Observations of the auroral width spectrum at kilometre-scale size

Abstract

Abstract. This study examines auroral colour camera data from the Canadian Dense Array Imaging SYstem (DAISY). The Dense Array consists of three imagers with different narrow (compared to all-sky view) field-of-view optics. The main scientific motivation arises from an earlier study by Knudsen et al. (2001) who used All-Sky Imager (ASI) combined with even earlier TV camera observations (Maggs and Davis, 1968) to suggest that there is a gap in the distribution of auroral arc widths at around 1 km. With DAISY observations we are able to show that the gap is an instrument artifact and due to limited spatial resolution and coverage of commonly used instrumentation, namely ASIs and TV cameras. If the auroral scale size spectrum is indeed continuous, the mechanisms forming these structures should be able to produce all of the different scale sizes. So far, such a single process has not been proposed in the literature and very few models are designed to interact with each other even though the range of their favourable conditions do overlap. All scale-sizes should be considered in the future studies of auroral forms and electron acceleration regions, both in observational and theoretical approaches.

Observations of GPS scintillation during an isolated auroral substorm

“…We derived the phase scintillation index every minute using 3,000 data points sampled at a temporal resolution of 50 Hz, which means that the enhancement of σ φ is caused by fluctuations in the phase whose temporal scale ranges from 0.04 to 60 s. If we assume the speed of the horizontal motion of the auroral arcs to be approximately 1 km s −1 , which is often the case at the onset of the expansion phase (e.g., Hosokawa et al 2013), 0.04 to 60 s phase fluctuations correspond to electron density structures of 40 m to 60 km in size. These scales are in good agreement with that of the discrete auroral arcs typically present during the substorm interval (e.g., Partamies et al 2010).…”

ULF Waves above the Nightside Auroral Oval during Substorm Onset

“…In auroral physics, some compromise exists between large-scale measurements from global auroral imaging (e.g., Polar, IMAGE) that provide long periods of entire hemispheric coverage at ~100 km spatial resolution at low (~30-120 s) cadence, localized in situ measurements of auroral measurements from LEO satellites (e.g., REIMEI) that rapidly pass over regions of interests, and groundmeasured high cadence and local fast auroral imaging [e.g., Trondsen et al, 1997;Knudsen et al, 2001;Semeter et al, 2005;Partamies et al, 2010] that provide only local observations of the aurora. The advent of the groundbased component of the NASA THEMIS mission presented an unique opportunity to probe the aurora across unprecedented spatial and temporal scales.…”

A new automatic method for estimating the peak auroral emission height from all-sky camera images

“…The range of arc widths (5-35 km) is representitive of a continuous distribution with a lower limit cutoff at 5 km due to the instrument resolution (Partamies et al, 2010). While Knudsen et al (2001) found arc widths larger than 35 km, this result is likely biased towards larger widths due to the aspect angle problem (Semeter et al, 2008;Partamies et al, 2010). Also, Knudsen et al (2001) defined arc widths at 135 km altitude, and so these widths will be slightly larger than those here which are defined at 100 km.…”

“…real observations; arcs of length 700 km reach close to the edge of the field-of-view at both ends of the arc and at both stations, representing large-scale structures which are only partially observed by the cameras, while the shorter lengths represent smaller, isolated structures. The range of arc widths (5-35 km) is representitive of a continuous distribution with a lower limit cutoff at 5 km due to the instrument resolution (Partamies et al, 2010). While Knudsen et al (2001) found arc widths larger than 35 km, this result is likely biased towards larger widths due to the aspect angle problem (Semeter et al, 2008;Partamies et al, 2010).…”

“…Kaila, 1989;Knudsen et al, 2001;Whiter et al, 2008;Dahlgren et al, 2009;Partamies et al, 2010;Obuchi et al, 2011). However, the height of the aurora is rarely measured; often when a numerical value is required it is assumed, typically at about 110-130 km for the most prominant emission at 557.7 nm.…”