Tom Ashcraft article
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Richard Lowenberg
Jun 18, 2012, 5:17:06 PM6/18/12
to dis...@sfcomplex.org, David Dunn
Local artist/scientist makes news.
The edge of magical: Astronomer captures images of otherworldly red sprites
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Roger Critchlow
Jun 18, 2012, 5:34:44 PM6/18/12
to dis...@sfcomplex.org
Brilliant journalism, the picture of the sprites has the caption over the sprites in the lower left corner of the image. I didn't even see them until I got to http://www.heliotown.com/Table_of_Contents.html.
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Gary Schiltz
Jun 18, 2012, 5:38:23 PM6/18/12
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Hey, thanks for clearing that up! I kept thinking that the line in the upper right hand of the photo was a pointer to direct my attention to a single tiny spot, instead of looking underneath the caption.
;; Gary
Bruce Sherwood
Jun 18, 2012, 10:09:04 PM6/18/12
to dis...@sfcomplex.org
Yes, thanks! The actual photo is gorgeous. It would be interesting to
know from how far away the photo was taken, as sprites are found very
high above thunderstorms.
I can't resist telling about the mechanism for these wonderful
phenomena. They are a form of spark, in which something ionizes the
air, making it a conductor, so that large currents (moving charges)
can flow, The charges (electrons and positively charged ions of air
molecules and atoms) sometimes recombine, and in the process of
dropping to the ground state of the neutral object, light is emitted
that is characteristic of the molecule or atom. Sprites were predicted
many decades ago by Wilson, the inventor of the Wilson cloud chamber,
but not observed until long after the prediction had been forgotten.
The lack of observations was due in large part to the very high
altitude of sprites.
How does the ionization occur? I'm very glad you asked that. The
question can be phrased in the form, how does a spark form in air?
Suppose you bring a negatively charged metal sphere near a neutral
metal sphere. When you get close enough, you may see (and hear) a
brief spark. There are a couple of plausible explanations that are
both intuitively appealing and both very wrong, quantitatively.
1) Electrons jump directly from the negatively charged sphere to the
other sphere. This explanation fails because the mean free path of a
particle in air is very short, something like half a micron (which is
a millionth of a meter).
2) If you apply a large enough electric field, you can pull electrons
out of the air molecules, thereby ionizing the air. This changes the
air from an insulator (no mobile free charges) into a conductor. It's
not difficult to estimate the electric field strength needed to remove
an outer electron from a typical-sized atom, and it's more than 1e11
volts/meter (that's 1 times 10 to the 11). However, we observe that an
electric field of 3e6 volts/meter is sufficient to trigger a spark, so
this model fails spectacularly. You can't by minor fiddling with the
model account for 5 orders of magnitude discrepancy between theory and
experiment.
A model that works involves invaders from outer space. Suppose that
somewhere in the air there is a free electron, an electron that has
been knocked out of an air molecule (there's also a positive ion --
the molecule that lost the electron). If the electric field is large
enough that in one mean free path motion of the electron, the electron
gains more than the ionization energy of an air molecule, the electron
can knock another electron out of a molecule. Now there are two
electrons, neither with much kinetic energy. In one mean free path,
each of these electrons can knock out electrons from two molecules.
Now there are four free electrons.
You get what's aptly called an "avalanche", and suddenly this whole
region of air is full of positive ions and negative electrons. These
charged particles move due to the applied electric field. Sometimes
they recombine, and light is emitted (whose colors are characteristic
of the particular energy levels of the atoms or molecules involved).
The air is heated explosively, so you get sound as well as light.
Incidentally, because of their low mass the electrons move a lot
faster than the ions, so one typically talks only of the role of the
electrons.
With an electric field of magnitude E and an electron charge e, there
is a force eE acting through approximately one mean free path d to do
an amount of work eEd which gives the electron a kinetic energy of
eEd, which must be greater than or equal to the ionization energy.
Plugging in the numbers, this simple model predicts that the field
strength E must be greater than or equal to 3e7 volts/meter, off by an
order of magnitude from the observed value of 3e6 volts/meter.
