Duration and extent of the great auroral storm of 1859.

The great geomagnetic storm of August 28 through September 3, 1859 is, arguably, the greatest and most famous space weather event in the last two hundred years. For the first time observations showed that the sun and aurora were connected and that auroras generated strong ionospheric currents. A significant portion of the world's 200,000 km of telegraph lines were adversely affected, many of which were unusable for 8 h or more which had a real economic impact. In addition to published scientific measurements, newspapers, ship logs, and other records of that era provide an untapped wealth of first hand observations giving time and location along with reports of the auroral forms and colors. At its height, the aurora was described as a blood or deep crimson red that was so bright that one "could read a newspaper by." At its peak, the Type A red aurora lasted for several hours and was observed to reach extremely low geomagnetic latitudes on August 28-29 (~25°) and on September 2-3 (~18°). Auroral forms of all types and colors were observed below 50° latitude for ~24 h on August 28-29 and ~42 h on September 2-3. From a large database of ground-based observations the extent of the aurora in corrected geomagnetic coordinates is presented over the duration of the storm event.