Andrew Yee
February 22nd 06, 06:39 PM
News Service
Stanford University
Stanford, California
Contact:
Dawn Levy, News Service
(650) 725-1944
Comment:
Umran S. Inan, Electrical Engineering
(650) 723-4994 office
February 17, 2006
Rare gamma-ray flare from a distant star disturbs Earth's daytime
ionosphere
By Dawn Levy
On Dec. 27, 2004, scientists detected the largest gamma-ray burst ever
recorded. It came from a magnetar -- a neutron star with an enormous
magnetic field -- 50,000 light years away. Its powerful rays penetrated
deep into the ionosphere, the electrically conductive layer encircling
Earth. On Feb. 19 in St. Louis at the annual meeting of the American
Association for the Advancement of Science (AAAS), Stanford electrical
engineering Professor Umran Inan will describe what scientists learned
from this rare and dramatic atmospheric disturbance.
"Enormous gamma-ray flares -- such as this giant flare from magnetar SGR
1806-20 -- affect our lower ionosphere to such a massive degree that by
simply watching and measuring its response to and recovery from the flare,
we are bound to learn more about the dynamics of these upper atmospheric
regions, which are ultimately so important for our quantitative
understanding of space weather, as well as communication and navigation
systems," said Inan in a recent interview. With more than 250 technical
publications, he is a pioneer in the discovery of atmospheric electrical
phenomena known as "elves" (horizontally expanding discharges at high
altitudes), "red sprites" (diffuse blobs that begin at the base of the
ionosphere) and "blue jets" (branches that shoot up from cloud tops).
His talk is part of a symposium, "A Giant Flare from a Magnetar: Blitzing
the Earth from Across the Galaxy," that includes astrophysicists Kevin
Hurley of the University of California-Berkeley, David Palmer of Los
Alamos National Laboratory, Bryan Gaensler of the Harvard-Smithsonian
Center for Astrophysics and Lynn Cominsky of Sonoma State University.
For the astrophysicists, the colossal flare is a window into the workings
of a neutron star. They observed the gamma-ray flare using two orbiting
spacecrafts and will use new knowledge about the event to hone their
theories about these distant objects.
Inan, in contrast, employs Earth-based equipment to measure
very-low-frequency (VLF) radio waves that remotely detect ionospheric
effects produced by lightning discharges, including precipitation of
high-energy electrons from the Van Allen belts, and luminous high-altitude
discharges such as elves, sprites or jets. He and his VLF research group
in Stanford's Space, Telecommunications and Radioscience Laboratory
(a.k.a. STAR Lab) continuously monitor the ionosphere for localized
effects. They didn't expect to see the massive effect the flare had on the
ionosphere -- illuminating an entire half of the global ionosphere -- but
their vigilance enabled them to capture it nonetheless. "In the course of
our studies of ionospheric effects produced by lightning of this type --
sprites, elves and electron precipitation -- this effect basically fell on
our lap," Inan said.
Probing the 'ignore-o-sphere'
Solar wind is an important component of space weather. When the Sun acts
up through flares and coronal mass ejections, it transmits streaming
energetic particles toward the Earth's magnetosphere, greatly increasing
the fluxes of energetic electrons trapped in the Van Allen radiation
belts, and also causing large changes in the Earth's ionosphere. The
ionosphere is the highest region of the upper atmosphere, which is
maintained by ionization of neutral air by solar photons and cosmic rays.
When cosmic rays hit the two nitrogen atoms bound together in a nitrogen
gas molecule, the molecular components separate into positively charged
nitrogen atoms and negatively charged electrons.
"At higher altitudes, there isn't enough air for ionized molecules to
combine and become neutral again, so the region stays ionized," Inan said.
"That's what the ionosphere is."
It's 60 to 90 kilometers up, where the atmospheric drag is too great for
satellites to orbit. At the same time, the air is too thin for aircraft or
research balloons. Ionization is too weak to provide detectable echoes for
even the largest radars.
"This region has been called the 'ignore-o-sphere' because it isn't an
easy region to measure," Inan said. "The VLF technique that we have
developed is particularly suitable for looking at this altitude range,
which is not otherwise measurable."
