This optical spectrum is measured on a small vertical strip between inclinations of 20° and 35° on the Northern sky. White triangles mark spectral lines characteristic for aurora. These are foremost the prominent 557.7 nm green oxygen line, the 630.0/636.4 nm red oxygen doublet, and the nitrogen lines at 391.4 nm and 427.1 nm. The Hydrogen α line at 656.3 nm is also typical for aurora, particularly for proton aurora, but can also have other origins.
The pronounced peak around 593 nm, as well as the peak at 819 nm, is caused by sodium streetlights in the area. Further artificial sodium and mercury lights are marked with orange and violet triangles, respectively. The strength of these disturbers can be used as measure of cloudiness of the sky.
For more information about the device, just read on or see a more detailed description of it.
Currently, an intensity calibration is still missing. First experience shows that from an intensity of ≈ 20 on, the aurora can be detected using photographic cameras ("photographic aurora"). Above an intensity of ≈ 30, the aurora should be visible to the human eye ("visual aurora"). Fast changes in intensity usually hint at moving structures in the aurora, indicating the presence of a substorm. In case of high background light it could also result due to moving clouds.
This experimental spectrometer measures the night sky spectrum and has the main purpose to allow for a real-time detection of northern lights, specifically of the strong and destinctive 5577Å line. It observes the Northern suburban sky at Viby/Sollentuna near Stockholm/Sweden (59°27' N, 17°54' E) at inclinations between about 20° and 36° above the horizon, directed at 5°N. Aurora, even if not yet visible to the unaided eye, will be detected. Any observer north of Stockholm and even south of Stockholm, down to southern Sweden should be able to detect this aurora, albeit at lower inclinations in the sky.
There is a fair amount of artificial background light, mostly by sodium vapor lamps and few mercury vapor lamps. Therefore the measurements do not represent the dark-sky spectrum.
Please note: The spectrometer is operated sporadically only, when conditions are favorable for northern lights and local weather is favorable and local skies are reasonably clear. I am also running a weather station which estimates sky clearness using a pyrometer. The spectrometer is a tool for nowcasting auroral presence. If you're interested in predictions concerning northern lights over central and southern Sweden, over Denmark and northern Germany, please check out my magnetometer, which issues alerts via Twitter. There's also a Magnetometer Line available, covering over 1790 km from Southern Germany to Central Sweden.
The spectrometer design is inspired by Michael Theusner's aurora monitor based in Bremerhaven, Germany (52° North). The optical system is based on a blaze transmission grating. The 300 grooves/mm grating is illuminated by a single slit. It is observed by a ZWO ASI120MM CMOS camera with a 16mm f/1.2 CS-mount lens. The spectrometer has been calibrated using several low-power lasers, and a daytime/twilight Fraunhofer-calibration is in preparation.
Data acquisition and analysis is performed by a Raspberry Pi 3 Model B+. The acquisition code is based on Thomas Jacquin's Wireless All Sky Camera project. The analysis code is self-developed and based on Mark Kness' Colorpy package. A night's worth of data amounts to 3.8 GiB (can be compressed losslessly to below 1 GiB), which are reduced on-line to around 120 MiB of spectral plots and 300 kiB of analytic spectral data.
The spectrometer takes regular exposures of 45 seconds duration. Those are analyzed and evaluated individually online, integrating over the full inclination range. Later, offline analyses stack frames to exposures of 3 minutes. The spectra are also analyzed differentially in inclination to produce keograms.
The spectrometer saw its first aurora on 2017 September 22: Here is the first aurora borelis spectrum recorded with this device, the spectrum of a fluorescent lamp with its many spectral lines, and here is a picture of the prototype housing built in summer 2017.
Since October 2017 the spectrometer has been operated on regular basis. During an aurora display on the evening of 2017 December 4, some Results beyond the online plots were produced, in particular a timelapse of the 2017 December 04 evening with aurora borealis and a keogram of that night. Another aurora was recorded on 2018 October 7.