RED SPRITES AND GREEN GRAVITY WAVES: As northern summer comes to a close, electrical storms are rumbling across the USA. After nightfall, red sprites can be seen dancing across the cloudtops. On Aug. 20th, Tom A. Warner photographed these speciments above New Underwood, South Dakota:.
On the night of Aug 20th, intense storms developed in north central South Dakota, along with an ongoing mesoscale convective complex in south central North Dakota. Skies cleared out to the west and offered a chance to capture some sprites from the northern activity. I did a time-lapse from western South Dakota and captured not only sprites, but also observed convectively-generated air glow gravity waves which are luminosity ripples in the air glow region which is present due to chemiluminescence near the mesopause around 85 km. These ripples are thought to be caused by the cyclical overshooting updrafts of strong thunderstorm complexes which push momentum upward in a localized area. These air glow gravity waves, which are currently being studied by lightning research scientists, are believed to be capable of modifying the luminosity patterns of transient LUMINOUS events such as elves and halos. Canon 5D Mark III, ISO 3200, f/2, 24 mm, 10 sec exposures
"On the night of Aug 20th, intense storms developed in north central South Dakota," says Warner. "Skies cleared out to the west and offered a chance to capture some sprites from the northern activity."
He saw not only sprites, but also green-glowing gravity waves. The waves are, literally, the ripple effect of a powerful thunderstorm on the mesosphere some 80 km above Earth's surface. From space, these waves look like a giant atmospheric bull's eye. From the ground, they appear to be green ripples in the sky, as shown in Warner's images.
Left to themselves, gravity waves would be invisible to the human eye. We see them, however, because they are colored green by an aurora-like phenomenon called "airglow." Airglow is caused by an assortment of chemical reactions in the upper atmosphere driven mainly by solar ultraviolet radiation. Gravity waves rippling away from the central axis of a thunderstorm cause temperature and density perturbations in the upper atmosphere. Speaking simplistically, those perturbations alter the chemical reaction rates of airglow, leading to more-bright or less-bright bands depending on whether the rates are boosted or diminished, respectively.
Inhabiting the upper reaches of Earth's atmosphere alongside meteors, noctilucent clouds and some auroras, sprites and mesospheric gravity waves are true space weather phenomena. Now is a good time to see them.