Start of the Northern Lights Season

Click on map for better resolution.

Almost three weeks after the summer solstice, we start to get questions regarding when the Northern Lights can be seen again after the Polar Day. The answer depends on where you are: the further south you are, the earlier it will get dark enough.

How dark does it need to be? Our scientific cameras operate when the Sun is at least 10° below the horizon. However, if there are particularly bright Northern Lights, you might be able to see them already earlier.

The map above shows the days on which the Sun will be at least 10° below the horizon in Finland. Be aware, however, that due to summer time (daylight saving time), the darkest time of the night is around 01:00h in Finland.

Note further, that our scientific cameras will not necessarily be back on location on the first day that it is dark enough. Their annual maintenance might well delay the schedule.

Map and text: Thomas Ulich, SGO.

Paper on energetic electron precipitation and auroral morphology accepted in JGR

A recent collaborative study between ISEE /Nagoya University and SGO/University of Oulu (+ other institutes) has been accepted to be published in Journal of Geophysical Research: Space Physics, please have a look:

http://onlinelibrary.wiley.com/doi/10.1002/2016JA023484/full

This paper shows a linkage between different auroral morphological structures and the energy spectra of the associated electron precipitation. The study is based on a selected set of case studies combining the EISCAT radar and the KAIRA spectral riometer measurements with the auroral camera observations.

Observation of pulsating aurora signatures in cosmic noise absorption data

A paper was recently accepted for publication in Geophysical Research Letters. It presents the results of a comparison between optical data from an all-sky camera and cosmic noise absorption data measured by KAIRA above Kilpisjärvi, during a pulsating aurora event. 

The accepted version of this article can presently be accessed here; the preliminary reference is:

Grandin, M., A. Kero, N. Partamies, D. McKay, D. Whiter, A. Kozlovsky, and Y. Miyoshi (2017), Observation of pulsating aurora signatures in cosmic noise absorption data, Geophys. Res. Lett., doi:10.1002/2017GL073901.

Abstract
This study investigates the contribution of energetic (E > 30 keV) particle precipitation during a pulsating aurora event over Kilpisjärvi (L = 6.2) on 26 February 2014. It is based on the comparison of auroral blue-line emission (427.8 nm) data from an all-sky camera and cosmic noise absorption (CNA) data obtained from a multi-beam experiment of the Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) riometer. The data sets are compared for three KAIRA beams close to magnetic zenith. Results show a clear correlation between the measured CNA and the auroral blue-line emission during the event, for each beam. In addition, individual pulsations are observed for the first time in the CNA data measured by KAIRA, and are found to be close-to-identical to the optical pulsations. This suggests that the modulation of electron precipitation during pulsating aurora takes place in a consistent way over a broad range of energies.

Northern Lights 31st January / 1st February 2017

[Helsingin Sanomissa 7.2.2017: "Revontulet sykkivät mystisesti" -artikkeli SGO:n ja Japanilaistutkijoiden välisestä tutkimusyhteistyöstä.]

Last week beautiful northern lights were seen across large parts of Northern Finland and Scandinavia. Already early in the evening after 19:00 EET (17:00 UTC), the lights appeared and by 21:00 EET the sky was filled with what we call "pulsating aurora". This form of aurora displays large patches of diffuse green light, which are flickering at various speeds, some flicker very fast. Pulsating aurora is very much a topic of active research, and our Japanese colleagues just launched the ARASE (ERG) satellite to study these specific lights from space. These efforts are combined with optical studies on the ground, and just in time for last week's northern lights, a special high-speed camera was installed at SGO, which can take up to 100 photos of the aurora per second!

From Sodankylä, four films of the aurora during the night of 31st January / 1st February have recently been published, two of which are from SGO:

The video above was made by taking all of the images of the regular all-sky camera of SGO, which are black-and-white images taken through filters for the auroral green, red, and blue lines, and combine these to create RGB (false) colour images. Images are taken every 20 seconds, and thus the video linked above is a time-lapse of the whole night, which is just 1min 30 sec long. (©2017 by SGO)

The video linked here is a result of a collaboration between SGO and Site-Eye Ltd., UK. Site-Eye have installed two long-term time-lapse cameras at SGO, one looking at the sky at a northerly direction, the other looking west across the river Kitinen. This video combines images from both cameras. (©2017 by SGO and Site-Eye Ltd)

