Simultaneous appearance of isolated auroral arcs and Pc 1 geomagnetic pulsations at subauroral latitudes

Abstract

[1] We have been conducting observations of aurora and geomagnetic pulsations at Athabasca, Canada, located at a subauroral latitude (magnetic latitude: 62°, L $ 4.6), using an all-sky imager and an induction magnetometer. Isolated auroral arcs at wavelengths of 557.7 nm, 630.0 nm, and 486.1 nm (H b ) were often observed at latitudes separated equatorward from the main auroral oval. From a 1-year observation (4 September 2005 to 3 September 2006), we found 13 isolated arc events. All these isolated arcs occurred coincidentally with Pc 1 geomagnetic pulsations, although there were nine other Pc 1 events without isolated arcs in the field of view of the imager. The arcs were observed in both pre-and post-midnight sectors and tended to appear during the late recovery phase of geomagnetic storms. The isolated arcs had limited latitudinal and longitudinal widths of less than 230 km and 250-800 km, respectively. We found that as isolated arcs moved equatorward (poleward), the frequencies of the simultaneous Pc 1 pulsations increased (decreased). Using the Tsyganenko-02 magnetic field model, the observed Pc 1 frequencies were almost the same as the frequencies of He + electromagnetic ion cyclotron (EMIC) waves at the equatorial plane connected to observed isolated arcs. These results indicate that interactions of spatially localized EMIC waves with ring current ions cause high-energy ion precipitation and associated isolated auroras at subauroral latitudes. These results also imply that the dynamics and instabilities in the inner magnetosphere can be monitored as low-latitude auroral emissions away from the ordinary auroral oval.

Global characteristics of Pc1 magnetic pulsations during solar cycle 23 deduced from CHAMP data

“…The geophysical importance of Pc1 pulsations consists in their close connection to (1) subauroral proton aurora with proton precipitation (e.g. Sakaguchi et al, 2008;Yahnin et al, 2009), (2) rapid dropout of relativistic electrons from the radiation belt (e.g. Miyoshi et al, 2008), and (3) traveling convection vortices .…”

Limitation of energetic ring current ion spectra

“…In fact, there is still much current interest in ion pitch angle scattering by EMIC waves in the context of ring current dynamics. This is evidenced by the substantial literature that relates EMIC waves to ion precipitation resulting in ring current decay [e.g., Cornwall et al, 1970;Erlandson and Ukhorskiy, 2001;Summers, 2005;Fraser et al, 2006;Jordanova et al, 2007;Yahnin and Yahnina, 2007;Engebretson et al, 2008;Sakaguchi et al, 2008;Spasojevic et al, 2011;Xiao et al, 2012;Yuan et al, 2014]. In the present work, we treat EMIC wave-ion interactions as linear or quasi-linear, in common with most previous studies.…”

Pc1–2 auroral pulsations

“…A relationship between the appearance of geomagnetic Pc1 pulsations on the ground and the occurrence of patchy proton auroras over an observatory by IMAGE satellite observations was reported by Yahnin et al [2007] and Yahnina et al [2008], but there is no data about the intensity of the aurora. A simultaneous occurrence of proton auroras and Pc1 pulsations at subauroral latitudes has been also reported by Sakaguchi et al [2007Sakaguchi et al [ , 2008 and Nomura et al [2012].…”