However, this is a statistical process. Often an electron will go more
than one mean free path before colliding with an air molecule, in
which case E can be smaller because d is bigger. We can be happy that
a very simple model gives a reasonable result, and we can see why it
is too simple a model to give a good value for the critical field.
Why does the spark last only a very short time? In the two-sphere
case, electrons drift onto the originally neutral sphere, and now the
electric field between the spheres has two opposing contributions,
from the two negatively charged spheres. Eventually enough charge is
transferred that the field gets smaller than the critical 3e6
volts/meter, recombination now swamps avalanche production, and the
spark extinguishes.
Where did the free electron come from? High-energy protons hit nuclei
in air molecules at the top of the atmosphere and create a shower of
particles, most of which have short ranges because they too interact
strongly with nuclei. However, the pions can decay into muons (and
neutrinos), and muons (like their light-weight cousins the electrons)
don't interact strongly with nuclear matter, so the muons have long
ranges. The muon lifetime is only about 2 microseconds, so even if
they are moving at the speed of light (3e8 m/s) on average they'll go
only about 600 m toward the Earth. However, at high speeds their time
runs slowly compared to our clocks, and only because of this
relativistic effect lots of muons go through your body every minute.
(Another way of saying it is that for the high-speed muon, the
distance from the top of the atmosphere to the ground is greatly
shortened, so they can go that short distance in their short
lifetimes). Free electrons can also be liberated from air molecules by
particles and high-energy photons emitted by radioactive objects and
walls around you.
A really good model explains things it was not developed to explain,
and that's the case here. The failing model (2) above, direct
ionization by applying an extremely large electric field, predicts
that increasing the density of the gas would not affect the critical
field strength. What about the avalanche model? If the density is
twice as great as that of our atmosphere (at sea level), the mean free
path will be half as great, which means we'll need twice the field
strength to accelerate the electron in one mean free path enough to
give it a kinetic energy equal to the ionization energy of an air
molecule. It turns out that for air of double density you need 6e6
volts/meter instead of 3e6 volt/meter to trigger a spark. In fact,
high-density gas is used as an insulator inside high-voltage
transformer housings.
Finally, why do sprites form above thunderstorms, and why so high?
We've seen that there is a critical field to cause the ionization
(which leads to recombination and emission of light), and that the
critical field is doubled when the gas density is doubled. Conversely,
at very low gas density you don't need a big electric field to ionize
the air (because the mean free path is long). Gas discharge tubes such
as neon lights have very low-density gas inside so that it's easy to
ionize the gas. So we look for a mechanism for producing even weak
electric fields at high altitudes.
In a thunderstorm there is charge separation. If a cloud acquires
(say) a negative charge, it will drive negative charge in the ground
below away, making the ground below have a net positive charge. The
electric field above the cloud is that of an "electric dipole", in
which there is a competition between the field pointing up due to the
positive charge of the ground and a somewhat larger field pointing
down due to the closer negative charge of the cloud. This competition
results in an electric field far above the cloud that decreases with
increasing height y in proportion to 1/y^3 instead of the familiar
1/y^2 for a single charge.
In a simple model of the atmosphere the air density falls
exponentially with increasing height: e raised to the power (-mgy/kT),
where m = mass of a molecule, g = 9.8 N/kg, k = 1.38e-23, Boltzmann's
constant, and T = absolute temperature are nearly constants.
Mathematically, any exponential falls faster than any 1/y^n function;
you can always find a big enough y for which the exponential is
smaller than the 1/y^n function. Therefore you can expect that at a
great enough height above the cloud, the air density will be so small
that the electric field there is greater than the critical value, and
the air will ionize. Thus, sprites.
The mechanism for sparks is treated in detail in the textbook "Matter
& Interactions" that Ruth Chabay and I have written (volume 2, pages
866-875; see matterandinteractions.org).
Bruce
know from how far away the photo was taken, as sprites are found very
high above thunderstorms.
I can't resist telling about the mechanism for these wonderful
phenomena. They are a form of spark, in which something ionizes the
air, making it a conductor, so that large currents (moving charges)
can flow, The charges (electrons and positively charged ions of air
molecules and atoms) sometimes recombine, and in the process of
dropping to the ground state of the neutral object, light is emitted
that is characteristic of the molecule or atom. Sprites were predicted
many decades ago by Wilson, the inventor of the Wilson cloud chamber,
but not observed until long after the prediction had been forgotten.
