February 21, 2005
New satellite observations of terrestrial
gamma-ray flashes reveal surprising features of mysterious blasts
By Tim Stephens
A particle accelerator operates in Earth's upper atmosphere
above major thunderstorms at energies comparable to some of
the most exotic environments in the universe, according to new
satellite observations of terrestrial gamma-ray flashes.
The RHESSI spacecraft studies radiation from the Sun in
the form of x-rays and gamma rays. RHESSI's detectors also
pick up gamma rays from other sources, including the Earth.
conception of lightning strikes above the clouds triggering
gamma-ray bursts. The red spark is a red sprite, blue
jets are the short sparks, and the purple flash is the
terrestrial gamma-ray flash (TGF). Video: NASA
On this map
of the world, red areas are regions that receive a
lot of lightning, as recorded by satellites, while the
flashes are individual TGFs observed by RHESSI. Note that
the TGFs occur more frequently in regions with higher
lightning probability. Video: NASA/NASDA.
Terrestrial gamma-ray flashes (TGFs) are very short blasts
of gamma rays, lasting about one millisecond, that are emitted
into space from Earth's upper atmosphere.
The gamma rays are thought to be emitted by electrons traveling
at near the speed of light when they scatter off of atoms and
decelerate in the upper atmosphere.
TGFs were first discovered in 1994 by the Burst and Transient
Source Experiment (BATSE) at the Compton Gamma-Ray Observatory.
BATSE could only detect TGFs in a special observing mode and
was limited in its ability to count them or measure their peak
energies. New observations from the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) satellite raise the maximum
recorded energy of TGFs by a factor of ten and indicate that
the Earth gives off about 50 TGFs every day, and possibly many
more. The findings were reported in the February 18 issue of
Science by a team of researchers from UCSC, UC Berkeley,
and the University of British Columbia (UBC).
"The idea that the Earth, a fairly small and tame planet,
can be an accelerator of particles to ultrarelativistic energies
is fascinating to me," said David Smith, an assistant professor
of physics at UCSC and first author of the paper. "The
energies we see are as high as those of gamma rays emitted from
black holes and neutron stars," Smith said.
The exact mechanism that accelerates the electron beams to
produce TGFs is still uncertain, he said, but it probably involves
the build-up of electric charge at the tops of thunder clouds
due to lightning discharges, resulting in a powerful electric
field between the cloudtops and the ionosphere, the outer layer
of Earth's atmosphere.
"Regardless of the exact mechanism, there is some enormous
particle accelerator in the upper atmosphere that is accelerating
electrons to these very high energies, so they emit gamma rays
when they hit the sparse atoms of the upper atmosphere,"
Smith said. "What's exciting is that we are now getting
data good enough for the theorists to really test their models."
TGFs have been correlated with lightning strikes and may be
related to visible phenomena that occur in the upper atmosphere
over thunderstorms, such as red sprites and blue jets. Just
how these various phenomena are related is a question the RHESSI
investigators plan to pursue in collaboration with other researchers
around the world, Smith said.
The Science paper presents the first analysis of RHESSI
data for TGFs. RHESSI, a NASA Small Explorer spacecraft, was
launched in 2002 to study x-rays and gamma-rays from solar flares.
But RHESSI's detectors pick up gamma rays from a variety of
sources. Smith worked with RHESSI principal investigator Robert
Lin at UC Berkeley and Christopher Barrington-Leigh, now at
UBC, to plan ways they could use the satellite for a range of
investigations in addition to studying solar flares.
Liliana Lopez, a UC Berkeley undergraduate, has been working
with Smith to analyze the RHESSI data for TGFs. The Science
paper presents the results from a search of three months of
RHESSI data, and the analysis of additional data is ongoing.
The authors estimated a global average rate of about 50 TGFs
a day, but the rate could be up to 100 times higher if, as some
models indicate, TGFs are emitted as narrowly focused beams
that would only be detected when the satellite is directly in
The duration of TGFs recorded by RHESSI ranged from 0.2 to
3.5 milliseconds. The most energetic TGF photons detected by
RHESSI were in the range of 10 to 20 million electron volts
(10-20 MeV), or about 300 times as energetic as medical x-rays.
The electrons that emitted these gamma rays would have been
traveling at 99.99 percent of the speed of light, with energies
on the order of 35 MeV.
The findings raise many interesting questions, including whether
the electrons that emit TGFs ultimately contribute to the high-energy
electrons in Earth's radiation belts, Smith said. "This
is a very interesting process involving extreme physics right
here on Earth, and if we can understand the process here it
might give us insights into similar processes in less accessible
parts of the universe."
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