Sporadic Aurora near Geomagnetic Equator: In the Philippines, on 27 October 1856

TL;DR

This paper presents a historical case study of a rare aurora observed near the geomagnetic equator in the Philippines in 1856, challenging the assumption that low-latitude auroras only occur during large magnetic storms.

Contribution

It provides evidence of extremely low-latitude auroras occurring independently of major magnetic storms, supported by historical report analysis and a proposed physical explanation.

Findings

The aurora was observed at an unusually low magnetic latitude.

Low-latitude auroras can occur during moderate magnetic activity.

A possible physical mechanism involves ephemeral magnetospheric disturbances.

Abstract

While low latitude auroral displays are normally considered to be a manifestation of magnetic storms of considerable size, Silverman (2003, JGR, 108, A4) reported numerous "sporadic auroras" which appear locally at relatively low magnetic latitudes during times of just moderate magnetic activity. Here, a case study is presented of an aurora near the geomagnetic equator based on a report from the Philippine Islands on 27 October 1856. An analysis of this report shows it to be consistent with the known cases of sporadic aurorae except for its considerably low magnetic latitude. The record also suggests that extremely low-latitude aurora is not always accompanied with large magnetic storms. The description of its brief appearance leads to a possible physical explanation based on an ephemeral magnetospheric disturbance provoking this sporadic aurora.