FAEs

the basics of fragmented aurora-like emissions

Soooo what exactly are FAEs? It stands for fragmented aurora-like emission(s), a name that Dan Whiter suggested due to it invoking the mythical beings from many European cultures, which are often described as magical tricksters. This name seemed like a perfect fit for these small, short-lived, hard-to-predict, and mystifying aurora-like features. Even though they sure look a bit like aurora, we deliberately refer to them as aurora-like, since they seem to be locally generated in the ionosphere, and thus do not qualify as “aurora” per se (aurorae are the result of particle precipitation from the magnetosphere).

Most FAEs seem to appear in groups close to auroral arcs, which I referred to as “category 2” in my master’s thesis. Due to their fairly regular spacing and close proximity to the arc, some kind of instability- or wave-driven generation mechanism seems highly likely, see also the articles listed below. Determining the exact type of wave that generates FAEs will require more work, so stay tuned for news!

Some very good examples of category 2 FAEs can be seen below:

These images of category 2 FAEs near Kjell Henriksen Observatory (KHO) on the Breinosa mountain on Svalbard were taken by Ida Svenningsson on 25 November 2022, towards the end of a rather spectacular substorm.

We had the privilege to observe these FAEs while walking down the Breinosa mountain after a fieldwork evening at KHO - the first time I was able to see them in person, rather than on all-sky camera (ASC) images. They appeared after a particularly dynamic arc showed signs of wave activity, towards the end of this substorm event. After spotting the first group, it was remarkably easy to find the next ones - I had previously presumed that FAEs might be somewhat difficult to spot in person due to their small size and short lifetime, but am happy to report that it is very much possible with knowledge what to look for (and some decent luck)!

These images of category 2 FAEs above the town of Longyearbyen on Svalbard were taken by Ida Svenningsson on 25 November 2022, towards the end of substorm recovery, approximately half an hour after the two pictures above.

This all-sky camera (ASC) image taken at the Kjell Henriksen Observatory is a great example of a category 2 FAE group with regular spacing.

These FAE groups generally appear, drift close to the arc and then disappear within a few seconds to a minute.

This group of cat 2 FAEs appears, drifts westward on the poleward side of the auroral arc and then fades away in approximately 30 seconds.

While many of the FAEs we have observed so far fall into category 2, they sometimes occur outside of these groups, seemingly random across the sky and not close to auroral arcs. We refer to this type of FAE as category 1, and some examples can be seen on the all-sky camera image below.

These ASC images taken at the Kjell Henriksen Observatory show examples of category 1 FAEs, which appear irregular and spread out across the field of view.

I will expand these examples and description in the near future, check back later for more!

References

Theses

2019

MSc Thesis

A detailed study of auroral fragments

Joshua Dreyer

Uppsala UniversityUppsala University, Department of Physics and Astronomy, Swedish Institute of Space Physics, Uppsala Division, Jun 2019

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Aurora occurs in various shapes, one of which is the hitherto unreported phenomenon of auroral fragments. For three periods of occurrence of these fragments their properties were studied in detail during this master’s thesis, using mainly ground-based instrumentation located near Longyearbyen on Svalbard, Norway. A base dataset was constructed from 103 all-sky camera images, manually marking 305 fragments for further analysis. This thesis reports and describes the fragment observations during the observed events, including the auroral and geomagnetic context. Fragments generally seem to fall into two categories, the first being singular, apparently randomly distributed fragments, and the second being periodic fragments that occur in groups with a regular spacing close to auroral arcs. A typical fragment has a small horizontal size below 20 km, a short lifetime of less than a minute and shows no field-aligned extent in the emission. The fragments appear mainly west of zenith (73%) during the three observation nights, whereas their north-south distribution is symmetric around the zenith. Almost all of them exhibit westward drift, the estimated speed for one of the fragments passing the field of view of ASK is ∼1 km/s. A spectral signature can be seen in the green auroral wavelength of O at 557.7 nm and red emission line of N2 at 673.0 nm, but no emission enhancement was observed in the blue wavelengths. One fragment passing the EISCAT Svalbard radar’s field of view shows a local ion temperature increase in a small altitude range of ∼15 km, whereas there is no visible increase in electron density. This could be explained by fragment generation due to locally strong horizontal electric fields. A potential mechanism for this might be electric fields of atmospheric waves superposing with the converging electric fields of auroral arcs created by particle precipitation and the corresponding field-aligned currents. The resulting field would be perpendicular to the magnetic field and the auroral arcs, leading to wave-like density variations of excited plasma close to the arcs. Further study is required to verify this hypothesis and improve the understanding of fragment properties determined from the limited dataset used for this thesis.

