The aurora can only be seen against a dark sky, and the chances of observing an Aurora are obviously much better in winter when nights are also dark at auroral latitudes. There are two basic types of Aurora, Diffuse and Discrete aurora.
Ø DIFFUSE AURORA:
Unlike the discrete aurora, which is due to the parallel acceleration of electrons down along the earth's magnetic field lines into the atmosphere, diffuse aurora "dribbles" in from the plasma sheet. This dribbling comes from wave-particle interactions that scatter electrons and give them velocities that are more parallel to the magnetic field line.
A photograph of the diffuse aurora by Jan Curtis in Fairbanks, Alaska. The dark regions covering the aurora are clouds.
This change of velocity will put the electrons in the loss cone in phase space and they will precipitate into the atmosphere, exciting atoms and molecules to produce the diffuse aurora. The quantitative study of the wave-particle interaction with different types of waves and with electrons having different energies is a complicated matter.
Ø DISCRETE AURORA:
Discrete auroras are the most intense auroral types, where field-aligned acceleration plays an important role, forming the so-called inverted-V precipitation.
Discrete auroral types include
Observers on the ground, however, don't see that "diffuse aurora," which is dim and lacks sharp outlines. The bright arcs of the "discrete aurora" which they do see--ribbons in the sky, often shimmering, folding, and constantly changing--have a different origin. Satellites flying through the electron beams which create such arcs see evidence that their electrons have actually been accelerated. It is as if a giant "electron gun" existed somewhere up in space, like the one found in TV picture tubes, and was shooting electrons down field lines.
This is believed to be the origin of the discrete aurora.
