Re: Aircraft and lightning

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: William Beaty <billb@xxxxxxxxxx>

On Thu, 4 Aug 2005, Tesla list wrote:

>
> The one I observed was essentially silent (or, at least, you couldn't
> hear it over the relatively high background noise in the plane).

Yes!  I always suspected that aircraft-initiated lightning would be
silent, or at least not loud like ground strikes.  After all, with ground
strikes the lightning leaders aren't the source of the thunder, instead
the big "pow" occurs when the leader finally touches the earth.  For an
aircraft-initiated lightning, the leaders would launch outwards from
wingtips or tail, repeatedly branching as they "explore" the high-field
regions of the falling charged raindrops.  The current at the aircraft
should ramp smoothly upwards, very different than with a ground strike.

> It struck near the end of the wing.  I have no idea if the stroke
> exited somewhere else.

If it was long at all, it would have to exit.  It's like letting a plasma
streamer strike any metal object: the object is a conductor and the plasma
is a conductor, so for objects which are very large relative to the
streamer, another streamer might not break out again from a second place
(the metal object would behave as a capacitor plate.)  But for long
streamers and small objects, the metal object acts as a short segment of
the long streamer; the object is in series with the long streamer.

>From lightning studies where they flew military fighters through
thunderstorms, they saw the lightning leaders start at sharp points on the
airframe, then be dragged backwards by the relative wind.

> As far as the sound of lightning "up close and personal" goes, the
> three times I've been near a stroke, it's sounded more like a very
> loud crack rather than a boom.  More precisely, it sounds like "hiss,
> buzz, crackle, BANG" as the field comes up before hand.  I'm not sure
> what the buzz comes from,

Ooo, I wonder if that's the steps in the stepped leader.  Or rather, the
electrophonic sound broadcast by the stepped leader The sudden changes in
the shape of the network of conductive plasma filaments would create
sudden changes in the surrounding e-field.  If the vegetation on the
ground and the falling rain near the ground is highly charged (it is,)
then the sudden changes in the leader and in the e-fields would cause
sudden jerks in the grass, shrubs, and raindrops.  If the steps in the
stepped leader happen at, say, 10Hz, you'd hear a loud buzz coming from
everywhere around you.   (I wonder if the frequency of the buzz would
increase just before the lightning makes ground contact?)


> but it's been there all three times, and
> it's very distinct from the usual corona hiss and crackle we're all
> familiar with.  Someone suggested that it's some sort of relaxation
> oscillation

I haven't seen anyone try to explain the steps in the stepped leader.  I
always assumed it was a relaxtion oscillator where the plasma filaments
act as resistors, and the plasma filaments with surrounding e-fields also
act as capacitor plates.  If a plasma filament grows too fast, then as the
capacitance increases, the charge on the new portion of plasma would need
to increase in order to keep the voltage about constant...  but if the
resistance of the long plasma filament is too high, then instead the
voltage on the growing plasma filament would fall until the filament stops
growing.  But the charges keep redistributing themselves through the
resistor, so once the plasma has stopped lengthening, the voltage will
rise again...  and the plasma would suddenly launch itself outwards as a
longer filament.   It would be like an NE-2 RC oscillator, but where the
resistor and capacitor is also made of plasma.

 as a metal object in the field charges up and discharges
> with a streamer.  The whole time sequence is maybe 3 or 4 seconds.
> Except for the hiss and buzz, which can last a lot longer, and occur
> whenever the field is high, even without a long stroke, and is really
> really creepy, because you just know something big might happen.  You
> also get to feel your hair standing on end.

BTW, "electrophonic" sound should be cool to play with.  If you can create
a high voltage music signal on a thick metal plate, you should hear
nothing.  But if you hold a charged balloon near that plate, suddenly the
balloon starts playing music.    And suppose you used a "Zerostat" gun to
squirt some ions onto the hair and ear-whiskers of a nearby person.
Suddenly that person can hear the music, since their charged hair acts as
a transducer to change the e-field AC signals into sound waves.  But
bystanders who lack the charged hair hear nothing.   This would almost be
like the tinfoil-hat topic of "microwave-beam speech broadcast into your
head."   But in this case tinfoil hats would not shield it.


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William J. Beaty                            SCIENCE HOBBYIST website
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