The Van Allen belts consist of energetic electrons trapped in the Earth's
magnetic field, which extends into space and shields the Earth from cosmic
radiation.
"One of the things that lightning does is to remove electrons from the
radiation belts," Inan explained. Without electron removal, the belts
would become more and more intense, he said.
When lightning flashes on the Earth's surface, it launches electromagnetic
waves up to the Van Allen radiation belts. En route, the electromagnetic
waves interact with energetic electrons trapped along the magnetic field
lines of the Earth in the Van Allen belts and scatter these electrons into
the ionosphere. Interactions between the electromagnetic waves change the
energy and direction of momentum of the electrons, causing them to
precipitate from the belts as a result of energy input by lightning
discharges. The precipitating electrons in turn produce patches of
enhanced electricity in the ionosphere.
Scientists detect these localized disturbances with VLF radio waves
propagating along the Earth's surface. The ionosphere, like a metal, is a
good electrical conductor. It acts as a guide for radio waves. That's why
the Earth's curvature is no barrier, as VLF radio waves bounce off the
ionosphere and can propagate to long distances around the globe, in the
so-called Earth-ionosphere wave guide.
"[Guglielmo] Marconi discovered global communication because of
reflections from the ionosphere," Inan explained. "Back at the turn of the
century, he sent signals from England to the United States by reflecting
them from the ionosphere."
Inan and his colleagues record VLF radio waves that propagate from one or
more transmitters on the Earth's surface to 25 receivers whose locations
include Japan, France, Israel, Greece, Turkey, Hawaii, Midway and
Kwajalein islands, the continental United States, Alaska and Antarctica.
The transmitters launch waves that propagate in the wave guide formed
between the surface of the Earth and the ionosphere.
When lightning strikes, it launches an electromagnetic signal that
propagates through the ionosphere to the Van Allen belts, and can
propagate from one hemisphere to another along the Earth's magnetic field
lines. When played through a loudspeaker, these signals have a distinct
sound and are called whistlers.
At night, lightning has a notable effect in causing ionization at
altitudes of 60 to 90 kilometers, as a result of precipitation of
energetic electrons by whistler waves. During the day, however, solar
ionization drowns out the effects of lightning-induced electron
precipitation and renders them negligible.
For 2007, "The International Heliospheric Year," the United Nations has
launched an initiative aimed at deployment of Inan's inexpensive receivers
in every member country so even scientists and students in developing
nations can host receivers, post data and access this rich data set.
Efforts are currently under way for fundraising from private foundations
to facilitate this global educational and outreach effort.
Flares hit Earth in 1998 and 2004
In a 1999 issue of the journal Geophysical Review Letters, Inan and his
STAR Lab colleagues reported the ionospheric effects of a giant gamma-ray
flare from another star. It occurred on Aug. 27, 1998, in the middle of
the night (as recorded at Stanford in the Pacific Daylight Time zone), but
it ionized the atmosphere to levels usually found only during daytime.
Like a lighthouse whose spinning beam hits a specific point on shore at
regular intervals, this neutron star had a periodicity. It spewed gamma
rays every 5.16 seconds. "We observed the ionosphere respond to that,"
Inan said. "The ionosphere was in fact pulsating at night."
The star responsible for the 2004 burst was about the same distance as the
star responsible for the 1998 burst but was within 5 degrees of the Sun as
viewed from Earth. Therefore its gamma rays arrived on the day side of our
planet. Neither star's gamma rays reached the Earth's surface, according
to Inan. Neither flare posed a danger to people, he said.
"The amazing part for the new [daytime] event is even during daytime, even
in this solar-illuminated ionosphere, the effect of the flare was huge,"
said Inan. "It was much, much more intense than the Sun in terms of
producing ionization."
Scientists didn't observe the ionosphere pulsating with the 2004 burst,
although they did see that the gamma rays arrived in pulses. "Because the
gamma rays were on the solar, day side of the ionosphere, we didn't see
the periodicity," Inan said. "We saw a massive effect that created new
ionization." The pulsing was at lower levels than the initial peak and was
drowned out by solar ionization, he said.
