Types of Red Sprites (Part 3)
This is part 3 of a series of forthcoming posts on red sprite lightning. This series is based on my term paper in the Mountain Meteorology course at the University of Utah.
The stereotypical sprite is the form that sprites were first photographed as in 1994. An amorphous, red blob with several thin roots reaching downward. Taking a step back in time, the stereotypical sprite is sudden twinkle of red in the sky far away. Not worth much thought. But since the U.S. campaigns in the 90s to document the existence of sprites, we have learned there are several distinct shapes that red sprites take when they form in the atmosphere, although there are no concrete delineations between the halo, jellyfish, columniform, and carrot sprites.
Each of these types of sprites has a distinguishing feature.
The halo is the basic sprite. It is the structureless sprite, appearing as a massive, diffuse sphere up to 80 kilometers in diameter. Halos are a key feature of jellyfish sprites.
The photo of the first photographed sprite as well as in the image on the left below are typical of the jellyfish form. They always have a bright halo in their upper regions that surround the sprite’s origin point. Jellyfish sprites also have the diverging tendrils, or streamers, reaching down from the halo down toward the ground. These streamers often turn blue and fade out between 30 and 50 kilometers above the surface.
Columniform sprites appear as a series of individual luminescent columns as in the photo on the right above. They are very similar to the streamers that descent from jellyfish and carrot sprites, but columniform sprites have no halo. Because of their distinct nature and similarity in sizes, columniform sprites were key in first measuring the dimensions of streamers. Following one thunderstorm in 1998 observed by Wescott et al that repeatedly produced columniform sprites, they determined that streamers and columniform sprites are a highly uniform cylinder of light up to a kilometer in diameter and 10s of kilometers long.
The headline image atop this page and that I have chosen to represent this series on red sprites is by far the best example of a carrot sprite that I have come across online. Carrot sprites are distinguished by their upward development. Most sprites start between 75 and 80 kilometers above the surface and grow downward with their streamers. Carrot sprites, although they do have some downward tendrils, are dominated by bright, energy dense streamers growing upward and outward. One recent study shows that for some unknown reason, the atmosphere allows carrot sprites to grow upward significantly faster than downward, so the majority of the energy travels up.
As with everything else relating to sprites, we don’t know exactly what causes the different shapes of a red sprite. One theory is that the thunderstorm that generates the sprite also changes the shape of the upper atmosphere through gravity waves (a wave motion caused by gravity pulling buoyant air downward).
I found the following graphic from a study by Qin et al to be very exciting. It’s a series of photos taken with a high speed camera of a single streamer forming below a sprite. It shows how quickly sprites begin to form after the sprite trigger to the peak of luminescence. Over 1.875 milliseconds, the sprite grows from around 90 km above the earth’s surface to 78 km.
