Did Alfred Russel Wallace See Aurora near the Equator?

Aurorae are visual manifestations of space weather and space climate. Their historical records occasionally help us to reconstruct the space weather and space climate in the past, while their records are scattered and forgotten in various archival documents. One of such cases was Alfred Russel Wallace’s travel account. Here, Wallace documented his observations of an unusual optical phenomenon and tentatively associated it with the aurora, but his doubts remain unanswered to this day. This study critically re-examines Wallace’s account of this unusual “aurora”, documenting his background and report. This study located and dated his observation at Muka in Waigiou Island (S00°26', E130°49') on 4 July 1860. The location and date allows us to calculate a magnetic latitude of −11.3°, with the phenomenon reported geomagnetically in the equatorward sky. This combination contradicts the auroral hypothesis for this phenomenon. Although the timing was close to a solar maximum, the contemporaneous geomagnetic measurements also did not show any significant geomagnetic disturbances. Morphologically, the reported phenomenon is more consistent with sprites or transient luminous phenomena; there was a constant succession of faint vertical flashings/flickerings and a great cloud that produced a storm two days after the observation. While we suggest the observation might be explained by sprites or transient luminous events, we cannot completely exclude other possibilities either. As such, Wallace’s concern was correct. This report in turn seems a probable account for sprites or transient luminous phenomena that predates their earliest known documentation in 1885.