- Upper-atmospheric lightning, also known as transient luminous events (TLEs), refers to a fascinating class of electrical phenomena that occur high above thunderstorms, reaching into the stratosphere and mesosphere—far above the altitudes of typical cloud-to-ground lightning.
- Unlike the familiar bolts of lightning that strike the Earth’s surface, these luminous discharges are fleeting, colorful, and largely invisible to the naked eye under normal conditions. First officially documented in the late 20th century, upper-atmospheric lightning has since become a major area of research in atmospheric physics, revealing new insights into how thunderstorms interact with the upper layers of the atmosphere and even space weather systems.
- The main types of upper-atmospheric lightning include sprites, blue jets, elves, trolls, and gigantic jets, each with distinct visual characteristics, altitudes, and physical mechanisms. Sprites are perhaps the most well-known and widely observed of these phenomena. They are large, reddish-orange flashes that occur at altitudes of about 50 to 90 kilometers (31 to 56 miles) and are often triggered by powerful positive cloud-to-ground lightning strikes. Sprites typically appear in clusters and can resemble jellyfish or carrot-shaped streaks, lasting just a few milliseconds.
- Blue jets, on the other hand, originate near the tops of thunderclouds and shoot upward into the stratosphere, reaching heights of about 40 to 50 kilometers (25 to 31 miles). They are conical in shape and glow with a distinct blue or violet hue, caused by the excitation of nitrogen molecules in the upper atmosphere. Unlike sprites, which are linked to electric field changes caused by lightning strikes, blue jets are thought to be caused by electrical breakdowns within the cloud tops themselves.
- Elves (an acronym for Emissions of Light and Very Low Frequency Perturbations due to Electromagnetic Pulse Sources) are even more fleeting. They are rapidly expanding rings of dim red or green light that appear at altitudes around 100 kilometers (62 miles), typically lasting less than a millisecond. Elves are generated by the electromagnetic pulse (EMP) emitted from intense cloud-to-ground lightning and can span hundreds of kilometers in diameter. They mark the boundary where atmospheric and space physics begin to overlap, as their occurrence often influences the lower ionosphere.
- Gigantic jets are relatively rare and striking phenomena that bridge the gap between cloud-to-ground lightning and sprites. These powerful discharges start in thunderclouds but shoot upward past the stratosphere, sometimes reaching the ionosphere at altitudes over 90 kilometers. Gigantic jets resemble lightning bolts in reverse, and their immense energy suggests a possible electrical connection between thunderstorms and the global electric circuit.
- Other TLEs such as trolls (Thunderstorm-Related Optical Luminous Lines) are less understood and still under investigation, but they add to the growing complexity and variety of upper-atmospheric lightning. These phenomena are often captured by sensitive low-light or high-speed cameras from mountaintops, aircraft, or space-based platforms like the International Space Station (ISS). Instruments like NASA’s ASIM (Atmosphere-Space Interactions Monitor) have greatly advanced our ability to observe and analyze these ephemeral events.
- The study of upper-atmospheric lightning is significant for several reasons. First, it helps scientists understand how energy and electrical charge are transported vertically in the atmosphere during storms. Second, these events influence the chemistry of the mesosphere and lower ionosphere, particularly through the production of nitrogen oxides and changes in ozone concentrations. Third, they can affect radio wave propagation and communications, as well as contribute to the electromagnetic coupling between the atmosphere and space.
