An aura is a continuous effect that emanates from a creature. Unless otherwise noted, an aura uses the following rules.
Fills an Area: The aura fills the creature’s space and each square that is both within a specified distance of the creature and within line of effect of it. For instance, an aura 1 affects each square adjacent to the creature. A creature is normally unaffected by its own aura.
Unaffected by the Environment: The aura is unaffected by environmental phenomena and terrain, although blocking terrain blocks an aura. For instance, an aura of fire is unaffected by an area of extreme cold.
Overlapping Auras: If auras overlap and impose penalties to the same roll or game statistic, a creature affected by the overlapping auras is subjected to the worst penalty; the penalties are not cumulative. For instance, if a creature is affected by three overlapping auras that each impose a -2 penalty to attack rolls, the creature takes a -2 penalty, not a -6 penalty.
Deactivating an Aura: A creature can take a minor action to deactivate or reactivate one of its auras. However, certain auras have set durations and cannot be reactivated after they end.
Death or Unconsciousness Ends: A creature’s auras end immediately when it falls unconscious or dies.
Published in Monster Manual 2, page(s) 6, Rules Compendium, page(s) 308.