More powerful and brighter than the nighttime flare, the daytime flare
pumped 1,000 times as much energy into the atmosphere, Inan said. "There's
nothing like this [the magnetar that delivered flares in 2004], I
understand from my astrophysics colleagues, in our part of the woods -- in
other words, near our galaxy," Inan said. If there was, he says, we would
be inundated with gamma rays, which are high-energy X-rays from which the
atmosphere shields us by creating ionization. "If the flare was intense
enough, then it would penetrate -- the atmosphere couldn't hold it."
The 2004 flare was brighter and more energetic than the Sun but lasted for
just a brief period. It ionized the atmosphere down to an altitude of 20
kilometers (about 50,000 feet), just above where airplanes fly. (Solar
photoionization is not effective at such low altitudes because the
atmosphere is too thick, Inan said.) Its most intense effects in ionizing
the atmosphere (called the "peak") lasted a few seconds. The
second-most-intense effects (the "oscillating tail") lasted five minutes.
And the least intense effects (the "afterglow") lasted an hour.
The flare changed the ionic density at an altitude of 60 kilometers from
0.1 to 10,000 free electrons per cubic foot -- an increase of six orders
of magnitude. Normally, it takes hundreds of seconds for the ionosphere to
recover from the electromagnetic waves launched by lightning.
"The remarkable thing is that it took an hour for it to come back from
this disturbance," Inan said. "It's a very unusual event and was three
orders of magnitude more intense than the [1998] one, which we thought was
very intense."
Of detectors, both satellite and planetary
In 1988, the journal Nature published a report of the first observed
effect of a gamma-ray burst on the ionosphere by Inan and Gerald Fishman
of NASA's Marshall Space Flight Center. On Dec. 27, 2004, it was Fishman
who contacted Inan to tell him that a satellite had detected a colossal
burst. The satellite's detector was designed to identify high-energy
X-rays and gamma rays from the Sun. That day, the detector counted a huge
amount of gamma rays, became saturated and stopped counting. When
impinging gamma rays from the flare began to wane, the detector began to
count again.
During the period the detector wasn't counting, the astrophysicists had no
data. But Inan's group, continuously monitoring VLF waves propagating
across the planet to measure the ionosphere, did, and therefore had data
to share. "Our response continued because the Earth of course didn't
saturate. The Earth is much too large a detector to saturate," Inan said.
The 1998 gamma-ray flare had a low-energy component, Inan said. While
satellite detectors look for high-energy rays (20keV and above), the
earthly VLF system could detect low-energy rays as well.
"Our modeling told us that without presuming a low-energy component that
was in place that was missed by our colleagues in the spacecraft
measurements, we couldn't explain the ionospheric effect," Inan said.
"That's not the case for this new event. For this new event, we are able
to explain the ionospheric disturbance using the fluxes and energies that
people have measured on spacecraft. So this particular flare might be
different from the previous flare in terms of its energy content. Not in
terms of its intensity -- which has to do with the number and energy of
photons -- because we know that this new one is much more intense; but in
terms of the energy of photons, the previous one may have had low-energy
photons as well as higher-energy photons."
The scientists also saw for the first time a phenomenon -- an intense,
short-burst (less than a second), low-frequency signal -- that they don't
yet understand. To better understand the phenomenon, Inan will model the
ionosphere and make a wish on a star -- but it's a star rarer than a blue
moon: "We are going to see whether we can get an effect like this
theoretically. But we don't have it yet. And of course another event would
be very useful."
-30-
Editor Note: Inan is speaking Sunday, Feb. 19, in a session titled "A
Giant Flare from a Magnetar: Blitzing the Earth from Across the Galaxy"
that runs from 10:30 a.m. to noon Central Time in the America's Center,
Level Two, Room 227. The title of his talk is "Ionospheric Disturbances."
