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.
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.
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.