Journal Articles

2021

AnnGeo

Characteristics of fragmented aurora-like emissions (FAEs) observed on Svalbard

J. Dreyer, N. Partamies, D. Whiter, P. G. Ellingsen, L. Baddeley, and S. C. Buchert

Annales Geophysicae, Mar 2021

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This study analyses the observations of a new type of small-scale aurora-like feature, which is further referred to as fragmented aurora-like emission(s) (FAEs). An all-sky camera captured these FAEs on three separate occasions in 2015 and 2017 at the Kjell Henriksen Observatory near the arctic town of Longyearbyen, Svalbard, Norway. A total of 305 FAE candidates were identified. They seem to appear in two categories - randomly occurring individual FAEs and wave-like structures with regular spacing between FAEs alongside auroral arcs. FAEs show horizontal sizes typically below 20 km, a lack of field-aligned emission extent, and short lifetimes of less than a minute. Emissions were observed at the 557.7 nm line of atomic oxygen and at 673.0 nm (N2; first positive band system) but not at the 427.8 nm emission of N2+ or the 777.4 nm line of atomic oxygen. This suggests an upper limit to the energy that can be produced by the generating mechanism. Their lack of field-aligned extent indicates a different generation mechanism than for aurorae, which are caused by particle precipitation. Instead, these FAEs could be the result of excitation by thermal ionospheric electrons. FAE observations are seemingly accompanied by elevated electron temperatures between 110-120 km and increased ion temperatures at F-region altitudes. One possible explanation for this is Farley-Buneman instabilities of strong local currents. In the present study, we provide an overview of the observations and discuss their characteristics and potential generation mechanisms.
AnnGeo

Fine-scale dynamics of fragmented aurora-like emissions

Daniel K. Whiter, Hanna Sundberg, Betty S. Lanchester, Joshua Dreyer, Noora Partamies, Nickolay Ivchenko, Marco Zaccaria Di Fraia, Rosie Oliver, and 3 more authors

Annales Geophysicae, Nov 2021

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Fragmented aurora-like emissions (FAEs) are small (few kilometres) optical structures which have been observed close to the poleward boundary of the aurora from the high-latitude location of Svalbard (magnetic latitude 75.3 °N). The FAEs are only visible in certain emissions, and their shape has no magnetic-field-aligned component, suggesting that they are not caused by energetic particle precipitation and are, therefore, not aurora in the normal sense of the word. The FAEs sometimes form wave-like structures parallel to an auroral arc, with regular spacing between each FAE. They drift at a constant speed and exhibit internal dynamics moving at a faster speed than the envelope structure. The formation mechanism of FAEs is currently unknown. We present an analysis of high-resolution optical observations of FAEs made during two separate events. Based on their appearance and dynamics, we make the assumption that the FAEs are a signature of a dispersive wave in the lower E-region ionosphere, co-located with enhanced electron and ion temperatures detected by incoherent scatter radar. Their drift speed (group speed) is found to be 580-700 m s-1, and the speed of their internal dynamics (phase speed) is found to be 2200-2500 m s-1, both for an assumed altitude of 100 km. The speeds are similar for both events which are observed during different auroral conditions. We consider two possible waves which could produce the FAEs, i.e. electrostatic ion cyclotron waves (EIC) and Farley-Buneman waves, and find that the observations could be consistent with either wave under certain assumptions. In the case of EIC waves, the FAEs must be located at an altitude above about 140 km, and our measured speeds scaled accordingly. In the case of Farley-Buneman waves a very strong electric field of about 365 mV m-1 is required to produce the observed speeds of the FAEs; such a strong electric field may be a requirement for FAEs to occur.