At 05:45 UT on 4 May , 1998 the magnetic field at Halley, Antarctica was perturbed by over 4000 nT or almost 10% of its ambient strength. This may be the largest ever recorded natural perturbation of the Earth's magnetic field.

A scientist at Halley, Matthew Paley, observed associated aurora: "The aurora was flickering all over the sky .... I could read the label on the back of my gloves by aurora light."

The magnetometer measurements from Halley between 0 UT and 9 UT on 4 May 1998 are shown in the figure below. The panels show, from top to bottom, the variations in the north-south (H), east-west (D), and vertical (Z) components of the geomagnetic field in nanoTesla. The peak magnetic deflection of over -4000 nT in the H component is seen between 05:45:42 UT and 05:47:27 UT.

Preliminary data from the Wind spacecraft show that the cause of the large magnetic perturbations was an exceptionally strong interplanetary magnetic field (IMF) that swept by the Earth in the solar wind. The IMF strength was 40 nT, about five times larger than the average.

We would like to lead an investigation of this amazing phenomenon. If you have any interesting data that you would like to share, we would be very pleased to hear from you.

Mervyn Freeman and Matt Paley.

M.P.Freeman@bas.ac.uk

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