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Friday, March 28, 2008

Lightning and the Runaway Breakdown Theory

What is Lightning?

Ever since the first humans walked on Earth, we have been fascinated with lightning. It’s the greatest mystery and phenomena on earth. What is lightning? Where does it come from? How is lightning initiated? Unfortunately, as fascinated as we are about lightning, we are still unable to answer our own questions. Lightning strikes the Earth as least 4 million times worldwide each day. One strike of lightning is the brightest light we see, the loudest noise we hear, and has as much energy as a nuclear bomb. Despite these facts, we know more about distant cosmos that are millions of light years away, than how lightning propagates 6 miles above our heads. However, we are aware of the fact that lightning does occur inside thunderclouds. What we don’t know is how lightning is initiated. Several ideas and theories have been suggested, including colliding raindrops, localized regions of concentrated charge, and avalanches of high-energy electrons initiated by cosmic rays from outer space. At least for now, no one knows how lightning is triggered. This is why at Florida Tech, with the help of the University of Florida and New Mexico Tech, we’re currently performing experiments and research to answer these questions.

Different Types of Lightning

In order to better understand lightning, one must know the similarities and difference between the different types of lightning. Although there are nine types of lightning, we’ll take a look at the more common forms of lightning for the sake of convenience and understanding.

One type of lightning is called cloud discharge. This type of lightning, also called heat lightning, occurs within a thunder cloud, between two thunder clouds, or from a thunder to the air. As scientists, we believe that most cloud discharges take place within an individual cloud. However, we have collected very few data to confirm this belief. Cloud discharges are far more common than cloud-to-ground lightning; 10 or more cloud flashes may occur before the first one that strikes earth.

Perhaps the most noticeable and fascinating phenomena on earth is cloud-to-ground lightning. This is the archetypical lightning bolt that arcs out of the sky and smites the ground with a great, flickering flash of light. Cloud-to-ground lightning is the sudden release of built-up charge stored in an electric field. However, what triggers this type of lightning remains a mystery. Hence the following form of lightning.

In order to learn what exactly triggers cloud-to-ground lightning, participants of the Florida Tech Lightning Research Team have created triggered lightning. Lightning expert Pierre Hubert writes, “Triggering lightning at will, at a predetermined place and time, is the old Promethean dream which seems more related to legend than to science.” The technique used at Florida Tech has taught scientists much about processes and effects of lightning. We launch rockets using a strong enough propellant to launch them 700 yards into the atmosphere in two seconds. The rockets are attached with long copper wires to conduct electricity (much like what Benjamin Franklin did) to create cloud-to-ground lightning. From 2000 feet up, the wires trigger lightning with a cloud-to-ground distance larger than the Empire State building. When the lightning strikes the ground, it creates over 100 million volts that zap the array of test equipment on the ground. This allows us to study the data collected and form new hypothesizes about Lightning.

Benjamin Franklin and a Early Theory of Lightning

Now with the idea of the different forms of lightning fresh in our head, let’s take a look at the experiment performed by Benjamin Franklin and the early theories of lightning. Franklin’s kite and key experiment performed in 1752 is arguably the most famous in scientific history. He wanted to prove that lightning was a form of electricity and not a form of punishment given by God. In his experiment, he tied a copper key to the bottom of the string attached to a kite. Then, he flew his kite during a thunder storm. Lightning struck the kite and caused the copper key to create a spark. The spark was generated because electricity has a negative charge and the key has a positive charge. Assuming that enough energy is present, two objects of different charges that interact with each other will create a spark. As a result, lightning is a form of electricity.

In more modern terms, a spark is generated when positive and negative charges build up enough energy so that they leap through the air to reach other. Take for example, a negatively charged ball and a positively charged rod. When the charge between the ball and the rod become strong enough, a spark is generated and cuts a pathway through the air.

Early scientists such as Thomas- Francois of France, and George Richmann of Russia, were able to take advantage of the results yielded by Franklin’s famous experiment. They proposed a new theory of how lightning is generated through the process of particle discharge. As ice and water particles within clouds collide with each other, the positively charged particles move to the top of the cloud, and the negatively charged particles move to the bottom of a cloud. When the charge above and the charge become strong enough, the particles leap through the air as a bolt of lightning.