Photos of Inan are available on the web at
http://newsphotos.stanford.edu/Inan_horiz.jpg (1.1MB)
http://newsphotos.stanford.edu/Inan_vert.jpg (1.4MB)
Relevant Web URLs:
* Umran S. Inan
http://www-star.stanford.edu/starlab_web_20030912/people/inan.html
* Very Low Frequency Group
http://www-star.stanford.edu/~vlf/
* STAR Lab
http://www-star.stanford.edu/
* Listen to a whistler
http://www.stanford.edu/dept/news/pr/2006/service.stanford.edu/news/2006/february22/videos/whistler.html
Stanford University
Stanford, California
Contact:
Dawn Levy, News Service
(650) 725-1944
Comment:
Umran S. Inan, Electrical Engineering
(650) 723-4994 office
February 17, 2006
Rare gamma-ray flare from a distant star disturbs Earth's daytime
ionosphere
By Dawn Levy
On Dec. 27, 2004, scientists detected the largest gamma-ray burst ever
recorded. It came from a magnetar -- a neutron star with an enormous
magnetic field -- 50,000 light years away. Its powerful rays penetrated
deep into the ionosphere, the electrically conductive layer encircling
Earth. On Feb. 19 in St. Louis at the annual meeting of the American
Association for the Advancement of Science (AAAS), Stanford electrical
engineering Professor Umran Inan will describe what scientists learned
from this rare and dramatic atmospheric disturbance.
"Enormous gamma-ray flares -- such as this giant flare from magnetar SGR
1806-20 -- affect our lower ionosphere to such a massive degree that by
simply watching and measuring its response to and recovery from the flare,
we are bound to learn more about the dynamics of these upper atmospheric
regions, which are ultimately so important for our quantitative
understanding of space weather, as well as communication and navigation
systems," said Inan in a recent interview. With more than 250 technical
publications, he is a pioneer in the discovery of atmospheric electrical
phenomena known as "elves" (horizontally expanding discharges at high
altitudes), "red sprites" (diffuse blobs that begin at the base of the
ionosphere) and "blue jets" (branches that shoot up from cloud tops).
His talk is part of a symposium, "A Giant Flare from a Magnetar: Blitzing
the Earth from Across the Galaxy," that includes astrophysicists Kevin
Hurley of the University of California-Berkeley, David Palmer of Los
Alamos National Laboratory, Bryan Gaensler of the Harvard-Smithsonian
Center for Astrophysics and Lynn Cominsky of Sonoma State University.
For the astrophysicists, the colossal flare is a window into the workings
of a neutron star. They observed the gamma-ray flare using two orbiting
spacecrafts and will use new knowledge about the event to hone their
theories about these distant objects.
Inan, in contrast, employs Earth-based equipment to measure
very-low-frequency (VLF) radio waves that remotely detect ionospheric
effects produced by lightning discharges, including precipitation of
high-energy electrons from the Van Allen belts, and luminous high-altitude
discharges such as elves, sprites or jets. He and his VLF research group
in Stanford's Space, Telecommunications and Radioscience Laboratory
(a.k.a. STAR Lab) continuously monitor the ionosphere for localized
effects. They didn't expect to see the massive effect the flare had on the
ionosphere -- illuminating an entire half of the global ionosphere -- but
their vigilance enabled them to capture it nonetheless. "In the course of
our studies of ionospheric effects produced by lightning of this type --
sprites, elves and electron precipitation -- this effect basically fell on
our lap," Inan said.
Probing the 'ignore-o-sphere'
Solar wind is an important component of space weather. When the Sun acts
up through flares and coronal mass ejections, it transmits streaming
energetic particles toward the Earth's magnetosphere, greatly increasing
the fluxes of energetic electrons trapped in the Van Allen radiation
belts, and also causing large changes in the Earth's ionosphere. The
ionosphere is the highest region of the upper atmosphere, which is
maintained by ionization of neutral air by solar photons and cosmic rays.
When cosmic rays hit the two nitrogen atoms bound together in a nitrogen
gas molecule, the molecular components separate into positively charged
nitrogen atoms and negatively charged electrons.
"At higher altitudes, there isn't enough air for ionized molecules to
combine and become neutral again, so the region stays ionized," Inan said.
"That's what the ionosphere is."
It's 60 to 90 kilometers up, where the atmospheric drag is too great for
satellites to orbit. At the same time, the air is too thin for aircraft or
research balloons. Ionization is too weak to provide detectable echoes for
even the largest radars.
"This region has been called the 'ignore-o-sphere' because it isn't an
easy region to measure," Inan said. "The VLF technique that we have
developed is particularly suitable for looking at this altitude range,
which is not otherwise measurable."
The Van Allen belts consist of energetic electrons trapped in the Earth's
magnetic field, which extends into space and shields the Earth from cosmic
radiation.