The lack of observations was due in large part to the very high
altitude of sprites.
How does the ionization occur? I'm very glad you asked that. The
question can be phrased in the form, how does a spark form in air?
Suppose you bring a negatively charged metal sphere near a neutral
metal sphere. When you get close enough, you may see (and hear) a
brief spark. There are a couple of plausible explanations that are
both intuitively appealing and both very wrong, quantitatively.
1) Electrons jump directly from the negatively charged sphere to the
other sphere. This explanation fails because the mean free path of a
particle in air is very short, something like half a micron (which is
a millionth of a meter).
2) If you apply a large enough electric field, you can pull electrons
out of the air molecules, thereby ionizing the air. This changes the
air from an insulator (no mobile free charges) into a conductor. It's
not difficult to estimate the electric field strength needed to remove
an outer electron from a typical-sized atom, and it's more than 1e11
volts/meter (that's 1 times 10 to the 11). However, we observe that an
electric field of 3e6 volts/meter is sufficient to trigger a spark, so
this model fails spectacularly. You can't by minor fiddling with the
model account for 5 orders of magnitude discrepancy between theory and
experiment.
A model that works involves invaders from outer space. Suppose that
somewhere in the air there is a free electron, an electron that has
been knocked out of an air molecule (there's also a positive ion --
the molecule that lost the electron). If the electric field is large
enough that in one mean free path motion of the electron, the electron
gains more than the ionization energy of an air molecule, the electron
can knock another electron out of a molecule. Now there are two
electrons, neither with much kinetic energy. In one mean free path,
each of these electrons can knock out electrons from two molecules.
Now there are four free electrons.
You get what's aptly called an "avalanche", and suddenly this whole
region of air is full of positive ions and negative electrons. These
charged particles move due to the applied electric field. Sometimes
they recombine, and light is emitted (whose colors are characteristic
of the particular energy levels of the atoms or molecules involved).
The air is heated explosively, so you get sound as well as light.
Incidentally, because of their low mass the electrons move a lot
faster than the ions, so one typically talks only of the role of the
electrons.
With an electric field of magnitude E and an electron charge e, there
is a force eE acting through approximately one mean free path d to do
an amount of work eEd which gives the electron a kinetic energy of
eEd, which must be greater than or equal to the ionization energy.
Plugging in the numbers, this simple model predicts that the field
strength E must be greater than or equal to 3e7 volts/meter, off by an
order of magnitude from the observed value of 3e6 volts/meter.
However, this is a statistical process. Often an electron will go more
than one mean free path before colliding with an air molecule, in
which case E can be smaller because d is bigger. We can be happy that
a very simple model gives a reasonable result, and we can see why it
is too simple a model to give a good value for the critical field.
Why does the spark last only a very short time? In the two-sphere
case, electrons drift onto the originally neutral sphere, and now the
electric field between the spheres has two opposing contributions,
from the two negatively charged spheres. Eventually enough charge is
transferred that the field gets smaller than the critical 3e6
volts/meter, recombination now swamps avalanche production, and the
spark extinguishes.
Where did the free electron come from? High-energy protons hit nuclei
in air molecules at the top of the atmosphere and create a shower of
particles, most of which have short ranges because they too interact
strongly with nuclei. However, the pions can decay into muons (and
neutrinos), and muons (like their light-weight cousins the electrons)
don't interact strongly with nuclear matter, so the muons have long
ranges. The muon lifetime is only about 2 microseconds, so even if
they are moving at the speed of light (3e8 m/s) on average they'll go
only about 600 m toward the Earth. However, at high speeds their time
runs slowly compared to our clocks, and only because of this
relativistic effect lots of muons go through your body every minute.
(Another way of saying it is that for the high-speed muon, the
distance from the top of the atmosphere to the ground is greatly
shortened, so they can go that short distance in their short
lifetimes). Free electrons can also be liberated from air molecules by
particles and high-energy photons emitted by radioactive objects and
walls around you.
