-rays
from such an altitude due to the absorption in the atmosphere, but apparently the
gamma-ray intensity at the source was so large that some gamma-rays were able to
make it down to the ground. The observation suggests that massive runaway
breakdown must have occurred within the thundercloud in a process related to the
initiation of triggered lightning. Furthermore, these observations demonstrate
that it is possible to study this phenomenon on the ground, which is experimentally
We are now in the process of greatly expanding the number of x-ray instruments at
the ICLRT from 5 to over 36, covering the entire 1 square kilometer of the Camp
Blanding site. This will allow the practical study of natural lightning as well as
triggered lightning and should increase the odds of measuring more gamma-ray bursts
from the thunderclouds. The x-ray and gamma-ray emission should serve as a probe
to help determine the electric fields in regions that are otherwise very difficult
to measure and it should allow us to better understand the breakdown processes that
initiate lightning and facilitate its propagation over many kilometers through the
air. Using x-rays to study lightning is still new and consequently just about
every time we record x-rays from lightning we discover something we didn't know
before-a rare and exciting opportunity for a scientist like me. We have already
learned that lightning is not just an ordinary spark like we get when we touch a
doorknob. It involves a more exotic kind of discharge that produces runaway
electrons and x-rays. Since x-rays allow us to look at lightning in a novel way, I
believe that this research may help us finally solve the puzzle started by Benjamin
Franklin two and a half centuries ago.
|•Conventional Electrical Breakdown
|When positive and negative electrical charges are separated, they create a potential difference, measured in volts, abbreviated as V. For example, the potential
difference between the positive and negative terminals of an AA battery is 1.5 V.
The electric field is the rate at which the potential changes with distance. The
average electric field across a 5 cm long AA battery is about 0.3 V/cm or,
equivalently, 0.003 V/m.
In a conventional discharge, free electrons move under the influence of an electric
field, with frequent collisions with air molecules preventing the individual
electrons from gaining too much energy. Some of the electrons are absorbed by
attaching to oxygen atoms. Free electrons can also be created, for instance, when
an electron hits an air molecule hard enough to knock out an additional electron.
The original electrons plus the newly freed ones are then accelerated by the
electric field. For low electric fields, the rate of free electron loss through
attachment is greater than the rate of free electron gain through collisions. As
the electric field increases, however, the balance shifts. Above the so-called
breakdown field, free electrons are created faster then they are lost, and an
avalanche of free electrons is produced, with the number increasing exponentially
with time. Because moving electrons create an electrical current, above the
breakdown field air becomes a good conductor of electricity. For example, dry air
breaks down and becomes a conductor, e.g. it sparks, at about 3,000,000 V/m, so if
instead of 1.5 V, an AA battery had a voltage of roughly 150,000 V, it would spark
across its terminals.
|The runaway breakdown of air occurs when the rate of energy gain experienced by an electron in an electric field exceeds the rate of energy loss through collisions
with air molecules. The numerous collisions result in an effective drag force on
the electron, which works to slow it down. For subatomic particles like electrons
this drag force does not behaves according to our ordinary experience. When you
stick your hand outside your car window while driving down the road, you feel a drag
force due to the motion of your hand through the air. As you go faster, the drag
force increases, and as you slow down the drag force decreases. Electrons, on the
other hand, behave differently: when electrons are moving sufficiently fast, the
drag force actually decreases the faster the electron goes. As a result, if an
electron is moving very fast its drag force can become smaller then the force from
the electric field and the electron will run away, continuously gaining energy until
it is moving very close to the speed of light. The electric field necessary for
runaway breakdown to occur is about 300,000 V/m at sea level and lower at
thunderstorm altitudes, a factor of ten lower than the field needed for a
As electrons run away, they will occasionally bump hard into air molecules knocking
off other fast electrons. These "knock-off" electrons can subsequently run away,
producing more energetic "knock-off" electrons and so on. The result is a large
avalanche of high-energy electrons that increase exponentially with time and
distance. The whole process can be initiated by as little as one high-energy "seed"
electron, which are plentiful in our atmosphere due to the steady background from
cosmic-ray air showers and radioactive decays.
As the runaway electrons move through the air they create large amounts of
ionization, e.g. low-energy electrons and ions, plus they produce x-rays and
gamma-rays as a byproduct of collisions with the air molecules. This process is
known as bremsstrahlung (German for braking radiation) and is exactly how an x-ray
tube in a dentist office works.
More information on Runaway Breakdown
|Both natural and triggered lightning flashes are usually composed of several strokes. For triggered lightning, each stroke starts as a downward propagating
column of charge called a dart leader that, near ground, more or less follows the
path left by the rocket and triggering wire. The dart leader brings down charge
from the cloud and ionizes the channel as it moves. Once the dart leader connects
to the ground, a short circuit is created and a large pulse of current, called the
return stroke, flows through the channel. The current in the return stroke quickly
heats the channel causing the visible light that we see, and the subsequent rapid
expansion of the hot air causes the thunder that we hear. After the return stroke,
another dart leader can follow and the whole process repeats. The quick succession
of strokes is what causes the lightning channel to flicker. In natural lightning,
the role of the rocket is played by a stepped leader, which forges the ionized path,
extending from the cloud to the ground. However, the strokes of triggered lightning
are very similar to the subsequent strokes of natural lightning.
|The x-rays from lightning are detected using crystals of sodium-iodide (NaI). When sodium-iodide absorbs energetic radiation, such as x-rays, it emits a large optical
light pulse. This light is captured by a photomultiplier tube, which converts the
light pulse into a current pulse that is recorded. The size of the current pulse is
proportional to the size of the light pulse, which in turn is proportional to the
energy of the detected x-rays. As a result, measuring the size of the output
electrical signal from the photomultiplier tube gives a direct measurement of the
To prevent spurious signals from being detected, the sodium-iodide/photomultiplier
tube detectors are placed inside heavy aluminum boxes, which are completely sealed
to keep out light, moisture and radio noise, and only allow x-rays to enter. The
instruments are battery powered and transmit their data via fiber optics to a shielded trailer that contains the data acquisition system.
The Thunderstorm Energetic Radiation Array (TERA) is an experiment designed to measure energetic electromagnetic radiation (x-rays and gamma rays) from lightning. We are currently looking at the radiation from stepped leaders, dart leaders, and return strokes. We've even caught a gamma ray burst associated with a triggered CG flash! Ultimately, the goal is to determine what role runaway breakdown plays in lightning initiation and propagation.
More information on TERA