Category Archives: self-luminously phenomena
It was an ironic situation when during the night from 14th to 15th of July 2012 (at a weekend) a high number of observers and photographers were looking for a predicted aurora borealis and instead were confronted with a remarkable outburst of structured (or banded) green airglow. This phenomenon is well known and explored by professional geo-scientists but seemed to have slipped the attention of most amateur observers, including myself, up to then. Though it first seemed likely that the geomagnetic storm may have somehow triggered this event, later observations (e.g. July 23rd, 2012: http://www.polarlichter.info/airglow.htm) indicated that the traditional excitation mechanism (UV and X-Ray radiation from the sun) is capable of producing intense green airglow without the need for a geomagnetic anomaly.
Due to the fascination I felt during my own observation, I got interested in using the many available photographs from the July 14th/15th night for a height and position reconstruction. However, as I later found out from literature, the airglow height of 87-95 km (i.e. a quite thin layer, comparable to the NLC layer around 83 km) is already well established by professional measurements. It is remarkable that this value can in fact be reproduced by comparing amateur photographs from various locations in Germany by an un-biased analysis, which I want to present here.
The first task to do was to contact other observes via the well-known communication boards about atmospheric optics to gather suitable photographic material. Of course I had my own images at hand and intended to use them for this process, so I already had a list of time slots to find synchronous counterparts for. Even though I could find several pictures taken within a tolerance of < 1 minute with respect to my own photos, I had to drop most of them since a coarse analysis of the viewing directions yielded no overlapping fields of view. But through discussing my idea with several other photographers, I was able to identify other matching pairs independent from my own material. Finally, I ended up with two data sets (image pairs), 1 and 2, to work with:
1a) Frank and Sabine Wächter: July 15th, 00:55 CEST, 51° 12’ N, 13° 35’ E, 189 m above sea level (Meißen, Saxonia): https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/1a.jpg
1b) Jens Hackmann: July 15th, 00:55 CEST, 49° 29’ N, 9° 55’ E, 333 m above sea level (Weikersheim, Baden-Württemberg): https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/1b.jpg
2a) Franz Peter Pauzenberger: July 15th, 02:02 CEST, 49° 00’ N, 11° 30’ E, 518 m above sea level (Beilngries, Bavaria): https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/2a.jpg
2b) Alexander Haußmann: July 15th, 02:01 CEST, 51° 32’ N, 13° 58’ E, 110 m above sea level (Senftenberg, Brandenburg): https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/2b.jpg
For a detailed analysis, it is necessary to calibrate these photos, which means to precisely assign values for azimuth and elevation to each pixel. If the projection characteristics of the photographic lens are known, the positions of two stars in each picture are sufficient input for this purpose. However, the simple assumption of an ideal gnomonic (rectilinear) or equal-area projection (for ordinary and fisheye lenses, respectively) drastically limits the accuracy of the results. To overcome this, the projection characteristics for all four lenses were reconstructed by measuring the pixel distances of approximately 15 stars from the image center and compare these with the angular distance from the optical axis for each image.
After this calibration and assignment, longitude and latitude positions for each pixel can be calculated, allowing the projection of the photo onto a map if a certain height of the airglow layer is assumed. This method already proved to be very useful for the reconstruction of NLC positions (http://www.meteoros.de/php/viewtopic.php?t=8451). Since the goal is here to determine the layer height, this parameter is varied until the corresponding structures in both reconstructions of an image pair give the best fit. Indeed it was possible to find consistent height values for both data sets, 92 km for pair 1 (https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/1.gif) and 93 km for pair 2 (https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/2.gif). Here the traditional blink comparison technique was applied in a modern form using gif animations. It is fascinating to see how the airglow structures that look completely different in the original two photos of each pair coincide in the reconstruction on the map. Evidently, all non-airglow structures such as trees, background light, clouds, photographic violet aurora etc. have to be ignored in the reconstruction. It should be noted that more complex approaches (http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-51-7-963) are recently established in the professional field, allowing even to resolve finer structures within the thickness of the airglow layer.
Furthermore, these reconstructions show an undistorted view on the band structure of the green airglow layer. As already expected from the perspective view of the original photos, these bands are roughly aligned in the direction from West to East. Using the consistent height information obtained from the image pair comparisons, it is moreover justified to project a whole picture series from a single observation site onto the map in order gain insight in the airglow band dynamics. For this purpose I used a time lapse series that I took from July 14th, 23.16 CEST to July 15th, 01.20 MESZ at the Senftenberger See (51° 29’ N, 14° 01’ E), starting in the evening dawn and originally intended to capture the predicted aurora borealis (https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/3.avi). Due to the weak contrast of the airglow at this stage, strong image processing is needed to separate the bands from the background. Though this finally results in a rather poor signal to noise ratio, it can clearly be seen that the airglow bands move in northward direction (https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/4.avi), illustrating the recombination and/or matter transport dynamics in the mesopause region.
