Magnetic conjugacy of northern and southern auroral beads

Abstract

Auroral beads, i.e., azimuthally arrayed bright spots resembling a pearl necklace, have recently drawn attention as a possible precursor of auroral substorms. We used simultaneous, ground‐based, all‐sky camera observations from a geomagnetically conjugate Iceland‐Syowa Station pair to demonstrate that the auroral beads, whose wavelength is ∼30–50 km, evolve synchronously in the northern and southern hemispheres and have remarkable interhemispheric similarities. In both hemispheres: 1) they appeared almost at the same time; 2) their longitudinal wave number was similar ∼300–400, corresponding bead separation being ∼1° in longitude; 3) they started developing into a larger scale spiral form at the same time; 4) their propagation speeds and their temporal evolution were almost identical. These interhemispheric similarities provide strong evidence that there is a common driver in the magnetotail equatorial region that controls the major temporal evolution of the auroral beads; thus, the magnetosphere plays a primary role in structuring the initial brightening arc in this scale size.

Statistical azimuthal structuring of the substorm onset arc: Implications for the onset mechanism

“…This scale closely corresponds to the ion gyro radius of 1-10 keV protons in the plasma sheet at 10 R E , where the magnetic field strength is~12 nT. Motoba et al [2012] observed beads with wavelengths of 30-50 km at magnetically conjugate stations in the Northern and Southern hemispheres, confirming that the source of the beads is most likely in the equatorial magnetosphere. There have also been multiple substorm studies in which auroral beads were not reported [e.g., Angelopoulos et al, 2008;Nishimura et al, 2010] leading to a general consensus that auroral beads are only part of the auroral substorm sequence in a small subset of substorms.…”

“…Auroral beads can statistically be observed~2 min before auroral onset at low spatial scales and are the earliest signature of waves which exists in the near-Earth magnetosphere on closed field lines. In the minutes around auroral onset, the waves start to grow exponentially over a wide range of spatial scales; this is characteristic of the linear stage of the instability [Rae et al, 2010;Motoba et al, 2012;Murphy et al, 2014;Kalmoni et al, 2015]. During the transition from preonset to postonset times, the consistent median statistical characteristic spatial scale of the instability is indicative that a single instability is active at these times.…”

“…Magnetosphere dynamics can be remotely sensed through space and ground-based auroral campaigns [Shiokawa et al, 1996;Mende et al, 2000Mende et al, , 2008Angelopoulos, 2008;Motoba et al, 2012]. These have enabled detailed studies of the morphology of substorm aurora and revealed the complexity of substorm dynamics initially reported by Akasofu [1964Akasofu [ , 1977.…”

“…Structures known as auroral rays or beads have been observed to form azimuthally along the substorm onset arc in the minutes before auroral substorm onset [Davis, 1962;Henderson, 1994]. In recent years there have been many studies of auroral beads from data acquired by high resolution (temporal and spatial) ground-based all-sky imagers (ASIs) [e.g., Friedrich et al, 2001;Kepko et al, 2009;Sakaguchi et al, 2009;Rae et al, 2009Rae et al, , 2010Motoba et al, 2012;Murphy et al, 2014;Kalmoni et al, 2015] in which beads are reported to have spatial scales, or wavelengths, in the range 30-150 km in the ionosphere. Sakaguchi et al [2009] reported two case studies of substorm onset arcs which exhibited beading with spatial scales of 30-60 km in the ionosphere.…”

Pulsating aurora and cosmic noise absorption associated with growth-phase arcs

“…Optical pulsating aurora electrons originate from near the magnetic equator with possible mechanisms being injection via bursts of, for instance, chorus wave energy (Nishimura et al, 2010) or electromagnetic ion cyclotron (EMIC) wave energy (e.g. Miyoshi et al, 2008).…”