Problems Concerning the Early Theory of Lightning

The theory of particle discharge has remained an acceptable explanation for Lightning until recent years. As already noted, there must be enough energy present in order for the different charges to leap through the air and create a spark. This is the problem with the particle discharge theory. After examination of a storm cloud, the strength of the positive and negative charges, and the electric field around them, isn’t enough to create a spark or a bolt of lightning. Dr. Joe Dwyer of Florida Tech has addressed this issue by saying: “The problem is after decades and decades of measurements up in thunderstorms, nobody has ever managed to find an electric field anywhere near that big (to create an electric spark).” If this is the case, where does the extra energy come from needed to create an electric spark or a bolt of lightning?

A New Theory: Runaway Theory

As previously explained, the triggered lightning created at Florida Tech allows physicists to test new hypothesizes. One such theory called runaway breakdown, explains how a thundercloud gains extra energy needed in order to create an electric spark or a lightning bolt. Using the runaway breakdown theory, scientists can make a new model of how lightning is initiated. This model states that the energy inside of a thundercloud, that force of the positive and negative particles, is too weak to generate a spark to initiate lightning. Therefore, the thundercloud must be struck by outside particles. These outside particles are burst of electrons that carry very high energy. With this added energy, a spark can be generated to initiate lightning. Dr. Joe Dwyer describes this process by saying, “You end up with an avalanche of electrons moving near the speed of light. This model will work as long as you have one fast electron to start it off. Similar to the finger that pushes the first domino to get the whole thing started.”

Where on Earth are the Outside Particles?

Through the theory of runaway breakdown, we now know that thunderclouds are struck by outside particles to generate the energy needed to initiate lightning. However, what we don’t know is where these outside particles of a thunderstorm come from. This is where things become interesting. Do they come from molecules splitting apart in our atmosphere? Or do they come from free electrons floating around in the air that collide with a thundercloud? Scientist and researches like Dr. Joe Dwyer, believe that these outsides particles don’t come from the clouds above, or anywhere else on earth for that matter; but from cosmic rays. These cosmic rays are tiny, sub-atomic particles that are ejected from dying stars millions of years ago, and billions of years ago. However, the origin of these cosmic rays is just an idea. How do we test such an idea?

A Unique Signature

We now understand that the idea of lightning is triggered when cosmic rays strike a thundercloud and provides enough energy needed in order to initiate lightning. But how can we prove that cosmic rays, and not some phenomena on earth, provide that energy boost needed to initiate lightning? It turns out that when cosmic rays strike the Earth’s atmosphere, they leave a unique signature in the form of x-rays and gamma rays. Much like how we can test two signatures to find forgery, we can test for the unique signature of x-rays and gamma rays.

At the lightning research center, there are ten sodium iodide detectors strategically positioned so that the triggered lightning will strike them. Once lightning strikes one of these detectors, a crystal inside of it absorbs the x-rays and gamma rays. From there, it’s just a matter of the detector’s instruments measuring the cosmic rays.

Results

After the instruments measure the x-rays and gamma rays, the data is transferred to a computer where it can be viewed. After collecting and viewing the first data results a detector, Dr. Dwyer recalled his first thoughts:

“I actually didn’t think that we were going to see x-rays. The first plot we brought up, there was a nice, little pulse that looked just like an x-ray (figure 1), right at the time the lightning occurred. Well, that’s interesting. That’s probably just a coincidence. What’s the chance of that? So we looked at the next lightning strike, and there was even a bigger pulse (figure 2) , and the next one (figure 3), and the next one (figure 4). Every one had these pulses that looked exactly like x-rays.”

NOTE: Figures and Graphs will be posted as soon as possible


The data is plotted on a graph with the time (measured in microseconds) on the x-axis, and the signal strength (measured in volts) on the y-axis. The red line indicates the amount and magnitude of an x-ray. If a big negative voltage pulse is shown on the graph, it means that a big burst of electrons were absorbed in the detector. With a big burst of electrons detected, it concludes that cosmic rays are in the form of x-rays and gamma rays from outer space.

For instance, in figure 1 the red line has a very small pulse. This indicates that this particular lightning strike had a very low level of x-ray emission. Thus, suggesting that x-rays do not coincide with lightning. However, in figure two, the red line slightly dips and then jumps off of the graph. This strongly suggests that x-rays might coincide with lightning. In figures 3 and 4, the red line dips even more and jumps toward the top of the graph. A greater change in magnitude of the red line indicates that there are x-rays present in lightning. The results measured in figures 2,3, and 4, indicate that figure 1 was just a fluke and that x-rays are linked to the triggering of lightning.

