Have you ever been on a plane during a thunderstorm that experienced a direct lightning strike? While most commercial airliner will do their best to avoid thunderclouds delivering the wrath of the atmosphere, it's estimated that every plane in the U.S. is struck more than once per year.
Large commercial planes are equipped to route the electrical current from a lightning strike so that it avoids sensitive electronics, and most passengers may not even realize that a plane has been struck when it does occur. However, the electrical current and loud clap of thunder are not all that is produced by a bolt of lightning. It's only within the past 20 years that research has confirmed that lightning also emits x-rays and gamma-rays.
One source of x-rays is normal lightning, under normal atmospheric pressure that occurs near the ground. These x-rays are measured at strengths analogous to the energy range commonly emitted by CT scanning devices used in the health care industry. Then there are gamma-rays, high energy x-rays usually seen emitted by particle accelerators, exploding stars, and black holes, that have been detected as a continuous kind of glow within clouds. Additionally, a separate class of gamma-rays, called terrestrial gamma-ray flashes or TGFs, are even more powerful, brief bursts that can be seen by spacecraft and satellites in low earth orbit. TGFs are the most energetic phenomena on the planet, and are thought to be caused by intracloud lightning (lightning that occurs between clouds). TGFs appear all over the world where there are thunderstorms, but nobody understands exactly why or how.
Scientists at the University of Alabama in Huntsville reported in a press conference today at the 2014 American Geophysical Union that they have been delving into the world of these high energy particles associated with the bursts of light, and have concluded that these TGFs can be produced by any type of storm from the "garden variety" to more extreme events. That weaker or moderate strength storms would produce TGFs was totally unexpected based on earlier theories.
While lightning strikes some 50 times per second around the globe, TGFs fire up to 1100 times per day based on data from NASA's FERMI Gamma-ray Burst Monitor (GBM). Separation of positive and negative charges between layer in the clouds leads to lighting, and sometimes when lighting does fire a surge of electrons is deflected upward. Those energetic electrons cause gamma ray flashes when they bump into other particles in the atmosphere. The TGFs that are measured by craft in low-earth orbit like NASA's FERMI GBM form between 7-9 miles high, but TGFs are likely to form at lower altitudes well. Although, because of attenuation in the atmosphere those at lower altitude are harder to detect from space, so the total numbers of TGFs are probably vastly underestimated.
In order to address this issue, researchers attempted a gutsy experiment in which they actually flew an Airbus plane into a thunderstorm. The plane was equipped with a system called ILDAS to measure various properties of lightning strikes. The plane entered the thunderstorm to measure the radiation coming from the storm at approximately 10,000 feet, and over five hours experienced extreme turbulence and more than 20 direct lightning strikes. The data from the flight corroborates previous ground-based measurements linking gamma-rays and x-rays to lightning strikes. As long as there are enough brave pilots, this type of in-storm research will be one way that the scientists can get a better understanding of lower altitude TGFs.
There are many questions remaining about how and why these high energy particles are produced by storms, and how they influence other atmospheric phenomena. What we do know now is that x-rays and gamma-rays that are detected can be tracked back to their source, the lightning, to help us learn more about the dangerous, dramatic, and mesmerizing flashes of light.