Lightning

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Lightning

Lightning

Forget anything you have been told about lightning discharges in opposite directions, or positive lightning. They just don't exist!

The explanation of lighning is quite simple. It is the movement of electrons down a gradient from a region of  more negative charge to a regionof less negative charge.
A free electron has a much higher velocity than an atom or molecule. When an electron moves down a charge gradient it accelerates further - untila close encounter with an atom or molecule deflects it. A cluster of electrons moving in aproximately the same direction produce electromagnetic fieldsthat, because of their high velocity, draw these electrons together morestrongly than their charge pushed them apart.
These fast moving electrons ionise the air, heating it up and causeingit to glow (but see the involvement of other particles present later).

These moving electrons are also conveying their own charge, and will only continue moving down the charge gradient if this is sufficiently steep.They cannot be pushed from the most negative end, because this raises thecharge in the region into which they are being pushed, and reduces the gradient. This is self terminating.

A lightning discharge develops by a process that I have termed 'a reverse cascade'. The discharge starts in a small region where the charge gradient is sufficiently high, and spreads back into the region of more negativecharge. At the less negative end, electrons are spreading out again, distributing the charge and raising the charge in the surrounding region. At the morenegative end, electrons are cascading into the discharge path, which nowhas a lowered resistance to their flow because of the ionisation.
As these cascading electrons accelerate, they experience an electromagnetic pinch - and the hot ions in their path emit light. We have lightning.
As the source of the cascade extends back into the more negatively charged region, the lightning also appears to extend from the less negative region into the more negative region.
As electrons that have moved into the discharge are redistributed, thecharge potential of the destination end of the discharge rises, until thecharge gradient from end to end of the discharge is no longer sufficientto support the discharge, and the reverse cascade stops.

Animation of a reverse cascade:
animation of reverse cascade lightning discharge


The discharge will leave a trail of ionised gas behind, and will have changed the distribution of charge electrons in the cloud. This may be enough tostart another reverse cascade elsewhere in the cloud.
In high speed photography, the lightning discharge can be seen developing in a less negative region, and propagating back into the more negative region, but this does not mean it is positive electricity, or that it breaks allthe known laws of physics. As you can see from the animation, the electronsall flow as you would expect, from a more negative region to a less negativeregion. It is only the wave front of the reverse cascade  - and thestarting point of the lightning - that propagates back into the more negativeregion!





Sequence of a lightning strike

These diagrams are a bit rough and ready, but I hope you get the picture.
The blue shading represents the charge density of the lower region of athundercloud. The dark blue has the greater negative charge. The red is theslightly positive ground charge.

The first picture shows a reverse cascade discharge initiated at the region of greatest charge gradient. Electrons are flowing in the direction of the arrows, and a small region starts to glow.
lightning 4
The glowing region is extending upwards into themore negative region of the cloud as more electrons cascade into the discharge zone.
Charge density gets more negative at the lower end and less negative atthe source end, but the charge gradient is still sufficient to maintain thedischarge.
lightning 5
The glowing region continues to extend upwards into the more negative region of the cloud as more electrons cascade into the dischargezone.
Charge density gets more negative at the lower end and less negative at the source end, but the charge gradient is still sufficient to maintain the discharge.
lightning 6
The charge gradient now is getting to the point where it can no longer sustain the discharge.
lightning 8
And the initial discharge ( A stepped leader) now ceases, leaving a trail of ionised air to mark its path.

The charge gradient in a lower part of the air mass is now steep enoughto start a new reverse cascade. Electrons are flowing in the direction ofthe arrows, and a small region starts to glow.
lightning 9
The glowing region is extending upwards into themore negative region of the cloud as more electrons cascade into the discharge zone.
Charge density gets more negative at the lower end and less negative at the source end, but the charge gradient is still sufficient to maintain the discharge.
lightning 10
The charge gradient now is getting to the point where it can no longer sustain the discharge.
lightning 11
meanwhile, several areas further into the storm cloud now have a sufficient gradient to initiate a reverse cascade discharge.
lightning 12
In each discharge, the glowing region is extending upwards into the more negative region of the cloud as more electrons cascade into the discharge zone.
lightning 13
In each discharge, the glowing region is extending upwards into the more negative region of the cloud as more electrons cascade into the discharge zone.
lightning 14
The charge gradient between the ground and the air volume immediately above it has now developed sufficiently for a reverse cascadedischarge initiated at the ground to extend upwards.
lightning 15

lightning 16
The reverse cascade draining electrons to grounddevelops upwards, and new cascades are initiated as the charge gradient inthe mid level starts to increase.
lightning 17
These cascades are now joining together into a common pathway of ionised routes.

lightning 20
As the charge density in the discharge routes start to fall as electrons drain to ground, more and more reverse cascades are intiatedthat extend back into the charged region of the storm cloud. For a brieftime this discharge route drains an increasing volume of the storm cloudand the discharge current surges.

Because the charge distribution of the cloud is not uniform the discharge current fluctuates, rising sharply as a reverse cascade moves into a more negative region of the cloud, or falling sharply as feeder cascades are extinguished as the charge gradient drops below the threshold needed to maintain a discharge.

The main discharge may extinguish, only to re-establish itself as feeder discharges move into new regions of the cloud. Eventualy the whole discharge extinguishes long enough for the ionised paths to disappear
See thefull sequence as an animated gif


Thunder and lightning

The noise is not generated by the initial flash of light - it is a little more complicated than that.

The surge of fast moving electrons in the middle of the discharge are drawn togther by their own electromagnetic fields.
This produces a pinch that draws them even closer together. The electromagnetic force that draws them in is balanced by the electrostatic force that pushes them apart. (although along the length of the discharge it still holds them apart). This discharge path is highly negative, and induces a movement of charge in the air molecules through which it passes to form dipoles. In each molecule electrons are pushed away and the positive end of the molecule is drawn towards the discharge path.
This adds to a very powerful pressure wave moving inwards and compressing the discharge path, which gets very very hot, becoming a plasma.
Ionised molecules in this plasma have the negative ion pushed away, andthe positive ion retained.

As the pulse of fast moving electrons of the discharge starts to subside, the pressure wave reflects,  and the pinch now reverses, and the now very hot plasma can all contribute to an explosive expansion, which propagates outwards as waves of sound.