Investigating Characteristics of Lightning-Induced Transient Luminous Events Over South America
Sprites, halos, and elves are members of a family of short-lived, luminous phenomena known as Transient Luminous Events (TLEs), which occur in the middle atmosphere. Sprites are vertical glows occurring at altitudes typically ranging from ~40 to 90 km. In video imagery they exhibit a red color at their top, with blue tendril-like structure at low altitudes. Elves are disk-like glows that occur at the base of the ionosphere, with diameters of ~100-300 km, and have very short lifetimes (~2-3 ms). Halos are diffuse glows that occur at low altitudes, have diameters <100 >km, and have a duration that may last up to 10s of ms.
A majority of the studies of TLEs have taken place over the Midwestern U.S. where they were first discovered. This area produces large thunderstorms, which in turn generate large lightning discharges that have been associated with the formation of TLEs. Studies have used the low frequency radiation that initiates with these strokes to study characteristics of these events. This low frequency radiation has been used to determine where large numbers of TLEs may occur. Extreme southern Brazil is a region of the globe believed to have many TLEs, but few studies on these phenomena.
Two collaborative campaigns involving Utah State University proceeded in 2002-2003, and in 2006. Multiple TLE images were made, proving this is, indeed, a region of the globe where these types of events are prominent. In particular, one storm in February 2003 produced over 440 TLEs imaged by USU video cameras. Of these events, over 100 of them had associated halos. Statistical studies for halos previously had been performed in the U.S., but never abroad. Also, several events from the February storm have been associated with negative cloud to ground lightning, a surprising occurrence, as to date, less than a handful of such events have ever been witnessed or published.
In analyzing the TLEs from this campaign, we have shown the halos are similar to those seen in the U.S., even though the storms may be somewhat different. Also, detailed analyses of the negative events show both temporal and spatial morphology heretofore never reported on.