In this inaugural blog post by Dr. Maria-Theresia Walach, a Senior Research Associate in Lancaster’s Space and Planetary Physics group, we learn about how scientists study the aurora from space – including the recently revived IMAGE spacecraft.
When we go aurora hunting, we need to be in the right place, at the right time and then of course it needs to be dark, and the weather needs to be right.
Conditions that are tricky to come across.
But just because the aurora is not dancing in the night sky above our heads, does not mean that there is no auroral activity at all. During the day, sunlight washes out any auroral emission. As far as we know, auroral emission in the ultraviolet (UV) part of the light spectrum is always there, it just so happens that we cannot see it by eye. UV-aurora is generally hard to observe from the ground, as it is emitted above the layers of the Earth’s atmosphere which filter most of it out.
For many decades now we have been studying the aurora by observing it from space, downwards and sideways on, as well as from the ground upwards.
One of the first images of the aurora under sunlit conditions taken by the HILAT spacecraft on 23 July 1983 above the Arctic coast of Russia. The entire field of view was sunlit. The image is in false-colour where yellow shows the highest emission intensities recorded and blue the lowest (source: Meng et al. 1984; Paxton and Meng, 1999).
As you can see from this image, there is a faint band of emission draping over the very northern parts of Russia and like the aurora we see from the ground, the emission can be patchy and streaky!
Since the beginning of the space-age, many space-borne auroral imagers have been flown, with varying image resolution and the capability to observe the aurora at different wavelengths.
In 1971, the Canadian satellite ISIS-2 was launched, which observed something known as the diffuse aurora for the first time. This type of aurora forms a wide band around where we would usually see visible aurora from the ground. Unlike the arcs of green light we often see (the result of atomic oxygen), the diffuse aurora is almost ever-present but is too faint to be seen by eye from the ground.
Looking down at the Earth from space to observe the aurora has several advantages. Not only does it enable us to see aurora during the day and when it is cloudy, but it also allows us to establish a much more global picture than one camera on the ground ever could.
During the International Geophysical Year, or third International Polar Year (1957-1958), a large network of ground-based auroral cameras, also known as ‘All-Sky Imagers’, were set up. By setting up networks of cameras, a mosaic of the auroral activity allows to see the large-scale activity when the sky is not overcast. For the first time, they allowed us to see the large-scale dynamics of the aurora from the ground up. This led to the realization that the aurora forms as an oval around the Earth’s magnetic pole. Of course, we still have networks of ‘All-Sky Imagers’ nowadays and in fact we have one in Lancaster, but we are limited in coverage by where landmass is.
THEMIS-ASI Mosaic put together by NASA/Goddard Space Flight Center Scientific Visualization Studio (source: http://svs.gsfc.nasa.gov/3590).
False-colour auroral image from the Northern hemisphere taken by the Dynamics Explorer 1 at 0241 Universal Time on November 8, 1981 (source: Frank and Craven, 1988).
This geographical limitation for where ground based cameras can be set up, means that the entirety of the auroral oval was not seen until 1981, when another spacecraft, the Dynamics Explorer 1 was launched. This spacecraft had a much larger field of view than previous spacecraft allowing the whole auroral oval to be seen in one image.
Although observing the aurora from space has many advantages, it also comes with drawbacks: Some of our best resolution imagers must fly relatively close to the atmosphere to see all the intricate details.
The closer a satellite whizzes around the Earth, the faster it must go with respect to the ground to keep a stable orbit. This means that there is a trade-off: although some imagers give us extremely detailed views of the aurora, they are by no means comprehensive.
Imagers which fly further away from the Earth, on the other hand can see the entirety of the auroral oval for longer intervals, but the resolution is yet to match those of close-flying spacecraft! The SUOMI National Polar-orbiting Partnership (NPP) satellite for example has taken some phenomenal high-resolution pictures of the aurora, despite not being originally built to study the aurora.
Night-time view of the aurora above Canada taken by the SUOMI NPP satellite (source: NASA).
There is a trade-off: a satellite whizzing closer to the Earth give us extremely detailed views of the aurora; imagers which fly further away from the Earth can see the entirety of the auroral oval for longer intervals but in less detail.
Being able to see the aurora from above, allows us to study the extent of where and when it flares up on a larger scale, but there is another point of view which we can gain from space: a side-one view.
This allows us to see the vertical structure of the aurora. The videos and images taken by the astronauts on board the international space station are a good example of this.
One of the most recent science satellites built to study the aurora is the Cassiope small satellite. Even though it might not fit into your pocket, as it is just over 2 m3 in volume, it is comparatively small and combines two science missions with several instruments.
A side-ways view of the aurora taken by the Fast Auroral Imager, part of the e-POP instrument onboard the Canadian Cassiope small satellite (source: Cogger et al. 2015).
Amongst other things, the Cassiope satellite allow us to study the aurora as the spacecraft flies through the flickering lights, which is definitely something we cannot do from the ground!
And whilst these are just some examples of auroral imagery taken by space missions, there are more out there too.
The archive of auroral images taken by space missions is vast, but unfortunately, when the satellites stop working, they are not as easy to fix as a ground based camera.
The IMAGE satellite for example took auroral imagery for approximately 5 years, similar pictures to those from the Dynamics Explorer 1 before losing contact with Earth. After this happened in 2005, it was believed to still be in orbit by many scientists, but because the signal was lost, it was hard to tell how functional it really was. Just recently, the signal has been rediscovered by an amateur visual and radio astronomer, so perhaps this once-presumed dead space mission is still going after all!
