The Aurora Alarm detects the light from an aurora directly, rather than measuring some other correlated variable (like the amount of disturbance in the local magnetic field). This is the ultimate in real-time, "ground truth" data, but it does make the detection of the aurora contingent on good weather at the detector sites. This is actually a good thing, as long as you live close enough to the detector to experience similar cloud cover conditions.
How does the detector tell the difference between light from an aurora and light from other sources? The human eye perceives many colors in the aurora, but these are caused by emission occurring at a relatively small number of optical wavelengths. Like the glow from a neon sign, all of this light's energy is concentrated into very narrow spectral lines. By contrast, other natural light sources/reflectors in the night sky---the moon, stars, and planets---spread their energy over all visible wavelengths. The detectors for the Aurora Alarm take advantage of this by isolating a prominent green emission line in the aurora using a highly selective filter.
This faint green light is measured with a special type of vacuum tube called a photomultiplier tube, or PMT. (Unlike its obsolete cousins that were replaced by transistors years ago, the PMT is still widely used in physics and astronomy to sense extremely low light levels.) A microprocessor circuit continuously monitors the output level of the PMT and communicates this information through a built-in serial port. If the local site has Internet access, the data runs directly into an Internet-connected computer running Linux; otherwise it may take an indirect route to the Internet via a radio link.
At another site, potentially anywhere in the world but currently located in Pleasantville, New York, data from the detector(s) is received in real time and stored in a database. A computer program watches the data continuously and compares it with preset alarm thresholds, taking into account known times of sunrise/sunset/moonrise/moonset, brightness of the current moon phase, and sometimes other factors as well. When the sky brightness at a detector site passes an alarm threshold, email is automatically sent to about a thousand recipients on the public Aurora Alarm mailing lists. Mailing speed is checked from time to time with the goal of keeping the entire process to less than ten minutes.
Reference: Jesse Knight, Monitoring the aurora electronically. Sky and Telescope, June 1982.Technical Documents:
Home - News - FAQ - Tech - Sign up - Data - Gallery