The severe space weather event of 13-16 May 1921 produced some spectacular technological impacts, in some cases causing destructive fires. It was characterized by extreme solar and geomagnetic variations, and spectacular aurora, recorded at many locations around the world. A wealth of information is available in scientific journals, newspapers, and other sources, enabling us to reconstruct the storm timeline. This shows that a series of major coronal mass ejections (CMEs) bombarded Earth in May 1921. The first pair may have prepared the way for latter intense activity, clearing density from the region between Sun and Earth, and energizing Earth's magnetosphere. Thus, a subsequent CME could travel more quickly and drive even more energy into the already active magnetosphere. This CME arrived late on 14 May, driving very intense activity early on 15 May, and leading to the spectacular technological effects. However, some effects, attributed at the time to space weather, were probably coincidental with the storm, and due to more prosaic faults. The timeline of the 1921 event, including the confusion caused by prosaic faults, can be used to construct scenarios for use today by those emergency managers planning how to reduce the adverse impacts of future space weather events.

In the last decade, machine learning has achieved unforeseen results in industrial applications. In particular, the combination of massive data sets and computing with specialized processors (graphics processing units, or GPUs) can perform as well or better than humans in tasks like image classification and game playing. Space weather is a discipline that lives between academia and industry, given the relevant physical effects on satellites and power grids in a variety of applications, and the field therefore stands to benefit from the advances made in industrial applications. Today, machine learning poses both a challenge and an opportunity for the space weather community. The challenge is that the current data science revolution has not been fully embraced, possibly because space physicists remain skeptical of the gains achievable with machine learning. If the community can master the relevant technical skills, they should be able to appreciate what is possible within a few years time and what is possible within a decade. The clearest opportunity lies in creating space weather forecasting models that can respond in real time and that are built on both physics predictions and on observed data.

The Carrington event is considered to be one of the most extreme space weather events in observational history. In this article, we have studied the temporal and spatial evolutions of the source active region and visual low-latitude aurorae. We have also compared this storm with other extreme space weather events on the basis of the spatial evolution. We have compared the available sunspot drawings to reconstruct the morphology and evolution of sunspot groups at that time. We have surveyed visual auroral reports in the Russian Empire, Ireland, Iberian Peninsula, Oceania, and Japan and fill the spatial gap of auroral visibility and revised its time series. We have compared this time series with magnetic measurements and shown the correspondence between low-latitude to midlatitude aurorae and the phase of magnetic storms. We have compared the spatial evolution of the auroral oval with those of other extreme space weather events in 1872, 1909, 1921, and 1989 as well as their storm intensity and concluded that the Carrington event is one of the most extreme space weather events but is likely not unique.

The extreme space weather events of early August 1972 had significant impact on the U.S. Navy, which have not been widely reported. These effects, long buried in the Vietnam War archives, add credence to the severity of the storm: a nearly instantaneous, unintended detonation of dozens of sea mines south of Hai Phong, North Vietnam on 4 August 1972. This event occurred near the end of the Vietnam War. The U.S. Navy attributed the dramatic event to magnetic perturbations of solar storms. In researching these events we determined that the widespread electric- and communication-grid disturbances that plagued North America and the disturbances in southeast Asia late on 4 August likely resulted from propagation of major eruptive activity from the Sun to the Earth. The activity fits the description of a Carrington-class storm minus the low-latitude aurora reported in 1859. We provide insight into the solar, geophysical, and military circumstances of this extraordinary situation. In our view this storm deserves a scientific revisit as a grand challenge for the space weather community, as it provides space-age terrestrial observations of what was likely a Carrington-class storm.

Historical records of ground-level geomagnetic disturbance are analyzed for the magnetic superstorm of May 1921. This storm was almost certainly driven by a series of interplanetary coronal mass ejections of plasma from an active region on the Sun. The May 1921 storm was one of the most intense ever recorded by ground-level magnetometers. It exhibited violent levels of geomagnetic disturbance, caused widespread interference to telephone and telegraph systems in New York City and State, and brought spectacular aurorae to the nighttime sky. Results inform modern projects for assessing and mitigating the effects of magnetic storms that might occur in the future.