The Other Flashes in the Sky: GLM’s Ability to Observe Meteors
by Anita LeRoy, University of Alabama in Huntsville/NASA SPoRT
and Kris White, NWS Huntsville
Did you see that bright flash in the sky on Friday, February 1? No, not that flash of lightning. Did you see the meteor explode over Cuba? If you missed it, don’t worry. The GOES Geostationary Lightning Mapper (GLM) saw it.
When it launched in November 2016, NOAA and the rest of the weather world had great expectations for the GLM, NOAA’s new operational lightning mapping instrument in geostationary orbit. After all, the lightning research community has seen the value of lightning imagers in low earth orbit, like the Optical Transient Detector and TRMM-Lightning Imaging Sensor to diagnose convection on a global scale. GLM improves upon those efforts with a hemispheric field of view, near-continuous sampling, and several sophisticated algorithms and filters that map lightning in space and time, providing a two-dimensional view of lightning as it propagates through a cloud. But, unexpectedly, GLM can detect bright meteors, called bolides, as they burn up, break up, or explode in Earth’s atmosphere.
GLM detects light from meteors partly because meteors look more like lightning than noise. Long, bright flares of light produced as bolides break up in the atmosphere can resemble continuous current lightning flashes in which a charged channel lights up for tens to hundreds of milliseconds. In other cases, the meteors appear to be incredibly weak, unenergetic lightning flashes, nearing the bottom threshold of detection for the instrument. Many of these weaker events are filtered out in the operational datasets and must be reconstituted from the raw data.
The Cuba bolide from February 1 is certainly an example of the larger meteor events detected by GLM. It was a bright blast of energy, packing about 1.4 kilotons of impact energy according to U.S. government sensors that track and measure meteors. Some small meteorites survived all the way to Earth’s surface. GLM saw a highly energetic light source flash energy peaked at over 350 picojoules (pJ; 10-12 J) and the detection lasted about three milliseconds (Figure 1). The flashes from the nearby storms in the Gulf of Mexico that GLM also detected peaked at about 30 pJ. Most importantly, it was observed in the daylight given GLM’s unique ability to detect light emissions during the day as well as at night.
Initially, witnesses reported seeing the bright flash tracking northeast to southwest according to the American Meteor Society. But the actual trajectory calculated by the U.S. government sensor network appears to be closer to due north, and GLM observations confirm that trajectory (Figure 2). Eye witness reports can be unreliable for a number of reasons, but in the absence of global networks of meteor cameras, witnesses (and their ubiquitous pocket technology) tend to be the best sources of information about meteor activity. Now, GLM could be added to the mix of data sources for meteor scientists, meteorite enthusiasts, and government agencies seeking to characterize these events, and it can potentially do it in real-time.
Additionally, GLM is providing value in cases where the meteor activity is quite a bit less noteworthy. Smaller bolides from last August’s Perseid meteor shower disintegrate at altitudes exceeding 70 km. These meteors have brightnesses, or magnitudes, rivalling that of the planet Venus, but have also been detected by GLM. The energy that makes it to GLM from those bolides tends to be minimal, on the order of 15 to 45 femtojoules (fJ; 10-15 J), which is very near the dimmest activity that GLM will detect, comprising a handful of events that are condensed into single flashes. Soon, GLM might be used to help characterize those meteor events, too.
NWS forecaster Kris White (WFO Huntsville) has helped us understand what impact this new detection talent might have on operational forecasters by saving imagery from the Cuba bolide using GLM flash extent density (FED) in AWIPS, the data framework used by NWS forecasters (Figure 3). The animated AWIPS imagery shows some specific points about detecting bolides in AWIPS, or any other visual display of GLM. First, it happens fast. FED is displayed as 1-minute snapshots, and the three second bolide is, of course, only seen in a single frame. The bolide is also isolated, and FED shows a very focused series of “flashes” in one spot, instead of the branching cluster of flashes that electrified clouds often produce. Even when bright meteors break up in the atmosphere, most of them do not scatter far and wide or persist, making them look suspiciously unlike lightning in any product used by forecasters. In some instances, such as the western Cuba bolide, the meteorological environment may also not support the existence of lightning, with no convective type clouds present.
So, when your phone banks light up like an exploding meteor because everyone saw or posted about the latest bright light in the sky, there is a good chance that GLM saw it, too.
Read more articles from our NWA May Newsletter.