Named the auroral dunes, the phenomenon was discovered by citizen scientists in Finland; it occurs at a relatively low altitude of 100 km (62 miles), in the upper parts of the Earth’s mesosphere, and is believed to be caused by waves of oxygen atoms glowing due to a stream of particles released from the Sun.
The auroral dunes appear as a green-tinged and even pattern of waves resembling a striped veil of clouds or dunes on a sandy beach. Image credit: Kari Saari.
“The part of the auroral zone where the Earth’s electrically-neutral atmosphere meets the edge of space is an extremely challenging environment for satellites and other space-borne instruments,” said University of Helsinki’s Professor Minna Palmroth, who led the team of professional and citizen researchers.
“This is why it is one of the least studied places on our planet.”
“Due to the difficulties in measuring the atmospheric phenomena occurring between 80 and 120 km (50-75 miles) in altitude, we sometimes call this area the ignorosphere.”
The dunes were observed precisely in that particular region of the auroral zone.
The phenomenon guided the scientists towards a middle ground between atmospheric research and space research, as the usual methodology of space physics could not explain it alone.
“The differences in brightness within the dune waves could be due to either waves in the precipitating particles coming from space, or in the underlying atmospheric oxygen atoms,” Professor Palmroth said.
“We ended up proposing that the dunes are a result of increased oxygen atom density.”
Very rarely, a gravity wave rising up in the atmosphere can be filtered and bent to travel between the mesopause and an inversion layer intermittently formed below the mesopause. The mesopause and the inversion layer are colder than the other layers of the atmosphere. In the wave channel established between these two layers, gravity waves coming from below can travel long distances without subsiding. Dune-shaped auroral emissions are created when solar wind charges the oxygen atoms surging through the channel. Image credit: Jani Närhi.
Next, Professor Palmroth and colleagues had to determine how the variability in the density of the oxygen atoms caused by gravity waves in the atmosphere results in such an even and widespread field of waves.
Normally at the altitude of study there are many different kinds of gravity waves traveling in different directions at different wavelengths, which is why they do not easily form the even wavefields exhibited by the dunes.
The study suggests that the phenomenon in question is a mesospheric bore, a rare and little-studied phenomenon that takes place in the mesosphere.
The tidal bore phenomenon is a wave common to many rivers, where the tide travels up the river channel.
Various types of gravity wave are born in the atmosphere and then rise.
In very rare cases, gravity waves can get filtered as they rise between the mesopause and an inversion layer that is intermittently formed below the mesopause.
The inversion layer makes the filtered waves bend and enables them to travel long distances through the channel without attenuation.
When the oxygen atoms in the bore collide with the electrons precipitating down upon the atmosphere, they become excited.
When releasing this excitation, they create the auroral light. This is why mesospheric bores — a phenomenon thus far considered a very challenging subject of research — can occasionally be seen with the naked eye.
Prior to this discovery, mesospheric bores were not observed in the auroral zone, nor have they been investigated via auroral emissions.
“The auroral zone as a whole is usually discounted in studies focused on the bore, as auroral emissions impair the technique used to identify mesospheric bores,” Professor Palmroth said.
The team’s findings were published in the journal AGU Advances.
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M. Palmroth et al. Citizen Scientists Discover a New Auroral Form: Dunes Provide Insight into the Upper Atmosphere. AGU Advances, published online January 28, 2020; doi: 10.1029/2019AV000133
