The discovery of the auroral dunes: How one thing led to another

Guest blog post. Dr. Minna Palmroth is Professor of Computational Space Physics and the Director of the Finnish Centre of Excellence in Research of Sustainable Space at the University of Helsinki, Finland. On June 15, 2020, Dr. Palmroth gave a presentation to the Aurorasaurus Ambassadors about the discovery of dune aurora, viewable on our YouTube channel


The dune aurora may give new information from one of the last frontiers on Earth, a curious layer in the upper atmosphere where space touches the atmosphere. This place is so hard to measure that it is sometimes called the ignorosphere.


“A time lapse video recorded by a Scottish aurora borealis hobbyist Grame Whipps was used to determine the speed of the phenomenon at over 200 m/s.” (Pellinen, 2021).
 

On a cloudy and dark November morning in 2015 I received a very polite email – “could the good professor join our auroral enthusiast Facebook group and explain the physics of aurora for us …”.  This will cheer me up, I thought, and joined the group. Over the years I became the group’s staff scientist: explaining things, pointing toward data and indices, interpreting pictures. At some point I noticed that while the number of members grows (now over 17,000), I’m always explaining the same things. This led to the second step: the book project.

I listed the auroral forms known in science: arc, corona, omega band, diffuse aurora, etc, and asked the people in the group to find examples of the different forms. I contacted two friends, the legendary auroral photographer Jouni Jussila whom I knew already from grad school, and the legendary author Markus Hotakainen. They were immediately inspired. We wrote basically a birdwatching guidebook, but instead of birds we showed examples of the auroral forms, explained their background, and even added a star classification showing how rare the form is. Now, I thought, the most frequently asked questions were explained!

But no – that led to the next step: While choosing example pictures for the guidebook, we engaged some of the most active Facebook group members to help. One of them, Matti Helin asked “… but what are these stripes…” and showed us a picture with regular, horizontal waves in the diffuse green aurora. I had never seen anything like them, and we left the stripes out from the book. The book got published on 5 Oct 2018, and in the aftermath of the publication party, on 7 Oct 2018, while messaging with the picture selection group, Matti Helin suddenly shouted a message: “THE STRIPES ARE ON THE SKY!”

Two photos show wavelike auroral "dunes"

(a) Auroral dunes captured on 7 October 2018, at 17:40.59 UT in Laitila (see location in Figure 4b). The stars in the background indicate that the field of view is toward the northwest. The dunes (marked by magenta circles and numbers, which refer to section 2) extend equatorward from the discrete bright arc in the north. The local magnetic field direction is shown by the auroral pillars, the almost vertical purple structures. (b) Simultaneous picture of the dunes in Ruovesi, 172 km northeast from Laitila. The background star positions indicate that the field of view is roughly toward the west. (Palmroth et al., 2020).

The other aurora chasers immediately ran out to take pictures and I tried to coordinate things from my couch. Long story short, we got two simultaneous pictures from two different places. A postdoc from my group, Maxime Grandin, developed a clever trigonometric method to use the pictures as data, and we got an altitude and a wavelength to the structures. At some point one of the enthusiasts, Pirjo Koski, came up with a name: dunes, and that stuck.

Writing the dune discovery paper was like a rollercoaster of feelings, possibly analogous to an actor who repeatedly succeeds and fails on stage in front of the audience, although in our case the citizens were actively involved in the play. Finally we came to the conclusion that the dunes are a manifestation of a relatively rare type of atmospheric wave called a mesospheric bore. We suggested that the brighter stripes in the aurora would result from a larger density of excitation targets in atmospheric oxygen, while the dimmer parts in between bright stripes would indicate lack of emission from the atmospheric wave trough.

One of the proudest moments of the entire process was when we realized that a brand-new, high-impact journal, American Geophysical Union’s counterpart to Nature, called AGU Advances, published our article in its first volume, issue one, pages 1-12, and on the cover page. In this article we speculated that the atmosphere forms a horizontal waveguide for the bore to travel, and the aurora illuminates the wave crests. Since we didn’t have data on the location of the dunes, we couldn’t say for sure. In any case the discovery “bombed the bank” as we say in Finland; the press release reached over 3 billion people on all continents, and I spent two weeks commenting on the discovery nonstop for all kinds of journalists. 

Now we can continue the story, because on May 4, 2021, AGU Advances published our second dune paper! This is a paper by Maxime Grandin – this time featuring dunes from Finland to Scotland – and including in situ measurements linking the waveguide with the dunes. However, we still didn’t observe the bore itself, not because it was not potentially there, but because density measurements were not available, nor were airglow observations where bore signatures are typically present. So the story hopefully continues with new events.

A collage of 5 images show dune aurora

Selected pictures of the dune aurora by Citizen Scientists during the 20 January 2016 event. The pictures were taken from (a) Aura, Finland, at 17:23 UT, (b) Engerdal, Norway, at 20:13 UT, (c) Karmøy, Norway, around 20:30 UT, (d) the Isle of Mull, Scotland, at 20:57 UT, (e) Lendalfoot, Scotland, at 21:15 UT, and (f) Rattray, Scotland, at 21:15 UT. (g) (Grandin et al., 2021)

If you have seen the dunes, we would love to hear about it, because new observations would give us more opportunities to understand this incredibly fascinating phenomenon. The dune observations are important because they may give a new glance from the ignorosphere, the last frontier of unknown places on Earth where space touches the atmosphere. This place is extremely hard to measure, but with dune pictures we can explore it!

Minna Palmroth

University of Helsinki press releases:

Palmroth et al 2020: https://www2.helsinki.fi/en/news/science-news/citizen-science-discovers-a-new-form-of-the-northern-lights

Grandin et al 2021: https://www2.helsinki.fi/en/news/science-news/confirmation-of-an-auroral-phenomenon-discovered-by-finns

Pictures and timelapse in AGU press release: https://news.agu.org/press-release/citizen-scientists-discover-new-type-of-aurora-named-the-dunes-video-available/

Editor’s Highlight on EOS: https://eos.org/editor-highlights/dune-aurora-explained-by-satellite-ground-studies 

Taivaanvahti citizen science observation system https://www.taivaanvahti.fi/

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