AuroraWatch UK:an automated aurora alert system

The AuroraWatch UK aurora alert service uses a network of magnetometers from across the United Kingdom to measure the disturbance in the Earth's magnetic field caused by the aurora borealis (northern lights). The service has been measuring disturbances in the Earth's magnetic field from the UK and issuing auroral visibility alerts to its subscribers, since September 2000. These alerts have four levels, corresponding to the magnitude of disturbance measured, which indicate from where in the UK an auroral display might be seen. In the following, we describe the AuroraWatch UK system in detail and reprocess the historical magnetometer data using the current alert algorithm to compile an activity database. This data set is composed of over 150,000h (99.94% data availability) of magnetic disturbance measurements, including nearly 9,000h of enhanced geomagnetic activity. Plain Language Summary Witnessing the aurora borealis, more commonly known as the northern lights, is a much desired event, often featuring in people's "bucket lists." Although rarer than in more arctic regions, such as Scandinavia, Iceland, and Canada, the northern lights are seen from the UK too. To help with this aurora-hunting endeavor, the AuroraWatch UK service sends alerts to its followers when UK aurora sightings may be possible. The service has been running for 17 years and has over 100,000 subscribers. We have recorded over 150,000 h of magnetic field measurements including nearly 9,000 h where geomagnetic activity was large enough for an aurora to potentially be seen from at least some parts of the UK