Lightning Bolts - AC or DC?

It is DC....

I don't see how you can get an AC charge?

There is some confusion caused by which way the 'bolt' travels.

There are other things to add to the confusion... ionised gas? electrons...

You can't actually see electrons...what you see is probably the ionised gas?

I think there maybe many rapid multiple strikes...with electrons (negative) going one way and postive ions flowing the other.

This is just my vauge recolections...so any lightning buffs out there forgive any errors.

Del

My recolection is that Lightning is DC also.

It is essentially a huge static electrical charge caused by the relative movement of air currents, causing a charge to build up in the clouds.

I've seen pictures that show that the actual strike is from the ground up to the cloud to cause an ionization path down which the massive static charge from the cloud runs as a bolt of lightning.

Sailors used to see this ionization on the masts of their ships in storms, and called the aura st Elmo's fire. They thought this meant they were protected, when in fact it meant a strike was imminent!

Looks like it's more likely AC because of Re-strike:

High speed videos (examined frame-by frame) show that most lightning strikes are made up of multiple individual strokes. A typical strike is made of 3 to 4 strokes. There may be more.^[20]

Each re-strike is separated by a relatively large amount of time, typically 40 to 50 milliseconds. Re-strikes can cause a noticeable "strobe light" effect.^[20]

Each successive stroke is preceded by intermediate dart leader strokes akin to, but weaker than, the initial stepped leader. The stroke usually re-uses the discharge channel taken by the previous stroke.^{[citation needed]}

The variations in successive discharges are the result of smaller regions of charge within the cloud being depleted by successive strokes.^{[citation needed]}

The sound of thunder from a lightning strike is prolonged by successive strokes.

Nope... maybe pulsed DC... but switching DC On and Off doesn't make it AC.

Okay, how bout AOO, Alternating On, Off?

Oooh ah aye AOO

I would say DC, because as the potential difference discharges, in either direction, the momentary discharge results in momentrary direct current.

The flow of electrons would run from negative (potential-static charge) side to positive (potential-static charge) side, right? The conductor would be the air, ionized gas, moisture, i don't know.

The direction of travel depends on where the positive static charge and the negative static charge exist, and the number of discharges depends on the amount of discharges necessary to neutralize the potential difference.

As the electrons and holes of atoms in the air shift because of the great potential difference, photons are emitted with the electron energy transfer.

Please correct me if I am wrong.

-Andrew

Well, the lightning is really a spark caused by the build-up of electric charge in clouds, and in that sense, it is definitely direct current. Sparks can propagate an electromagnetic wave which "looks" like a broad spectrum of frequencies, but it just "looks" that way in the same sense that a half-wave of 1 MHz AC "looks" like DC for 500 nS and which can propagate a 1 MHz electromagnetic wave.

http://www.newton.dep.anl.gov/askasci/wea00/wea00250.htm

I would think it would be DC wih a high frequency of discharge strokes. kind of like a spark plug firing multiple times in short succession.

Check out Ewen Thompson from Florida. He studied lightning while a professor at the Miami campus of Florida Stae university . He now own a a company specilizing in lightning protection schemes. Seveal of his early paper are still posted on sail boat focused websites.

The high frequency content of a lightning strike has more to do with the harmonic rich content of a fast rise time. Crack open the books and check out the fourier series of a square pulse. Not sure about the fall time though I suspect it has a fairly fast fall time. Who wants to make that measurement?

Who wants to make that measurement?

Yeh, it's a real pig trying to hook your scope probe onto one of those suckers!

If anyone wants to play at home...get a piezo gas lighter....great for producing nasty glitches through circuit boards...

Too bad old Ben Franklin didnt have the scopes available at the time, just a Leyden Jar. I think we are all fairly much in agreement that the strikes are DC though.

Perhaps the puzzled looks aren't the result of the question, buy why?

Tesla Fathered AC, but not his Sphere.

Technically, it is an impulse voltage of unidirectional polarity (either positive or negative with respect to earth). About 90% of cloud to ground strokes are of negative polarity. A typical lightning stroke has an extremely steep wave front, which means that its voltage is rising at the rate of millions of volts per microsecond; in fact, 15% of lightning strokes crest in less than one microsecond. The steep wave front is followed by a short wave tail, which means that after the crest or peak voltage is reached, the surge voltage diminishes to half crest value in less than 200 microseconds and completely dissipates in less than 1,000 microseconds. There can also be restrikes or multiple discharges following the ionized path of the initial stroke. One study years ago found that 36% were single discharges while 21% had 6 or more discharges per stroke. The median was found to be 3 discharges per stroke.

The mechanism of lightning formation is interesting but probably beyond the scope of the question. One interesting fact however is that most observers believe when they see a cloud to ground lightning stroke they are observing stroke progression from the cloud to the ground when, in fact, what is visible is the return stroke discharge back to the cloud.

Lightning is simulated in the laboratory with what is called a Marx impulse generator, which consists of multiple banks of capacitors that are charged in parallel and discharged in series through firing gaps (typically sphere gaps).

The industry has defined a standard 1.2/50 impulse voltage waveform to be used for lightning studies on various apparatus and equipment. This designation means a front time to crest of 1.2 microseconds (actually defined by a line drawn between 30% and 90% of crest voltage on the front of the wave) x 50 microseconds time to half-crest on the tail of the wave.