A really good model explains things it was not developed to explain,
and that's the case here. The failing model (2) above, direct
ionization by applying an extremely large electric field, predicts
that increasing the density of the gas would not affect the critical
field strength. What about the avalanche model? If the density is
twice as great as that of our atmosphere (at sea level), the mean free
path will be half as great, which means we'll need twice the field
strength to accelerate the electron in one mean free path enough to
give it a kinetic energy equal to the ionization energy of an air
molecule. It turns out that for air of double density you need 6e6
volts/meter instead of 3e6 volt/meter to trigger a spark. In fact,
high-density gas is used as an insulator inside high-voltage
transformer housings.
Finally, why do sprites form above thunderstorms, and why so high?
We've seen that there is a critical field to cause the ionization
(which leads to recombination and emission of light), and that the
critical field is doubled when the gas density is doubled. Conversely,
at very low gas density you don't need a big electric field to ionize
the air (because the mean free path is long). Gas discharge tubes such
as neon lights have very low-density gas inside so that it's easy to
ionize the gas. So we look for a mechanism for producing even weak
electric fields at high altitudes.
In a thunderstorm there is charge separation. If a cloud acquires
(say) a negative charge, it will drive negative charge in the ground
below away, making the ground below have a net positive charge. The
electric field above the cloud is that of an "electric dipole", in
which there is a competition between the field pointing up due to the
positive charge of the ground and a somewhat larger field pointing
down due to the closer negative charge of the cloud. This competition
results in an electric field far above the cloud that decreases with
increasing height y in proportion to 1/y^3 instead of the familiar
1/y^2 for a single charge.
In a simple model of the atmosphere the air density falls
exponentially with increasing height: e raised to the power (-mgy/kT),
where m = mass of a molecule, g = 9.8 N/kg, k = 1.38e-23, Boltzmann's
constant, and T = absolute temperature are nearly constants.
Mathematically, any exponential falls faster than any 1/y^n function;
you can always find a big enough y for which the exponential is
smaller than the 1/y^n function. Therefore you can expect that at a
great enough height above the cloud, the air density will be so small
that the electric field there is greater than the critical value, and
the air will ionize. Thus, sprites.
The mechanism for sparks is treated in detail in the textbook "Matter
& Interactions" that Ruth Chabay and I have written (volume 2, pages
866-875; see matterandinteractions.org).
Bruce
Bruce Sherwood
Jun 19, 2012, 6:38:52 PM6/19/12
to dis...@sfcomplex.org
I see that I omitted a sentence. The muons that plunge down through
the atmosphere are electrically charged, so in their passage they can
rip electrons out of air molecules, as can the decay products of
radioactive materials in the neighborhood.
Note too that these charged muons can cause errors when they pass
through digital devices, and they can cause errors in DNA when they
pass through your body. There are of course error-detection and
error-correction mechanisms in digital circuits and in DNA, but they
aren't perfect.
Bruce
the atmosphere are electrically charged, so in their passage they can
rip electrons out of air molecules, as can the decay products of
radioactive materials in the neighborhood.
Note too that these charged muons can cause errors when they pass
through digital devices, and they can cause errors in DNA when they
pass through your body. There are of course error-detection and
error-correction mechanisms in digital circuits and in DNA, but they
aren't perfect.
Bruce
Thomas Ashcraft
Jun 19, 2012, 7:16:01 PM6/19/12
to dis...@sfcomplex.org
Greetings from Thomas Ashcraft. (Thanks to Richard L about the pointers
to this discussion list and the thoughtful feedback on the newspaper
article.)
Bruce Sherwood asked: "......It would be interesting to know from how
here: http://www.heliotown.com/Transient_Luminous_Events_2012.html
Distances from my observatory are included in the data. I have been
imaging sprites as far away as Wichita, Kansas and east of Oklahoma City
this year. Kind of amazes me.
I am quite curious about the intricate forms of sprites. Some appear
almost "biological" in a strange and beautiful way. I am not sure the
forms are fully explained as yet.
