Researchers have observed millisecond pulses of gamma-rays produced by thunderstorms using instruments onboard the International Space Station (ISS), which may reveal more about the process by which these terrestrial gamma-ray flashes (TGFs) are made. This was stated in a report in December 2019. The process by which TGFs are generated from thunderstorms, has been debated. The researchers say that they have observed an ultraviolet emission as well, which they have termed an ‘elve’.
While many are familiar with the brilliantly electric bolts of lightning that crack the sky below thunderstorm clouds, other types of luminous phenomena as well occur above storm clouds high in the Earth’s upper atmosphere. Elves, one such phenomenon, are expanding waves of ultraviolet and optical emission in the ionosphere above the thunderstorm. Elves are the highest of all the ‘transient luminous events’ known to date. In the blink of an eye, concentric rings appear as a dim, expanding glow hundreds of kilometres wide formed by electrons colliding and excited nitrogen molecules. They are triggered by electromagnetic pulse radiating from lightning discharges in the storm below, but not much is known about this process. It is also not known how thunderstorms lead to the generation of TGFs.
As the ISS flew over the coast of Sumatra, Indonesia, lightning from a thunderstorm reached the upper layers of the atmosphere and its light show was captured by ESA’s latest observatory in space. The instruments of the Atmosphere-Space Interactions Monitor (ASIM), also called the Space Storm Hunter, are mounted to the exterior of the ISS. The ASIM is a collection of optical cameras, photometers and an X- and gamma-ray detector designed to look for electrical discharges born in stormy weather conditions that extend above thunderstorms into the upper atmosphere. It was completing its initial tests after it was installed outside Europe’s Columbus laboratory.
The Columbus observatory points straight down at Earth so the atmosphere filters as little of the light as possible. The storm hunter’s photometers are hundreds of times more sensitive than an average camera on Earth. In the storm above Indonesia, the instruments recorded a spike across three wavelengths.
Setting up one of the most complex facilities ever flown on Columbus is a matter of trial and error. Each element is tested, including measures to avoid sunlight burning the sensors.
The Observations
Torsten Neubert, science team coordinator at the Technical University of Denmark, and his team collected 100,000 measurements per second of this phenomenon when they observed a TGF and an associated elve with the ASIM. The first images and data captured the strong signature of lightning with unprecedented accuracy 400 kilometres above Earth. The ASIM data captured high-speed observations of the event in optical, ultraviolet, x-ray and gamma-ray bands, which allowed them to identify the sequence of events that generated the TGF. The first images are from the visual cameras on the ASIM. A second suite of instruments detects high and low energy and had not finished calibration.
The images are surprisingly similar to a sequence captured by ESA astronaut Andreas Mogensen from the International Space Station in 2015. The data may allow scientists to investigate the phenomenon, and distinguish between layers of lightning and other high-energy discharges
The results show that the TGF was produced by the high-electric fields produced just prior to a lightning bolt within the thunderstorm cloud — occurring milliseconds after the onset of the lightning leader, which was key to the TGF’s formation. The subsequent lightning flash released an electromagnetic pulse, which induced the elve visible above the thunderstorm. The scenario may represent prerequisite conditions for generating elves.