Fine structures and dynamics in auroral initial brightening at substorm onsets

Abstract

Abstract. We show four auroral initial brightening events at substorm onsets focusing on fine structures and their longitudinal dynamics, which were observed by all-sky TV cameras (30-Hz sampling) on January 2008, in Canada. For two initial brightenings started in the field of views of the cameras, we found that they started at longitudinal segments with a size of less than ∼30-60 km. One brightening expanded with wavy structures and the other expanded as a straight arc. Although the two events had different structures, both brightening auroras expanded with an average speed of ∼20 km/s in the first 10 s, and ∼10 km/s in the following 10 s. The other two events show that brightening auroras developed with periodic structures, with longitudinal wavelengths of ∼100-200 km. Assuming that the brightening auroras are mapped to the physical processes occurring in the plasma sheet, we found that the scale size (30-60 km) and the expanding speed (20 km/s) of brightening auroras correspond to the order of ion gyro radii (∼500-1400 km) and Alfvén speed or fast ionflow speed (∼400 km/s), respectively, in the plasma sheet.

Magnetic conjugacy of northern and southern auroral beads

“…In particular, the slowly moving beads in the first stage have never been reported in previous studies [Donovan et al, 2006;Sakaguchi et al, 2009]. Very recently, Kataoka et al [2011], using an electron multiplier charge-coupled device (EMCCD) camera with a narrow FOV, demonstrated that smaller-scale auroral forms (on a scale of a few/several km) appear in a breakup arc in a few minutes before onset.…”

“…The scale of the beads and their separation are typically on the order of the gyroradius of 1-10 keV protons in the source magnetosphere. Thus, as suggested by Liang et al [2008] and Sakaguchi et al [2009], the plasma instability creating the beads in the primary stage of their development would not be a pure MHD scale process but include kinetic effect.…”

“…Recently, attention has been drawn to the temporal evolution of the initial brightening arc during the few minutes before the onset, because it could illustrate the process that triggers the expansion phase onset. Optical observations from the ground [Donovan et al, 2006;Liang et al, 2008;Sakaguchi et al, 2009] and from spacecraft [Elphinstone et al, 1995;Henderson, 2009] have shown that in the initial brightening arc, there exists a characteristic small-scale auroral structure, consisting of azimuthally arrayed wave-like forms (often referred to as "auroral beads") with a wavelength of 30-100 km and an azimuthal propagation speed of 5-8 km s À1 . As time progresses, the auroral beads evolve into wave-like undulations with a larger scale of 50-200 km, and finally lead to auroral breakup.…”

Turbulent microstructures and formation of folds in auroral breakup arc

“…This filter passes 844.6 nm prompt emission from Oxygen molecule, as a result of both direct excitation and dissociative excitation. Breakup aurora is known as "ray" aurora [Sakaguchi et al, 2009a[Sakaguchi et al, , 2009b which has electron precipitation with the broadband energy range [Mende et al, 2003]. The 844.6 nm is a prompt emission, capable of capturing the rapid variation of both hard and soft electrons associated with the auroral breakup.…”

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

“…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. 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.…”