Figure 1: Intensified video image from the Space Shuttle. Visible
are the Earth's limb, at least one star, the airglow region at ~100 km
altitude, and a wide (~500 km) enhancement to the airglow, overlying a
lightning-brightened cloud.In 1992, Boeck et al. reported observing a transient enhancement in the airglow layer above and coincident with some lightning activity, as viewed with a low-light video camera aboard the Space Shuttle. Figure 1 shows an image of this event, which was probably the first unambiguous recording of light from an elve. The existence of elves was actually predicted by Inan et al. [1991], shortly before it was observed.
Figure 1: Intensified video image from the Space Shuttle. Visible
are the Earth's limb, at least one star, the airglow region at ~100 km
altitude, and a wide (~500 km) enhancement to the airglow, overlying a
lightning-brightened cloud.
Since that one event, all subsequent elves until 1999 were recorded from the ground. In 1999, elves were reported from balloon and aeroplane observations.
The name "elves" first appeared in the peer-review literature in
Fukunishi et al.'s [1996] photometer measurement of an elve,
which was shortly followed by Inan et al.'s [1997] space-time
resolved measurements.
Unfortunately, without being able to resolve the time dynamics or
spatial structure of the flash, even a perceptible elve would be quite
indistinguishable to a human eye from the very common phenomenon of
"skyflash" due to distant lightning. This skyflash results from light
emitted by a thundercloud discharge; the light scatters in all
directions throughout the atmosphere and can therefore be seen (for
about one thousandth of a second) at great distances from which the
clouds themselves are well out of view below the horizon. This skyflash
can sometimes be perceived and has unfortunately been referred to as
"heat lightning," a name which leads to common misunderstanding of its
origin.
The light emitted by elves comes primarily from nitrogen molecules
(the primary constituent of our atmosphere) which have been excited by
a collision with an energized electron in the nighttime lower
ionosphere. The electrons, in turn, are energized by the
electromagnetic pulse which is launched by the huge currents which
flow (for less than a ten-thousandth of a second) in the brightest
phase of cloud-to-ground lightning. The loud static burst that you
hear in your A.M. radio (simultaneous with a lightning flash) is
evidence of this same electromagnetic pulse, whose total energy may
exceed 10 gigaWatts. Most of the electromagnetic pulse travels no
higher than the lower ionosphere, because it is absorbed in energizing
the electrons there. Only an unusually powerful lightning stroke, with
a peak current greater than 60,000 Amps, launches an electromagnetic
pulse strong enough to cause detectable emissions in elves.
The interaction of the lightning electromagnetic pulse with the lower
ionosphere can be simulated in detail by computer. By clicking on
Figure 2, you can launch an MPEG video sequence showing some features
of the lower ionosphere while it is being hit by an electromagnetic
pulse originating near the ground by a vertical lightning current.
There are two panels. Both show only the altitude range 70 km to 100
km, and they show a horizontal range of 350 km, where the horizontal
measure is the horizontal distance from the lightning. In this model,
the lightning is located (well out of view) between 0 and 10 km
altitude and at 0 km horizontal distance. In the top panel, the colour
scale shows the strength of the electric field in the electromagnetic
pulse (measured in V/m). To visualize the whole scene (rather than
just the 70 km to 100 km region), you must imagine a
spherically-shaped electromagnetic pulse expanding at the speed of
light from the lightning location. The pulse reaches the region shown
in Figure 2 only after 0.2 thousandths of a second (ms). The red
contour lines in the upper panel show the strength and location of
light that is emitted by excited nitrogen molecules. The entire
duration of the simulation is only about 1 thousandth of a second. You may
now see why the flash of light at any one point only lasts 50
millionths of a second, while the entire elve lasts almost 1
thousandth of a second: the elve consists of a rapidly expanding ring
of light emissions.
The bottom panel shows the same two-dimensional cross-section of
space, but now the colour scale indicates ionization enhancement. As
well as exciting nitrogen to produce light, the energized electrons
can ionize other neutral molecules, thus increasing the total number
of electrons (and ions) available in the lower ionosphere. The colour
scale shows the modified electron density as a fraction of the ambient
electron density. The electron density is nearly doubled in
places. These density enhancements are thought to persist for a good
fraction of a minute after an elve.
The vast majority of elves, however, have been measured from the ground, most effectively with an array photometer -- that is, a number (typically ~10) of very fast optical detectors looking at adjacent patches of the sky. We called our such array the "Flye's Eye".
Figure 4 shows the typical geometry involved in ground observation of elves. For more detailed interpretation, you may ask me, or refer to some primary literature.
Below, for comparison with Figure 2, is a similar simulation in
which the electric field is dominated not by the electromagnetic
pulse, but by the "quasi-static" electric field of a lightning stroke.
