All lightning on Earth may have its roots in space, new research suggests.
Lightning flashes on Earth about 100 times per second, but what triggers lightning in thunderstorms remains mostly unknown. Especially odd is the fact that decades of analysis suggest electrical fields within thunderclouds have only a tenth or so of the strength needed to spark a lightning bolt.
More than 20 years ago, physicist Alex Gurevich at the Russian Academy of Sciences in Moscow suggested lightning might be initiated by cosmic rays from outer space. These particles strike Earth with gargantuan amounts of energy, surpassing anything the most powerful atom smashers on the planet are capable of.
When cosmic rays slam into air molecules, they can make them spit out huge numbers of electrons. This shower of electrons would collide into still more air molecules, generating more electrons. All in all, cosmic rays could each set off an avalanche of electrons, a chain reaction Gurevich calls a runaway breakdown.
However, to kindle lightning, initial calculations suggested very high-energy cosmic rays were needed. These are relatively rare — thunderclouds should each see only one a day, not enough to account for the amount of lightning occurring daily. [Electric Earth: Stunning Images of Lightning]
The answer to this mystery might lie in how thunderclouds possess vast numbers of electrically charged water droplets and ice nuggets, which Gurevich and his colleagues call "hydro meteors." In such energetic surroundings, cosmic rays 10,000 to 100,000 times less energetic than thought could generate the cascades of electrons needed for lightning. Such cosmic rays hit Earth about as often as lightning flashes on the planet.
Gurevich and his colleague Anatoly Karashtin at the Radiophysical Research Institute in Nizhny Novgorod, Russia, analyzed radio pulses from nearly 3,800 lightning strikes detected in Russia and Kazakhstan. The nature of these pulses suggests they may be created by the kind of electrons one would expect to see in the runaway breakdowns from cosmic rays.
If correct, this work "could resurrect the notion that cosmic rays are involved in lightning initiation, an idea that has been questioned in recent years," said physicist Joseph Dwyer, a professor at Florida Institute of Technology, who did not take part in this research.
To helpconfirm or refute this idea, simultaneous measurements of the showers of energetic particles produced by cosmic rays and the radio pulses from lightning are needed, Dwyer explained. "Such experiments are already being done at several places," Dwyer told OurAmazingPlanet.
Gurevich and Karashtin detailed their findings May 2 in the journal Physical Review Letters.
The shock wave from the brightest stellar explosion ever seen with the naked eye in recorded history is revealing secrets about the origins of mysterious cosmic rays.
That explosion was seen all over Earth in the spring of 1006. At its peak, supernova SN 1006, which occurred some 7,100 light-years away, was about one-quarter the brightness of the moon, bright enough to cast shadows during the day and for people to read by its light at midnight. It was seen above the southern horizon of the night sky, in the constellation Lupus, the Wolf.
Two new studies find that the shock waves from such supernovas are responsible for cosmic rays.
Cosmic rays strike Earth with giant amounts of energy dwarfing anything humans currently are capable of, and they are of growing concern as humans plan manned space missions far from the protection of Earth's atmosphere. Such radiation could, for instance, harm the brains of astronauts in deep space by accelerating the development of Alzheimer's disease. [Photos: Cosmic Rays and Supernovas]
Sladjana Nikolic, an astrophysicist at the Max Planck Institute for Astronomy in Heidelberg, Germany, and fellow researchers used the European Southern Observatory's Very Large Telescope in Chile to examine the remnant of SN 1006 in detail in 133 locations in the sky. They employed a technique called integral-field unit spectroscopy that allowed them to see both what kind of radiation the shock waves there emitted, as well as where they came from, in high-resolution. Their observations yielded a "data cube."
"The idea (of) working on something new and something you never know what to expect from is already exciting and interesting, even without any further results," Nikolic told Space.com. "The instrument we used has a high spatial resolution, an order-of-magnitude level higher than the instruments used in all previous studies of optical shock emission. Such a precision gives a more detailed look at the processes happening in the shock."
The scientists focused on the northwestern rim of the remnant, which had the brightest visible shock wave radiation. Their data suggest the presence there of protons that may be potential seeds for high-energy cosmic rays. These protons are called "suprathermal," as they are moving much quicker than expected simply from the temperature of the material.
The shells of gas from these outbursts, known as supernova remnants, travel at speeds of about 2.2 million mph (3.6 million km/h), producing shock waves that make interstellar gas glow.
"Supernova remnants are thought to be laboratories for producing cosmic rays," said Nikolic, lead author of the study unveiled Thursday.
Greg Stewart / SLAC National Accelerator Laboratory
An artist's illustration of a supernova explosion, which sends off shock waves that accelerate protons to the point that they become cosmic rays, a process called Fermi acceleration. Many details of Fermi acceleration are unknown, but data from NASA's Fermi Gamma-ray Space Telescope provide overwhelming evidence that Fermi acceleration is responsible for cosmic rays.