To this date, every strike of lightning that has been measured has shown the presence of x-rays. Therefore, proving that the cosmic rays that provide the energy needed to initiate lightning, are in the form of x-rays and gamma rays from dying stars in space.

Conclusion

As you can see, our knowledge of distant cosmos, millions of miles away, has certainly helped us answer questions about lightning that propagates 6 miles above our heads. Even though they’re far apart from each other, lightning and space might be related to each other. In fact, Dr. Dwyer said, “These cosmic rays might be the link which will connect a dying star, halfway across the galaxy, with lightning.”

Hopefully, when future generations of humans walk on earth, they’ll ponder about the beauty of lightning like the first humans did. But thanks to Florida Tech, the University of Florida, and New Mexico Tech, they’ll be able to answer the question of how lightning is initiated with one phrase: runaway breakdown.

Through the Runaway Breakdown theory, we can conclude that lightning isn’t initiated by the particle discharge within a thundercloud or some strange phenomena on earth. Lightning is initiated by cosmic rays from space that strike a thundercloud and provides enough energy to generate an electric spark, which in effect creates a bolt of lightning.


1 comment:

alexh said...

As near as I can tell, most types of natural lightning start with a stepped
leader from cloud to ground (typically 20 mSec), followed by a return stroke
from the ground to the cloud (typically 60 uSec). Once a plasma channel is
established, multiple strokes follow at about 40mS intervals, consisting of
dart leaders (2 mSec) and return strokes (60 uSec).



Most current theories of lightning explain it as the result of a buildup of
negative charge (excess of electrons) at the bottom of a storm cloud, which
causes a positive charge of the earth below, resulting in a sudden current
flow, triggered by the development of a highly conductive plasma channel by
a sudden stepped leader strike. This raises my question: What mechanism
triggered the initial stepped leader strike in the first place.



High-speed photography seems to indicate the stepped leader appears first at
the bottom of the cloud and progress relatively slowly to the earth. One
might naturally assume the flow of electrons in the stepped leader would be
from the negatively charged cloud bottom, down toward the positively charged
earth. But one might also conclude, looking at the inverted tree appearance
of the stepped leader as it progresses toward the earth, that something is
being drawn upward into the cloud through the leader, just as streams of
water flow into rivers, which in turn empty into an ocean. This view would
suggest electrons are being drawn from the ionized air below the cloud,
until a branch of the leader contacts the earth, resulting in a sudden
reversal of electron flow, and discharging electrons stored in the cloud to
the highly positively charged earth.



But what could cause such a sudden deficit of electrons in the lower part of
the cloud that triggered the creation of the stepped ladder, that was
necessary to suck new electrons into the cloud from the atmosphere?



In the first place, it is not clear to me how a storm cloud develops and
maintains such a large positive charge on it's top, and a corresponding
negative charge at the bottom, but that is what experimental data show.
Second, I'll assume positrons and electrons can be created by cosmic rays in
a storm cloud, just as in a Wilson cloud chamber in the lab. From these
assumptions one might posit that there could large numbers of positrons and
electrons in the great Wilson cloud chambers overhead during a storm.

Third, somehow great numbers of these electrons and positrons are prevented
from immediate interaction within the cloud by some unknown process.

Accumulation of these plus particles at top and minus particles at the bottom of the cloud continues
until a positron-electron-photon chain reaction occurs within the cloud
(trigger unknown).

The result of such a reaction would be an immense gamma ray pulse, a large
release of energy and a sudden decrease of cloud charge (greatly decreasing
cloud to earth electric field).

As far as I can tell, all of these effects have already been observed:

Gamma ray pulses associated with lightning have been detected by satellites,
and were originally suspected to be the result of illegal nuclear testing.

Within, and between cloud "lightning" (or positron-electron annihilation?)
is common.

The sudden decrease of cloud charge as posited above is the trigger for
ordinary cloud-to-earth lightning (not cosmic rays as you suggest).

What do you think?

This is science !

When you are speaking to technically illiterate people you must resort to the plausible falsehood instead of the difficult truth.

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Astro-photographer? Send your photos to pics@exploreuniverse.com and have them featured on this blog with your name. Comet Mcnaught : Pictures taken with Nikon D100 on 19/1/07 from Manning Point, northern NSW, Australia by Mr. Peter Enright.
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