Abstract
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 latitude 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 auroras, except for its appearance at considerably low magnetic latitude. The record also suggests that an extremely low-latitude aurora is not always accompanied by 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.
Highlights
It is known that a low-latitude aurora is a manifestation of a magnetic storm caused by solar eruptions (e.g. Gonzalez et al, 1994; Yokoyama et al, 1998; Shiokawa et al, 2005; Willis et al, 2006; Odenwald, 2015)
Reconstruction of contemporary MLAT showed that Manila was situated at 3.3◦ MLAT, close to the geomagnetic equator
We could find no large sunspots or large geomagnetic storms associated with this auroral report
Summary
It is known that a low-latitude aurora is a manifestation of a magnetic storm caused by solar eruptions (e.g. Gonzalez et al, 1994; Yokoyama et al, 1998; Shiokawa et al, 2005; Willis et al, 2006; Odenwald, 2015). In August and September 1859, solar eruptions from large sunspots caused an intense magnetic storm reaching values as extreme as 1600 nT in the horizontal geomagnetic field at Colaba (Tsurutani et al, 2003; Nevanlinna, 2006; Ribeiro et al, 2011), with major auroral displays seen worldwide down to magnetic latitudes (hereafter, MLATs) as low as ∼ 20◦ (Kimball, 1960; Cliver and Svalgaard, 2004; Green and Boardsen, 2006; Farrona et al, 2011; Cliver and Dietrich, 2013; Hayakawa et al, 2016b; Lakhina and Tsurutani, 2016). Willis et al (2007) and Vaquero et al (2007, 2013) surveyed this kind of localized low-latitude auroral display in China, Spain, and Mexico to identify reports during low or moderate geomagnetic activity.
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