Midsummer night brings sprites — rare phenomenon caught on camera at La Silla

This unique and spectacular image of ESO’s La Silla Observatory features a cloudless sky stained with red and green airglow and bejewelled with celestial objects. Amongst the celestial cast are the Small and Large Magellanic Clouds just to the right of centre, the rosy-red glow of various star-forming regions and the faint green streak of a meteor just to the left of the Milky Way. These striking heavenly regulars are eclipsed however by the presence of something far more elusive and much closer to home.

The six panels below the main image magnify a series of extremely rare atmospheric phenomenon known as sprites. A few hours before daybreak — signalled by the spire of zodiacal light rising from the base of the Milky Way — a powerful thunderstorm appeared on the distant horizon, and the Earth’s upper atmosphere became a playground for these ephemeral events.

Named after Shakespeare’s mischievous sprites Puck, from A Midsummer Night’s Dream, and Ariel, from The Tempest, sprites are caused by irregularities in the ionosphere, high above storm clouds, at altitudes of about 80 kilometres. Typically seen as groups of red-orange flashes, they are triggered by positive cloud-to-ground lightning, which is rarer and more powerful than its negative counterpart, as the lightning discharge originates from the upper regions of the cloud, further from the ground. In a short burst, the sprite extends rapidly downwards, creating dangling red tendrils before disappearing.

The sprites pictured here occurred over the course of about 40 minutes and were most likely over 500 kilometres away (compare with a satellite image showing the storm over Argentina), with each one spanning a height of up to 80 kilometres and lasting only a fraction of a second. Notoriously difficult to capture with anything other than specialised scientific cameras, these airy spirits rewarded the patience of ESO Photo Ambassador Petr Horálek with this tremendous series of images.

Petr captured another beautiful sprite image at La Silla, and, a few days later at Paranal, he captured this amazing shot from the VLT platform (originating from this storm).