Can a Major Geomagnetic and Auroral Disturbance Originate From a Solar Active Region Close to the Limb?

When forecasting major geomagnetic storms, it is commonly assumed that coronal eruptions near the solar limb are not of concern. We here discuss two sequences of events: the Fátima storms of January 1938 and the Halloween storms of October/November 2003. In both cases, a highly active sunspot group generated a major eruption when it reached the western solar limb. In the 2003 events, this generated no terrestrial disturbance, despite being estimated to be the most powerful flare on record. In contrast, the only known solar eruption that could have given the second Fátima storm, was the major flare on the western limb. The geomagnetic and Dcx indices in this storm exceeded disturbance levels that have been seen for less than, respectively, 0.0015% and 0.0091% of the time and caused aurora to be seen at geomagnetic latitudes down to (at Tataouine, Tunisia), making it the 13th most extensive auroral event known since the Maunder minimum. We investigate if this storm was caused by the limb eruption and conclude that it would be a possible but rare occurrence, although the historic data do not allow us to fully eliminate other scenarios. We show that the longitude distribution of low-latitude auroral sightings depends upon the Universal Time of the impact of the coronal mass ejection and the subsequent timing of substorm activity is consistent with the theory of how low-latitude red aurora is generated. We show how the Dcx and indices give understanding of the substorm behavior during historic events.