Can lightning exist in a vacuum?

Lightning is caused by a current of electricity through a dense gas such as air. The current excites the atoms of the air to emit a brilliant light, that we see as lightning.

Very large electric currents also flow through space. For example the solar wind is a current of charged particles. However there is not enough gas in space to cause the flash that we recognise as lightning. When this current strikes the rarefied outer parts of the earth's atmosphere, it causes a soft glow known as the aurora.

Similar currents may emerge from other phenomena in space. When they strike a nebula they cause the beautiful coloured glows that we are familiar with from hubble pictures.

Incidently, lightning not only strikes down from earth's clouds, it also strikes upwards towards the ionosphere. Such a strike is seen as a sudden diffuse glow known as a "sprite" or "elf". here is a site that explains:

http://en.wikipedia.org/wiki/Sprites_(lightning)

Cheers!

In one sense, yes. It has been proposed that huge electrical charges could build up in very disperse gas clouds (near-vacuum) in space and eventually jump from one part of a cloud to another in a sort of giant lightning bolt billions of kilometers long. However, because the matter is so thin, this 'space lightning' probably would not produce a spectacular spark like we see in the Earth's relatively thick atmosphere, in fact it probably wouldn't even be visible to the naked eye, although instruments could detect it.

yes, electricity can flow through a vacuum, but it doesnt appear like a spark... more like a glow, as you might fnd in a neon sign. If the current values are high enough, power could be delivered through a vacuum. However if the current becomes very high you will experience the "pinch effect" where the self generated magnetic field will pinch the flow into a contorted shape l;ike a snake. If two planets in space were to discharge against each other , they would form just such a snakelike current in space. Immanuel . Velikovsky described such a scenario in his books about planetary collisions.

Lightning won't travel through a vacuum.

It must ionize the atmosphere or some other medium to leap across a space.

No. Your question is illogical and contradictory. There is no matter and no atoms or molecules in a vacuum. Lightening requires an electrical charge difference in baryonic matter with atoms and molecules made of protons, neutrons, and electrons for lightening to happen, so by logical deductive thinking, if there is baryonic dust, that is not a vacuum.

no. even light needs a ion to penetrate to if it travels in spaces faster than it.