Effects of lightning upon trees near telegraph wires

Abstract

Visual auroras at low latitudes are rare, documented for only five occasions since 1859. They are important from a theoretical point of view in terms of the lowest latitude at which particle precipitation can been observed. One of the great magnetic storms of the past 150 yr occurred on 25 September 1909, during the prolonged solar activity minimum around the beginning of the present century. A report of an auroral sighting at Singapore, not far from the magnetic equator, the lowest latitude ever reported for a visual aurora. has focussed attention on this event. This paper compiles and discusses the solar, magnetic, auroral, and telegraphic disturbance data related to the event. The data show that the lowest credible magnetic latitude for this event was between 30° and 36°. I conclude that the report of a visual auroral observation at Singapore, based on a single Australian newspaper item, was almost certainly not auroral at all, but rather resulted from cable disturbances, known to be associated with auroras, and confused with the aurora itself. Furthermore, consideration of the height at which particle precipitation would be observed leads to an estimate of a threshold magnetic latitude for a visual auroral event of the order of 15°, greater than that of 'Singapore. This result removes the necessity of theories of major storms to account for such a low latitude sighting.