"One of the things that lightning does is to remove electrons from the
radiation belts," Inan explained. Without electron removal, the belts
would become more and more intense, he said.
When lightning flashes on the Earth's surface, it launches electromagnetic
waves up to the Van Allen radiation belts. En route, the electromagnetic
waves interact with energetic electrons trapped along the magnetic field
lines of the Earth in the Van Allen belts and scatter these electrons into
the ionosphere. Interactions between the electromagnetic waves change the
energy and direction of momentum of the electrons, causing them to
precipitate from the belts as a result of energy input by lightning
discharges. The precipitating electrons in turn produce patches of
enhanced electricity in the ionosphere.
Scientists detect these localized disturbances with VLF radio waves
propagating along the Earth's surface. The ionosphere, like a metal, is a
good electrical conductor. It acts as a guide for radio waves. That's why
the Earth's curvature is no barrier, as VLF radio waves bounce off the
ionosphere and can propagate to long distances around the globe, in the
so-called Earth-ionosphere wave guide.
"[Guglielmo] Marconi discovered global communication because of
reflections from the ionosphere," Inan explained. "Back at the turn of the
century, he sent signals from England to the United States by reflecting
them from the ionosphere."
Inan and his colleagues record VLF radio waves that propagate from one or
more transmitters on the Earth's surface to 25 receivers whose locations
include Japan, France, Israel, Greece, Turkey, Hawaii, Midway and
Kwajalein islands, the continental United States, Alaska and Antarctica.
The transmitters launch waves that propagate in the wave guide formed
between the surface of the Earth and the ionosphere.
When lightning strikes, it launches an electromagnetic signal that
propagates through the ionosphere to the Van Allen belts, and can
propagate from one hemisphere to another along the Earth's magnetic field
lines. When played through a loudspeaker, these signals have a distinct
sound and are called whistlers.
At night, lightning has a notable effect in causing ionization at
altitudes of 60 to 90 kilometers, as a result of precipitation of
energetic electrons by whistler waves. During the day, however, solar
ionization drowns out the effects of lightning-induced electron
precipitation and renders them negligible.
For 2007, "The International Heliospheric Year," the United Nations has
launched an initiative aimed at deployment of Inan's inexpensive receivers
in every member country so even scientists and students in developing
nations can host receivers, post data and access this rich data set.
Efforts are currently under way for fundraising from private foundations
to facilitate this global educational and outreach effort.
Flares hit Earth in 1998 and 2004
In a 1999 issue of the journal Geophysical Review Letters, Inan and his
STAR Lab colleagues reported the ionospheric effects of a giant gamma-ray
flare from another star. It occurred on Aug. 27, 1998, in the middle of
the night (as recorded at Stanford in the Pacific Daylight Time zone), but
it ionized the atmosphere to levels usually found only during daytime.
Like a lighthouse whose spinning beam hits a specific point on shore at
regular intervals, this neutron star had a periodicity. It spewed gamma
rays every 5.16 seconds. "We observed the ionosphere respond to that,"
Inan said. "The ionosphere was in fact pulsating at night."
The star responsible for the 2004 burst was about the same distance as the
star responsible for the 1998 burst but was within 5 degrees of the Sun as
viewed from Earth. Therefore its gamma rays arrived on the day side of our
planet. Neither star's gamma rays reached the Earth's surface, according
to Inan. Neither flare posed a danger to people, he said.
"The amazing part for the new [daytime] event is even during daytime, even
in this solar-illuminated ionosphere, the effect of the flare was huge,"
said Inan. "It was much, much more intense than the Sun in terms of
producing ionization."
Scientists didn't observe the ionosphere pulsating with the 2004 burst,
although they did see that the gamma rays arrived in pulses. "Because the
gamma rays were on the solar, day side of the ionosphere, we didn't see
the periodicity," Inan said. "We saw a massive effect that created new
ionization." The pulsing was at lower levels than the initial peak and was
drowned out by solar ionization, he said.