After a century of mystery, scientists now have the first conclusive evidence that cosmic rays come from the violent aftermaths of exploding stars, researchers say.
Cosmic rays strike Earth from every direction in space with gargantuan amounts of energy, surpassing anything the most powerful atom smashers on Earth can produce. A wide variety of cosmic rays exist, from electrons to massive atomic nuclei to antimatter, but about 90 percent are protons.
Austrian scientist Victor Hess discovered these electrically charged particles from deep space after a high-altitude balloon flight in 1912. However, despite a century of research, the origins of cosmic rays had remained a mystery.
"Cosmic rays are a significant part of the total energy content of our galaxy, but so far we have had no incontrovertible evidence (of) where they come from," said study author Stefan Funk, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University.
Cosmic ray mystery Scientists have long suspected cosmic rays were linked to the aftermaths of supernovas, the most powerful exploding stars in the universe, which are visible at the farthest edges of the cosmos. Researchers speculated that cosmic rays are accelerated gradually and over long periods of time by the shells of gas that supernovas expel, known as supernova remnants.
However, since cosmic rays have electrical charges, they get deflected by any magnetic field they encounter. Since these rays likely careened around before reaching Earth, it's challenging to prove where they were born. [8 Baffling Mysteries of Astronomy]
To help solve the mystery of cosmic ray nurseries, researchers spent four years analyzing gamma rays with the Large Area Telescope onboard NASA's Fermi Gamma-ray Space Telescope. The scientists focused on two supernova remnants, both located within the Milky Way: IC 433, which is about 5,000 light-years away in the constellation Gemini, and W44, which is about 10,000 light-years away in the constellation Aquila.
NASA / DOE / Fermi LAT Collaboration
In order to understand the origin and acceleration of cosmic ray protons, researchers used data from the Fermi Gamma-ray Space Telescope, and targeted W44 and IC 443, two supernova remnants located thousands of light years away.
"We found, for the first time, sources in the universe that accelerate protons," Funk told Space.com.
Supernova clues The shockwaves from supernovas can, in principle, accelerate protons to cosmic ray energies through a process known as Fermi acceleration. In this phenomenon, protons get trapped by magnetic fields in the fast-moving shock waves and accelerated to near the speed of light. Collisions among faster and slower protons can generate subatomic particles called neutral pions, which in turn quickly decay into gamma-ray photons, the most energetic form of light.
Unlike cosmic rays, gamma rays are not affected by magnetic fields, which means they zip out in straight lines and can be traced back to their sources. As such, the researchers looked for these gamma rays as direct evidence of cosmic ray creation.
NASA / DOE / Fermi LAT Collaboration, Chandra X-ray Observatory, ESA; Herschel / XMM-Newton
Finding evidence for the acceleration of protons has long been a key issue in the efforts to explain the origin of cosmic rays. This pair of spectra from two supernova remnants (also shown visibly with data from various satellites and wavelengths) are the "smoking gun" that researchers have been looking for.
The gamma rays from Fermi acceleration come in a distinctive range of energies. The data the scientists gathered from the supernova remnants matched the characteristic energy signature of neutral pion decay, clearly linking supernovas to cosmic rays.
"This is a 100-year-old mystery and being able to see direct evidence of the accelerated protons felt great," Funk said.
"Until now, we had only theoretical calculations and common sense to guide us in the belief that cosmic rays were generated in supernova remnants," said astrophysicist Jerry Ostriker at Columbia University, who was not involved in the study. "The direct detection of pion-decay signatures in supernova remnants closes the loop and provides dramatic observational evidence for a significant component of cosmic rays."
Although this research shows that supernovas can generate cosmic rays, it remains uncertain whether the star explosions cause most cosmic rays, or if there are other potentially more important sources for these particles, Funk said. It is also unclear how exactly supernova remnants accelerate protons, and up to what energies they can speed the particles.
"The acceleration in the shock wave is a rather slow process and happens over the lifetime of the supernova remnants," Funk said. "We would like to understand the efficiency of the acceleration in different evolutionary stages and other details of the process."
In future research, scientists could also hunt for the origins of cosmic rays of even higher energy than these protons. "To do so, one needs to use ground-based telescopes, instruments that use the interaction of gamma rays with the Earth atmosphere, such as HESS or VERITAS or the future Cherenkov Telescope Array," Funk said.
Ultra–high-energy cosmic rays, ones high both in mass and energy, "are extremely rare and therefore one needs huge detection areas," Funk added. "One such installation is the Pierre Auger Array in Argentina, and in the future people are talking about installing an instrument on the International Space Station that would look for interactions in the Earth atmosphere."
The scientists will detail their findings in Friday's issue of the journal Science, as well as at the annual meeting of the American Association for the Advancement of Science in Boston Thursday.