Geomagnetic activity during the passage of the Earth through Halley's tail in 1910

Abstract

In 1910 Halley's comet as seen from the Earth transited the Sun and hence the Earth should have passed through the comet's tail. From our understanding of the nature of a cometary tail we would expect the tail to have shielded the Earth's magnetic field from the dynamic pressure of the solar wind except for a brief period in the centre of the tail when the Earth passed through the dense flowing ion tail. Such a disturbance occurred, but it appears to have been 12 h too early. A detailed study of the records reveals that the discrepancy is due to a change in the convention for determining the start of the day. In 1910 the commonly used Greenwich Mean Time referred to Astronomical Time at Greenwich, which was 12 h different from the Greenwich Mean Time, or Universal Time which is in use today. Hence the disturbance associated with Halley did in fact arrive at the expected time and no unusual aberration of the solar wind need be invoked to explain the timing. The disturbance consisted of two troughs in the horizontal component of the Earth's magnetic field separated by ∼ 14 h presumably associated with wakes in the solar wind momentum flux on either side of the ion tail. The disturbance was independent of latitude indicating that the responsible current system flowed far above the surface of the Earth. After the passage of the comet, the magnetosphere was left in a mildly disturbed condition with a weak ring current present.