An alert that the Northern Lights would be visible across all of Great Britain last night was wrongly issued because a sit-on lawnmower disturbed scientific instruments.

Subscribers to the AuroraWatch UK mailing list were sent a “Red Alert” yesterday; informing them that it would be possible to view the aurora borealis from anywhere in the UK.

Unfortunately, this extraordinary alert was withdrawn just four hours later when it was discovered that a groundskeeper driving a sit-on lawnmower had disturbed the readings of a local magnetometer.

AuroraWatch UK, which is operated by boffins from Lancaster University, collects geomagnetic activity measurements from magnetometers across the UK, the Faroe Islands, and even Russia, to detect when the aurora borealis is likely to be visible from the UK and other locations.

Yesterday afternoon it sent its excited subscribers a “Red Alert”, declaring that: “It is likely that aurora will be visible by eye and camera from anywhere in the UK. Viewing aurora requires clear, dark skies away from light pollution.”

The alert was automatically issued after a crazy reading was recorded, visible on AuroraWatch UK's statement. Between 12:00 and 15:00 the readings from the LAN2 magnetometer go so crazy as to look like a thick line scrawled across the plot.

The red alert was withdrawn when it was found that the reading had only affected LAN2. The Aurorawatch UK statement said: “Unfortunately, the readings were spurious and not related to geomagnetic activity. It, instead, appears that some local interference set off a massive spike in the data (no, it wasn’t the intern!).”

A later update red-facedly confesed to what had caused the huge anomaly: “We believe the interference was caused by university staff mowing the grass on a sit-on mower. We’ll work with the facilities team to try and avoid an incident such as this occurring in the future!”

A Lancaster University spokesman told El Reg:

AuroraWatch UK measures the changes in the earth’s magnetic field using instruments called magnetometers. When an aurora is potentially visible from the UK, our magnetometers record a disturbance in the magnetic field and alerts are automatically sent out to our followers. Unfortunately, auroras (Northern Lights) aren’t the only thing that can cause such changes to the measured magnetic field. In this instance, the ride-on lawnmower’s electric motor created its own local magnetic field and, because it was driven near to our magnetometer, we recorded this disturbance and sent out an alert.

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