Author: Alexander Haussmann
On November 28, the webcam of the Meteorological Observatory on the 3106 metres high Mt. Hoher Sonnblick in the Hohe Tauern Mountains in Austria could catch this unbelievable St. Elmo´s Fire. This photographic webcam, a Canon EOS 1100D, was installed by radio hams and takes one picture every 10 minutes.
Weather observer Hermann Scheer describes the meteorological situation as follows: “The day the pictures were taken ist was snowing, and there was a stormy southwesterly wind with wind speeds around 60 kph. There was no thunderstorm near. I did not notice any discharges, but I could clearly hear the crackling noise on the tower outside, which is always a sign that there is a certain voltage applied. When I went to the platform with my camera the first time, I could not catch St. Elmo´s Fire quite clearly and did not really notice how strong it was. Later, during the second photo shooting, the camera on the tripod began to sparkle, and I also noticed the tension in my short hair. Then I saw St.Elmo´s Fire on the suntracker. The interesting phenomenon lasted for about one hour.”
A high field strength under conditions with falling snow and strong wind is not very rare, as these example diagrams from a day with similar weather conditions show (1-2). When there is enough tension generated by wind friction and high humidity, an electric current begins to flow between the charged air and the point of the instrument. The air becomes ionized generating a flickering, pale blue light that looks like a flame. St. Elmo´s Fire is probably reported so rarely because only very few people look for it on mountain tops under such weather conditions, and during thunderstorms there are no people there.
But even if St. Elmo´s Fire is very beautiful to see, it is in any case a warning. If you see St. Elmo´s Fire near, it is probably a hint that a discharge is imminent. So, if you see it, you should look for shelter immediately.
During the night from July 14 to July 15, 2012, Tilo Schroth could observe and photograph in northern Saxony, Germany this rare phenomenon: “Chasing aurorae from northern Saxony is full of surprises. You are burning the midnight oil in the middle of nowhere during nights which potentially might bring a touch of aurora, pressing the release from time to time, controlling the monitor display, and then looking up to the sky again. A shooting star here, a small satellite there…there is a lot to see during those nights. The same was it during the night between July 14 and July 15, when the solar wind was about to hit earth directly. We were three persons on the Liebschützberg range (except from some couples in love). We were just about to call it a day when a slowly increasing green loom appeared on our monitors. At that moment I still was connected to the Meteoros-forum, but a short while later the storage battery of my mobile phone showed empty.
My two friends and me on the hill were sure that this was a green aurora. So we were very excited. We were not connected to the forum any more, where somebody already had talked of “airglow”. Nevertheless, we were not disappointed when learning that our guess was wrong the next day.
What we saw – and could not really interpret – was really airglow. Appearing as a grey veil to the naked eye, our reflex cameras reveiled its real structures.”
“Airglow” is the name for a faint, lamellar glowing of the night sky which is induced by electromagnetic radiation. In normal cases, this diffuse glooming can hardly be perceived, but its brightness can be influenced by strong ionospherical activities. So airglow could not only in this case be seen in connection with solar wind and aurorae. The latter were visible over Germany also in this night, but just very faintly.
An interesting fact is the far southward extension of the airglow shown on a map made by Stefan Krause, which shows all observations. This is probably due to the fact that in Northern Europe it does not really get dark at this season. So parts of the atmosphere might get some sunlight which may intensify the airglow.
In the Meteoros-forum are more pictures from this rare phenomenon.
On May 26, 2011, Martin Popek filmed sprites above an area with heavy thunderstorms preceding a cold front in Nydek (Eastern part of Czech Republic) with its video camera (Watec 902h2 ultimate + lens 8/1,3). The radar map (1-2) shows the position of the observer and the approaching thunderstorm front.
On Nov/14/2007 between 18:00 and 19:00 CET St. Elmo’s fire appeared on a tower at the summit of Mt. Gäbris in Switzerland. The phenomenon was captured with a high sensitive video camera operated by Mark Vornhusen of Meteomedia, a private weather company.
The St. Elmo’s fire developed during a snow thunderstorm. Only one lightning strike occurred during this storm. The strike is visible on the webcam images at 18:55 CET as a sudden flare of the image and was heard by the author, who lives 2km away from the summit. The lightning strike was not close to the tower, because the flare was not very bright. The cameras operate with a special software that is able to capture all bright objects on the night sky, even if the duration is only a fraction of a second (lightning, meteors).
The St. Elmo’s fire was first visible on the webcam images at 18:05 CET and lasted about two minutes. The second and most intense appearance occurred between 18:40 and 18:50 CET, followed by the lightning strike at 18:55. During the 10 minute interval between 18:40 and 18:50 CET the weather station, which is also located on top of the tower, measured a wind gust of 75 m/s (270 km/h). This is obviously not a real wind speed. More likely it is an interference of the high voltage and the St. Elmo’s fire to the anemometer. The ultrasonic anemometer uses sound speed to measure the wind speed (http://thiesclima.de/usanemo.htm).
Author: Mark Vornhusen, Gais, Switzerland