MAKES ME WONDER ABOUT THE
VALIDITY OF ELEMENTALS (SPRITES, PIXIES, KNOMES, BROWNIES, SALAMANDERS,
FAIRIES, SYLPHS, UNDINES)
Atmospheric sprites have been
known for nearly a century, but their origins were a mystery. Now, a team of
researchers has evidence that sprites form at plasma irregularities and may be
useful in remote sensing of the lower ionosphere.
"We are trying to understand
the origins of this phenomenon," said Victor Pasko, professor of
electrical engineering, Penn State. "We would like to know how sprites are
initiated and how they develop."
Sprites are an optical
phenomenon that occur above thunderstorms in the D region of the ionosphere,
the area of the atmosphere just above the dense lower atmosphere, about 37 to
56 miles above the Earth. The ionosphere is important because it facilitates
the long distance radio communication and any disturbances in the ionosphere
can affect radio transmission.
"In high-speed videos we
can see the dynamics of sprite formation and then use that information to model
and to reproduce the dynamics," said Jianqi Qin, Penn State.
Sprites occur above
thunderstorms, but thunderstorms, while necessary for the appearance of a
sprite, are not sufficient to initiate sprites. All thunderstorms and lightning
strikes do not produce sprites. Recent modeling studies show that plasma
irregularities in the ionosphere are a necessary condition for the initiation
of sprite streamers, but no solid proof of those irregularities existed.
The researchers studied video
observations of sprites, developed a model of how sprites evolve and disappear,
and tested the model to see if they could recreate sprite-forming conditions.
Sprites resemble reddish
orange jellyfish with bluish filamentary tendrils hanging down below. Careful
examination of videos of sprites forming showed that their downward hanging
filaments form much more rapidly than in the horizontal spread, leading the
researchers to suggest that localized plasma irregularities cause the streamers
to propagate.
Researchers used a
two-dimensional cylindrical symmetric plasma fluid model, a mathematical model
of the ionization movements in the sprite, to study sprite dynamics. They then
used the model to recreate optical sprite creation. From this recreation, the
researchers determined where the sprite streamers originated, and they could
estimate the size of the plasma irregularity.
Further analysis suggested
some potential causes of these plasma irregularities. The most obvious seems to
be the existence in that area of a previous sprite. For the sprites examined,
there were no previous sprites in that area that occurred close enough in time,
unless there were long-lasting irregularities. However, the researchers are
unsure how such long-lasting events could occur.
Another possible source for
the irregularities is meteor events. The D region of the ionosphere is in the
upper part of the atmosphere where most meteors can exist, because once they
enter the denser, lower atmosphere they burn up due to atmospheric friction.
"This technique can be
used for remote sensing in the ionosphere as well," said Pasko.
"Using high speed videos and fluid models we may be able to see other
things that go on in the ionosphere and better understand the effects of
various natural phenomena on very low frequency radio communications."
