|Lightning detector in the foreground with |
a cosmic ray detector in the background.
Credit: William Hanlon, University of Utah
When you shock yourself after reaching for a metal doorknob, you’re experiencing a similar process that leads to lightning. As long as the extra charge you accumulate from, for example, rubbing your feet across a carpeted surface reaches a minimum value, called the breakdown voltage, a shock will travel from you to the doorknob.
Storm clouds can also build up extra charge, which must go somewhere. Often times it will either strike the ground or branch outward across the sky in the form of a lightning bolt. However, scientists have yet to find a way to explain how storm clouds build up enough extra charge to electrically illuminate the sky.
“The cloud has to charge to a certain amount so spontaneous discharge can happen,” said Helio Takai, a physicist at Brookhaven National Laboratory. “What [scientists] measure is not enough charge for this spontaneous discharge to happen.”
The solution might lie with cosmic rays, high-energy particles that enter Earth’s atmosphere from far-off sources, most of which are outside of our solar system.
About twenty years ago, physicist Alex Gurevich at the Russian Academy of Sciences in Moscow proposed that cosmic rays could lead to lightning. So far, finding a correlation has been easier said than done and scientists are still uncertain whether the two phenomena are linked and at what point in the process cosmic rays might be important for lightning production.
Takai is hot on the hunt with help from a team of scientists at University of Utah, New Mexico Tech and Ritsumeikan University in Kyoto, Japan. The team is using the largest cosmic ray physics experiment in the northern hemisphere, the Telescope Array, and a series of nearby lightning monitor stations to see if measurements from both will throw some light on the mystery.
Takai will be presenting the team’s results this weekend at the APS April Meeting in Savannah, Georgia.
|Panorama of the lightning detector at the left and cosmic ray detector in the middle. Credit: William Hanlon, University of Utah.|
When cosmic rays enter the atmosphere, they interact with the molecules in the air by stripping them of some of their electrons. These free electrons go on to liberate yet more electrons in what scientists call a runaway breakdown effect. The remaining ions charge up the atmosphere.
But is that extra charge enough to account for the missing amount in storm clouds that scientists need to explain lightning formation?
The answer to that question might get help from the team’s research. The lightning monitors, built by scientists at New Mexico Tech, work by detecting radio pulses of lightning discharges. If the Telescope Array site based in Utah measures a shower of high-energy particles around the same time the lightning monitoring stations detect a lightning bolt, it could point to a correlation.
Utah is not the stormiest of states, especially in Millar County which is mostly covered by the Sevier Desert and where the Telescope Array is based. Last year the team collected what measurements they could from the few storms that hit the region. That data is still being analyzed, said Takai who remains optimistic about the work.
“New Mexico Tech … have developed the coolest lightning detection system I know. They can reconstruct lightning in 3D,” Takai said. “Telescope Array is the largest cosmic ray detector on the northern hemisphere and I think this is a good opportunity to perform this kind of study.”
Takai is also interested in exploring another component of lightning. This second aspect deals with what lightning produces instead of what produces lightning.
“The second part of this research has to do with the opposite phenomena that has been observed by satellites,” he said. “In this case what was observed is an increase in high energy gamma ray activity when lightning is formed.”
Last year, Joseph Dwyer, a physicist at Florida Institute of Technology, reported an explanation for this phenomenon and the potential threat such gamma-ray flashes pose to aircraft. He expressed excitement in Takai’s work.
“Our understanding of how cosmic-rays interact with thunderstorm electric fields is not perfect, and so it is certainly worth doing such observations," Dwyer said. "I will be very interested to learn about what they have seen.”