Did you know that aurora is not just green, red and blue? It can also be white!
Have the white auroral “GHOSTs” always been there – and why haven’t we seen them before? Traces exist in older data, but past instruments and methods overlooked them. Traditional auroral studies focused on narrow wavelengths, while GHOSTs appear as a broadband glow that can look white, gray, or faintly colored. Only now, with the right combination of instruments, can we confirm them as true GHOST aurora.
By Noora Partamies
For several years I had an auroral all-sky image on my desktop with something I thought looked like a strange light. I asked several colleagues and got different answers, mostly saying that the colours in my image overlapped in a suitable way to produce this weird appearance. Then in an international working group meeting I saw a talk on flat and gray auroral light among normal red, green and blue auroral emissions. That was different from my mysterious image but at the same time similar. We started looking at Svalbard data of my strange thing together in January 2024, and today we have one published article, one PhD project and one young researcher project starting on this topic — auroral continuum emission, white aurora or the GHOST aurora.
We are accustomed to seeing aurora mainly as green, sometimes red or purple light and structures. The spectrum of that is made of individual lines and bands, where in between there is darkness. By continuum emission, we mean a spectrum of light where there are no dark sections, but all colours co-exist at the same time summing up to white. This concept is known from the airglow — spontaneous atmospheric emission that is not caused by particle precipitation. When the spectrum of STEVE arc was measured for the first time it was reported to be a continuum. This would not concern Svalbard, because STEVE is known to occur solely at the equatorial side of the auroral zone. Nonetheless, the strange light in our images from Kjell Henriksen Observatory looked very much like the colour of STEVE: pale pink, off-white. Very much unlike STEVE, our observed structure was not a stable, well-defined arc, but rather a cloud of light changing and moving along with the normal aurora. This is what makes it appear like a GHOST, which a colleague backronymed as Geomagnetically-driven High-bandwidth Optical Spectrum from Thermosphere.
One other thing we observed during our GHOST events is heating of both plasma (electrons and ions) and the neutral gas in the thermosphere (upper part of the atmosphere). It is fortunate for us to have the EISCAT Svalbard radar and a full suite of optical observations from Kjell Henriksen Observatory practically co-located, so we can follow the light and upper atmospheric temperatures evolve together. The upper atmospheric heating is an interesting aspect, because if it is strong enough and expends upwards, it may cause “speed bumps” to the low-Earth orbiting spacecraft.
You might ask if these white auroral ghosts have always been there and why they were not observed before. We think they have always been there, because we can find some GHOST signatures in older measurements as well. One reason for not having observed these before is that a large part of auroral science has been done by studying optical measurements of very narrow wavelength ranges where known specific auroral emissions occur. While the specific wavelengths (colours) may still be there during the GHOST aurora, the broadband enhancement causing the white light remains unnoticed. Another reason is that full-colour images may show GHOSTs as white, off-white, gray, orange, pale pink depending on the background illumination. A colour combination of normal auroral red and green may also appear gray or white without being associated with a broadband spectral enhancement. A suspiciously GHOST-y colour will therefore need a spectral measurement of the light to be confirmed as GHOST aurora. We now have this favourable combination of instrumentation at the observatory collecting data in a favourably synchronised sequence. This was not the case 10 years ago.
What’s next? With our preliminary findings, ideas, new team members and international collaboration we are starting our journey to solve the puzzle. By hunting more GHOSTs we will be able to answer some of the where’s and how’s and why’s, find evidences and turn speculations into facts. This will be great fun!
p.s. We encourage citizen scientists and photographers to report on their potential GHOST observations to SkyWarden (taivaanvahti.fi) worldwide. More observations becomes more coherent knowledge.
More about continuum, STEVE and other auroral structures as well as reporting on our Auroral Field Guide and Handbook: https://kherli.github.io/Aurora-Field-Guide-And-Handbook/
Photo: all-sky photo of the “strange light” on 3 January 2020 at 9 UT (KHO)