This field results from the huge excess of charge left behind in a
cloud after a lightning current removes either positive or negative
charge to ground. In this case, the location and dynamics of
emissions and ionization changes is quite different, and the
phenomenon is known as a "sprite." A lightning stroke causing sprites
has a very large amount of total charge moved, while one causing elves
has a very large instantaneous current.
A sprite may
include not just the sprite halo, modeled above, but also a filamentary
(corona streamer) discharge which may propagate to much lower
altitudes, and typically endures for several to many thousandths of a second.
Last modified: Sun Jan 3 13:00:53 PST 2001
Is it possible to see an elve with your bare eyes?
No. The brightness of a surface at a given wavelength may be measured
in Rayleighs. While an exceptional elve may be as bright as 10
million Rayleighs (much brighter than aurora) over a ~1 degree field
of view, this luminosity only occurs for about 50 millionths of a
second. Thus a human eye might receive a few thousand photons, only
some of which would be in the visible spectrum.
What colour are elves?
Elves are NOT green! If viewed from the ground, and at a great
distance, elves would appear reddish even if they were coloured white,
simply as a result of atmospheric scattering (the same reason the sun
appears red when it is low on the horizon). However, the optical
emissions constituting elves are largely red and near-infrared, with a
lesser component of near-ultraviolet. Their true colour in a visible
sense, then, is certainly red.
What causes elves?
Figure 2 (Click for MPEG video). Simulation of electromagnetic pulse
propagation, absorption, and excitation of emissions in the top panel,
and resultant ionization enhancement in the bottom panel.
How do scientists observe or measure elves?
Very occasionally, elves are bright enough to be recorded by special
image-intensified video equipment. The most remarkable such case
apart from that shown in Figure 1, as of 2000, is the image below. The
scale is not marked, but the view is looking up from an aircraft over
Europe, and the elve is likely >400 km in diameter. The central "hole"
is due to the dipole radiation pattern of the vertical lightning
current.
Figure 3:
An unusual image of an "elve" captured by M.J. Taylor
and L.C. Gardner of the Space Dynamics Lab., Utah
State University, during the night of the Leonids storm
at 02:10:00 UT, 1998. Click on the image for a full
size version.
Figure 4: Geometry of ground-based observation of elves.How do elves relate to sprites?
Figure 5 (Click for MPEG video). Simulation of a sprite halo electric
field, optical emissions, and ionization enhancement.
Why were elves and sprite halos confused?
The answer to this question is dealt with in great detail by a paper
published in January, 2001. Video instruments with 17 or 33 ms time
resolution are poorly suited to resolving sprite halos, which
typically last ~1 ms, and are extremely poorly suited to resolving
elves. For years, any diffuse "halo" seen in video data atop a
filamentary sprite was assumed to be the signature of an elve, even
though it had a horizontal spatial scale blatantly incompatible with
an elve. In late 1999, this mistake became clear during analysis of
some higher speed (0.3 ms resolution) video data. Now we know that
halos often occur without the development of filamentary sprite
structure (and vice versa). By the way, in my observations, any lightning
discharge which causes a bright sprite halo has also been impulsive enough
to cause an elve.
Do elves move faster than the speed of light?
Nothing with mass or energy moves faster than the speed of light.
However, the size of the luminous ring in an elve begins by growing
over 100 km in radius in a small fraction of a millisecond; thus an
elve can be said to expand faster than the speed of light.
For more detail, see What causes elves?
Do elves occur during the day, too?
No. During the day, the lower boundary of the ionosphere is much lower
than at night. As a result, the lightning electromagnetic pulse is
absorbed at a lower altitude. Because the neutral atmosphere is also
much denser at these altitudes, electrons collide with neutral
molecules too frequently for the electric field to be able to
accelerate (energize) them enough to excite optical emissions or to
cause enhanced ionization.
Are any scientific publications (primary literature) about elves
available online?
The figures and explanations provided above are derived from the
original publications reporting these new discoveries to the
scientific community. More detail on the history of elves, as well as
on almost all of these topics, is available in my PhD dissertation,
which is available for online viewing in PDF
format.
Additionally, most of the published
papers focusing on experimental observations of elves came from
our group at Stanford and are also available online.
Have elves been reported by online popular press?
Yes. Newspapers around the world have taken an interest in elves and
sprites. In addition, a number of popular
science articles were published in electronic format.
Who else is studying elves?
A number of
groups, mostly in the U.S. and Japan, contributed to our understanding
of elves in the 1990s. Searching on the web for "elves" and
"lightning" will find references to materials from some of these
efforts, but will also lead you to lots of poorly informed sources.
Let me know if you want to have your elves-related site listed here. Feel free to pose science questions regarding elves to me by email.