Atmospheric Fireworks in the Solar System
Our speaker for the evening was a physicist from the Rutherford Appleton Laboratory, Dr Karen Aplin, who specializes in atmospheric electricity. In her talk entitled "Atmospheric Fireworks in the Solar System" she told us of the role that electricity plays in helping to generate a planetīs weather.
She began at our own planet which, apparently, acts as a giant battery with the surface becoming negatively charged with respect to a part of the upper atmosphere called the ionosphere. The ionosphere is the uppermost part of the atmosphere where incoming radiation hits molecules and turns them into ions. The other planets also have this atmospheric layer as they are subject to the same shower of particles.
The conductivity of the atmosphere (or ability of electric current to flow) is rather poor at sea level due to the rarity of ions but increases with height up to and through the ionosphere as ion density increases. It is also lower at the equator than the poles, as at higher latitudes particles from space spiral down the Earthīs magnetic field lines and crash into air molecules producing ions. Surprisingly large voltages can be present even during good weather. For example from 250,000 to 500,000 Volts measured between the surface and ionosphere. This is known as the "fair weather current" and leads to a steady flow of electrons from the surface of 1 microWatt per square metre.
Even greater voltages are found in thunderstorms of which there are an estimated 2,000 continually active around the globe. The voltage between the ends of a lightning bolt depends on its length (e.g. one thousand million volts for a 300 m-long lightning bolt). What makes them deadly is that a bolt can allow thousands of Amperes of current to flow.
Lightning has only been directly observed on Jupiter and Earth but has been inferred on Saturn by the Cassini spacecraft picking up the resultant emission of radio waves. It has also been indirectly concluded that Venus may have cloud to cloud lightning from observations by the Venus Express spacecraft but ground to surface bolts are very unlikely due the high atmospheric pressure near the surface.
On planets and moons with thinner atmospheres the more prevalent form of electricity is electrostatic. Again this involves a separation of charges but is on a highly localized basis. It is most often encountered in our everyday experience when clothes cling rather than drape or you get an electric shock after walking across a nylon carpet and then touching a metal object.
The Apollo astronauts reported terrible problems coping with the persistent static cling of moon dust. For ultraviolet rays from the Sun drive electrons out of lunar dust by day, while the solar wind bombards it with electrons by night. Some scientists even suggest that the UV bombardment leads to such high positive charges that the dust grains are repelled from the lunar surface to a height of a few kilometres before falling back to the surface.