Simple version:
Visualize a rainbow encircling the planet. The color closest to the Earth is violet.

More complex version:
Visualize the planet earth with a pair of holes at key areas -- wherever you feel drawn to work. The holes reach from the surface of the planet to the center of the earth. In the center of the earth is a raging fire. Visualize all negative astral forms; born from events, thoughts, emotions, memories are gathered from the atmosphere and are sucked through one of the holes into the center of the earth . The negativity is purified in the fire, by the mother earth into positive energy of light and love. Visualize this purified energy expelled out of the other hole and sent throughout the atmosphere. As the energy circles the planet it becomes a rainbow until there is a strong luminous rainbow encircling the planet.