The video above was taken by SGO's Thomas Ulich, who placed a camera looking almost straight up, with a slight tilt towards the south. This is the best direction to see what is called the auroral corona, a display, where the auroral rays all seem to originate from the same point in space. This is, however, just a result of perspective: the rays are in fact parallel. Images taken over 9 hours at a rate of four photos per minute were combined into a time-lapse film of about one minute duration. (©2017 by Thomas Ulich, see his blog post)

In the final video of this series, which was taken by SGO's Esa Turunen using a fish-eye lens to cover as much sky as possible, you can see the shapes of pulsating aurora especially well. They are patches drifting across the sky and flickering at the same time. The images were taken at a rate of approximately one per second. (©2017 by Esa Turunen)

Please note that in all videos linked here, the flickering of the aurora is aliased by the number of time per minute the images were taken as well as by the frames-per-second rate of the final time-lapse films.

Calibration of the All-Sky Cameras

As nights are getting dark enough to observe the northern light, it is time to perform the yearly absolute calibration of the cameras and imagers which monitor the aurora. This week, scientists from the Finnish Meteorological Institute, EISCAT, the University of Southampton and the University Centre in Svalbard (UNIS) are calibrating their instruments in Tähtelä.

Four all-sky imagers are being calibrated: two ICCDs (Sodankylä and Longyearbyen) and two EMCCDs (Kilpisjärvi and Abisko). The calibration of each instrument takes a couple of hours. It consists in measuring the response curve of the sensor to a known source of light obtained from three lamps illuminating an integrating sphere. The purpose is to be able to convert the number of counts received in each pixel of the sensor into photon flux in rayleigh (1 R = 795,774,716 photons.m^-2.s^-1.sr^-1).

Integrating sphere used to produce the calibrating light source.
Photo: M. Grandin

Since the instruments measure the auroral emission in three wave lenghts (427.8 nm "blue", 557.7 nm "green" and 630.0 nm "red"), the calibration must be done for each of these three channels. In practice, the green and red channels are calibrated during a same procedure, and the blue channel calibration takes place separately. The whole calibration procedure is of course done in a dark room and is controlled remotely.

The Sodankylä ICCD imager set up for calibration.
Photo: M. Grandin

May this auroral season be rich in substorms with clear-sky conditions!

Polar Day Ahead!

The nights are getting brighter, and we are rapidly approaching the Polar Day, when the Sun does not set anymore for many weeks. The All-Sky Camera of SGO requires that the Sun has set at least 10° below the horizon before it will start up to take images of the night sky and the aurora if there's any. On Friday, 24th April, the Sun will not set below –10° elevation, and thus the camera operations have now been stopped for the summer. The next time the Sun goes low enough in the night is on 19th August. Thus from our auroral camera's point of view, the Polar Day is about to start, and it will last for 117 days!

Please note that for operational reasons, SGO's All-Sky Camera will not be immediately available from 19th August onwards, but of course we try to make the real-time images available as soon as feasible.

The image above (click to get a larger version), shows the light conditions in 2015 for the location of our All-Sky Camera. On the horizontal axis there are key dates for this year. On the vertical axis are the number of hours of darkness. Yellow refers to daytime, i.e. the Sun is above the horizon, and in the middle of the year, there are some 44 days when the Sun does not set at all here. The darkest blue refers to the time when the Sun is 18° or more below the horizon. This is considered total darkness.

The remaining shades of blue refer to the twilight: the lightest blue represents civil twilight, when the Sun is between 0° and 6° below the horizon, i.e. the time just before sunrise or after sunset. The next darker blue with the green line refers to nautical twilight when the Sun is between 6° and 12° below the horizon. The green line shows the limit for the All-Sky Camera, when the Sun is 10° below the horizon. The remaining shade of blue refers to astronomical twilight when the Sun is between 12° and 18° below the horizon.

The green line goes to zero (no darkness anymore) on 24th April and rises again on 19th August. In the middle of winter, the Sun remains for more than 16 hours low enough for our camera to operate.

Furthermore, we can see that just around winter solstice, the Sun does not rise at all during four days. However, it is not totally dark thanks to the prolonged civil twilight time and usually bright, snow-covered land.

Graphic and solar data: Thomas Ulich.