When Størmer Met STEVE

A summary of a groundbreaking new paper by Dr. Michael Hunnekuhl and Dr. Liz MacDonald, published in Space Weather with open access: “Early Ground-Based Work by Auroral Pioneer Carl Størmer on the High Altitude Detached Subauroral Arcs now known as STEVE.” 

In 2018, STEVE took the world by (solar) storm. The quirky little subauroral arc connected aurora scientists with the general public. Community scientists’ critical role in its classification and research reminded people that we all have the power to advance scientific knowledge. One citizen scientist, Dr. Michael Hunnekuhl, was inspired to turn to the past, to one of the most prominent early 20th-century aurora explorers: Carl Størmer. What he found was extraordinary and overlooked in our recent work. 

Pioneering Aurora Science

Norwegian mathematician Carl Størmer (1874-1957) became fascinated by aurora science in 1903 when he chanced to watch a science demonstration by aurora pioneer Kristian Birkeland. Størmer threw himself into researching the aurora, publishing a book in 1955. His many accomplishments included:

• Working out how charged particles in the Earth’s magnetic field behave
• Introducing the concept of the ring current, a large electric current that circles the Earth’s equator, significantly intensified during solar storms
• Developing the first quick-exposure aurora camera
• Figuring out how to accurately measure the heights of auroras with over 12,000 aurora height measurements
• Classifying different shapes & types of aurora

In order to better study aurora, Størmer put together a network of field stations in Norway specially equipped to observe the Northern Lights. By using early telephones to coordinate observations between stations, the scientists could take simultaneous photos and triangulate the height of the aurora—a huge advance at the time. During his decades-long research, on only seven days, Størmer noticed what he called “feeble homogenous arcs of great altitude” with distinct characteristics. He described them in 1935: 

“….There occurs in Oslo on rare occasions another type, which consists of a thin sharply limited homogeneous arc, some degree wide and with very feeble luminosity. This arc is, in general, situated near zenith, lasts from half to about one hour, and during its appearance is far from other aurorae, which, if any, occur only near the northern horizon. The height of these remarkable arcs is about twice that of the height of the common arcs mentioned above….”

Although he did not mention any visible color in his observations, in 1911 Størmer used two types of photo plates to triangulate the height of the aurora. The photos came out very faint on the plates, which were only sensitive to blue and near-ultraviolet wavelengths of light. For this reason, he initially thought that the spectra of these arches might only contain the prominent green aurora color. After his second observation in 1930, he changed his mind since he was not able to see any aurora typical spectral lines using an instrument called a spectroscope. These are optical tools that focus light into barcode-like signatures which can be used to identify the atoms and molecules that make up glowing phenomena. While Størmer had difficulty drawing conclusions from his spectral data on the arcs, his and other explorers’ findings match what we now know about them. Størmer classified the arcs as a rare form of aurora, but now we have given them a new name: STEVE. The point of the new paper is to bring this previously overlooked hundred-year-old work to light. 

A Short History of STEVE

In the early and mid-2010s, reports of a mysterious “proton arc” appearing equatorward of the aurora circulated online. While for years observers had occasionally seen faint, gray, east-west arcs, the phenomenon had never been fully investigated by modern science. Citizen scientists affectionately called the phenomenon “STEVE” in homage to the animated film Over the Hedge, and the name was eventually formalized to “Strong Thermal Emission Velocity Enhancement.” In collaboration with citizen scientists and other scientists around the world, Aurorasaurus published the first paper classifying the phenomenon in 2018. The discovery was not that the STEVE arc was new, but that satellite observations confirmed it was the visible result of the ‘subauroral ion drift’, a charged east-to-west flowing river of particles. When the news broke, STEVE went viral on a global scale, charming the world and illustrating the importance of community science. 

New Discoveries in Old Records

Laser physicist by day, aurora historian by night: Dr. Michael Hunnekuhl contributes a wealth of knowledge and a unique perspective to citizen science. While investigating rare aurora science in spring 2018, he ran across Carl Størmer’s descriptions of “feeble homogeneous arcs of great altitude.” These are described as rare, dim, thin, sharp arcs, at twice the height and separated from other auroras, situated near the zenith of the sky, and in the Earth’s shadow. Michael remembers the experience being “like a shock” because the news about STEVE had just gone viral and he was seeing similar descriptions in century-old writings. 

Michael delved into archives, using his knowledge of aurora records to sift through journals for possible glimmers of STEVE in historical narratives. While Størmer was aware of a few other explorers who had seen the arcs, Michael discovered references from many more and began to create a list of archival mentions. At the same time, he trawled modern citizen scientist reports, working to develop a set of criteria that would help identify and describe the phenomenon. Michael’s data on STEVE candidates in the historical record, list of present-day sightings, and poster for AGU 2019 on his work are available on OSF. He contacted Aurorasaurus in January 2019 and we have collaborated closely over the last year, culminating in jointly authoring this Space Weather Commentary feature and continuing with other work in progress

The Power of Citizen Science

According to Michael, Størmer put out calls in newspapers and over the radio for amateur aurora observations and data. These were integral to his work in the same way that citizen scientists’ Aurorasaurus reports are to the scientific community today. From the earliest studies, STEVE has been closely connected with citizen science. The rarity of the phenomenon and the uniqueness of the way it shows up play to the strengths of aurora enthusiast communities and photographers. Scientists continue to study STEVE, but it is citizen scientists, braving the elements, who provide the snow-boots-on-the-ground data that make it possible. STEVE pushes the limits of our understanding, and community scientists are continually unlocking new discoveries. 

Curious for more details? Read the full, open-access paper in Space Weather! 

For further reading on Carl Størmer, Michael suggests:

• “Remarkable Aurora-Forms from Southern Norway” by Carl Størmer (1935)
• “Remarkable Aurora-Forms from Southern Norway, III-IX” by Carl Størmer (1942)
• The Polar Aurora by Carl Størmer (1955)
• Fredrik Carl Mülertz Størmer by S. Chapman (1958)
• Carl Størmer—Auroral Pioneer by Alv Egeland and William Burke (2013)

Dr. Michael Hunnekuhl works as a laser physicist. For years he has also been interested in aurora research, including its history. His interest in aurora and aurora-like phenomena is focused on the STEVE phenomenon, and on linking the knowledge gathered by the aurora observer community with that of the scientific community. He began collaborating with the Aurorasaurus citizen-science project in 2019 and has gotten a chance to present his work at the European Space Weather Week meeting in 2019, be part of two presentations at AGU (presented by Dr. MacDonald), and will be attending and presenting at the upcoming HamSci workshop. This is his first peer-reviewed space science article.