Best regards,
Thomas
PS: I am also capturing sprites reflected at very high radio frequencies
using forward scatter radio methods. This may give clues into the plasma
"envelopes" that are not optically accessible.
http://www.heliotown.com/Radio_Sprites_Ashcraft.html
to this discussion list and the thoughtful feedback on the newspaper
article.)
Bruce Sherwood asked: "......It would be interesting to know from how
far away the photo was taken, as sprites are found very high above
thunderstorms."
I have some sample specimen movies of sprites with radio emissions
thunderstorms."
here: http://www.heliotown.com/Transient_Luminous_Events_2012.html
Distances from my observatory are included in the data. I have been
imaging sprites as far away as Wichita, Kansas and east of Oklahoma City
this year. Kind of amazes me.
I am quite curious about the intricate forms of sprites. Some appear
almost "biological" in a strange and beautiful way. I am not sure the
forms are fully explained as yet.
Best regards,
Thomas
PS: I am also capturing sprites reflected at very high radio frequencies
using forward scatter radio methods. This may give clues into the plasma
"envelopes" that are not optically accessible.
http://www.heliotown.com/Radio_Sprites_Ashcraft.html
Bruce Sherwood
Jun 19, 2012, 11:44:43 PM6/19/12
to dis...@sfcomplex.org
Thanks so much for the link to your wonderful images and videos. In
retrospect I guess I should have realized that the phenomena would be
fast, but I didn't know this.
I'm puzzled about what is meant by "2.5MHz radio
emissions".accompanying the videos. Presumably you don't have a radio
telescope whose diameter is many times the wavelength, which is about
100 meters, so you can't be making images of the sprites in anything
but the visual or near-infrared wavelengths. Can you clarify? Thanks.
Bruce
retrospect I guess I should have realized that the phenomena would be
fast, but I didn't know this.
I'm puzzled about what is meant by "2.5MHz radio
emissions".accompanying the videos. Presumably you don't have a radio
telescope whose diameter is many times the wavelength, which is about
100 meters, so you can't be making images of the sprites in anything
but the visual or near-infrared wavelengths. Can you clarify? Thanks.
Bruce
Bruce Sherwood
Jun 19, 2012, 11:45:45 PM6/19/12
to dis...@sfcomplex.org
Oh, also: You give the distances to the sprites, but from the angle
above the horizon can you estimate the approximate height of the
sprites above the ground?
Bruce
above the horizon can you estimate the approximate height of the
sprites above the ground?
Bruce
Thomas Ashcraft
Jun 20, 2012, 3:59:12 AM6/20/12
to dis...@sfcomplex.org
On 6/19/12 5:44
PM, Bruce Sherwood wrote:
Thanks so much for the link to your wonderful images and videos. In retrospect I guess I should have realized that the phenomena would be fast, but I didn't know this.
TA: Yes, transient luminous events are just a little too fast for
the average eye and nervous system to process although they can be
"glimpsed".
Bruce S: I'm puzzled about what is meant by "2.5MHz radio emissions".accompanying the videos. Presumably you don't have a radio telescope whose diameter is many times the wavelength, which is about 100 meters, so you can't be making images of the sprites in anything but the visual or near-infrared wavelengths. Can you clarify? Thanks.TA: I am working at multiple radio frequencies - ELF-VLF to VHF- and lightning emits throughout the radio spectrum. And I also use 2.5 MHz which is one of the frequencies where co-ordinated universal time signals are broadcast on radio time station WWV. It turns out that lightning also shows very well at this frequency and so I am simultaneously time-stamping with the time beeps and also audio recording and spectrographing the lightning/sprite radio emission there as well. For 2.5 MHz all I use is an old short wave radio and a simple dipole antenna and it is plenty sensitive for lightning for many hundreds of miles distant.
Bruce S: Oh, also: You give the distances to the sprites, but from the angle above the horizon can you estimate the approximate height of the sprites above the ground? TA: Well, at the moment I can only estimate heights from my single observatory but sprites - in general - extend in altitude from ~50 to 90 km. I am now correlating with a few other researchers and camera stations and we are just now beginning to get mutual captures. Just in the past week or so. Soon precise altitudes and distances will be determined by triangulation. Thomas
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