The Nature and Origin of Polar Cap Aurora from Observations by the Gjøa and Discovery Expeditions

The Gjoa and Discovery Expeditions to the northern and southern polar caps (1901-1906) carried out visual auroral and geomagnetic observations from Gjøahavn (GH) and Cape Armitage (CA).  These stations were geomagnetically conjugate within one degree of 78° geomagnetic latitude and 30min of geomagnetic time.  CA is 6.5hr in solar time behind GH.  Unique geomagnetic signatures of polar cap aurora were found using visual observations from the expeditions and modern optical and satellite observations.  The three main types of polar cap aurora are transpolar arcs (TPA), poleward substorm arcs (PSA) and sun-aligned arcs (SAA).  The first two are infrequent polar cap features originating in the plasma sheet and related to activity in the auroral oval.  SAA occur daily, as a part of the geomagnetic solar quiet variation (Sq).  SAA are contiguous to the morning and afternoon auroral oval and appear to emanate from the merging of the interplanetary magnetic field (IMF Bz) at both high and low latitudes.  The intensity of the SAA and the annual, diurnal and solar cyclical occurrence frequency suggest that they are embedded in “polar rain” and dependent on the density and energy of local photoelectrons.  The east or west currents associated with SAA around magnetic noon, appear to be the source of the Svalgaard-Mansurov Effect.