More powerful and brighter than the nighttime flare, the daytime flare
pumped 1,000 times as much energy into the atmosphere, Inan said. "There's
nothing like this [the magnetar that delivered flares in 2004], I
understand from my astrophysics colleagues, in our part of the woods -- in
other words, near our galaxy," Inan said. If there was, he says, we would
be inundated with gamma rays, which are high-energy X-rays from which the
atmosphere shields us by creating ionization. "If the flare was intense
enough, then it would penetrate -- the atmosphere couldn't hold it."
The 2004 flare was brighter and more energetic than the Sun but lasted for
just a brief period. It ionized the atmosphere down to an altitude of 20
kilometers (about 50,000 feet), just above where airplanes fly. (Solar
photoionization is not effective at such low altitudes because the
atmosphere is too thick, Inan said.) Its most intense effects in ionizing
the atmosphere (called the "peak") lasted a few seconds. The
second-most-intense effects (the "oscillating tail") lasted five minutes.
And the least intense effects (the "afterglow") lasted an hour.
The flare changed the ionic density at an altitude of 60 kilometers from
0.1 to 10,000 free electrons per cubic foot -- an increase of six orders
of magnitude. Normally, it takes hundreds of seconds for the ionosphere to
recover from the electromagnetic waves launched by lightning.
"The remarkable thing is that it took an hour for it to come back from
this disturbance," Inan said. "It's a very unusual event and was three
orders of magnitude more intense than the [1998] one, which we thought was
very intense."
Of detectors, both satellite and planetary
In 1988, the journal Nature published a report of the first observed
effect of a gamma-ray burst on the ionosphere by Inan and Gerald Fishman
of NASA's Marshall Space Flight Center. On Dec. 27, 2004, it was Fishman
who contacted Inan to tell him that a satellite had detected a colossal
burst. The satellite's detector was designed to identify high-energy
X-rays and gamma rays from the Sun. That day, the detector counted a huge
amount of gamma rays, became saturated and stopped counting. When
impinging gamma rays from the flare began to wane, the detector began to
count again.
During the period the detector wasn't counting, the astrophysicists had no
data. But Inan's group, continuously monitoring VLF waves propagating
across the planet to measure the ionosphere, did, and therefore had data
to share. "Our response continued because the Earth of course didn't
saturate. The Earth is much too large a detector to saturate," Inan said.
The 1998 gamma-ray flare had a low-energy component, Inan said. While
satellite detectors look for high-energy rays (20keV and above), the
earthly VLF system could detect low-energy rays as well.
"Our modeling told us that without presuming a low-energy component that
was in place that was missed by our colleagues in the spacecraft
measurements, we couldn't explain the ionospheric effect," Inan said.
"That's not the case for this new event. For this new event, we are able
to explain the ionospheric disturbance using the fluxes and energies that
people have measured on spacecraft. So this particular flare might be
different from the previous flare in terms of its energy content. Not in
terms of its intensity -- which has to do with the number and energy of
photons -- because we know that this new one is much more intense; but in
terms of the energy of photons, the previous one may have had low-energy
photons as well as higher-energy photons."
The scientists also saw for the first time a phenomenon -- an intense,
short-burst (less than a second), low-frequency signal -- that they don't
yet understand. To better understand the phenomenon, Inan will model the
ionosphere and make a wish on a star -- but it's a star rarer than a blue
moon: "We are going to see whether we can get an effect like this
theoretically. But we don't have it yet. And of course another event would
be very useful."
-30-
Editor Note: Inan is speaking Sunday, Feb. 19, in a session titled "A
Giant Flare from a Magnetar: Blitzing the Earth from Across the Galaxy"
that runs from 10:30 a.m. to noon Central Time in the America's Center,
Level Two, Room 227. The title of his talk is "Ionospheric Disturbances."
Photos of Inan are available on the web at
http://newsphotos.stanford.edu/Inan_horiz.jpg (1.1MB)
http://newsphotos.stanford.edu/Inan_vert.jpg (1.4MB)
Relevant Web URLs:
* Umran S. Inan
http://www-star.stanford.edu/starlab_web_20030912/people/inan.html
* Very Low Frequency Group
http://www-star.stanford.edu/~vlf/
* STAR Lab
http://www-star.stanford.edu/
* Listen to a whistler
http://www.stanford.edu/dept/news/pr/2006/service.stanford.edu/news/2006/february22/videos/whistler.html