Electrodynamics and energy characteristics of aurora at high resolution by optical methods

Abstract

Technological advances leading to improved sensitivity of optical detectors have revealed that aurora contains a richness of dynamic and thin filamentary structures, but the source of the structured emissions is not fully understood. In addition, high‐resolution radar data have indicated that thin auroral arcs can be correlated with highly varying and large electric fields, but the detailed picture of the electrodynamics of auroral filaments is yet incomplete. The Auroral Structure and Kinetics (ASK) instrument is a state‐of‐the‐art ground‐based instrument designed to investigate these smallest auroral features at very high spatial and temporal resolution, by using three electron multiplying CCDs in parallel for three different narrow spectral regions. ASK is specifically designed to utilize a new optical technique to determine the ionospheric electric fields. By imaging the long‐lived O+ line at 732 nm, the plasma flow in the region can be traced, and since the plasma motion is controlled by the electric field, the field strength and direction can be estimated at unprecedented resolution. The method is a powerful tool to investigate the detailed electrodynamics and current systems around the thin auroral filaments. The two other ASK cameras provide information on the precipitation by imaging prompt emissions, and the emission brightness ratio of the two emissions, together with ion chemistry modeling, is used to give information on the energy and energy flux of the precipitating electrons. In this paper, we discuss these measuring techniques and give a few examples of how they are used to reveal the nature and source of fine‐scale structuring in the aurora.

Multiscale Observation of Two Polar Cap Arcs Occurring on Different Magnetic Field Topologies

“…(2010). The highly structured dynamic auroral observations are compariable to previous ASK observations when it was situated in Tromsø under the main auroral oval (e.g., Dahlgren et al., 2010, 2011, 2016; Lanchester et al., 2009) and are therefore consistent with formation on closed field lines.…”

“…An estimate of the energy flux can be obtained by dividing the brightness measured by ASK1 by the mean modeled brightness within the energy range of each event. This method of estimating the energy and flux has been used in many studies and has been verified by comparison with other ground based instruments, for example, the European Incoherent Scatter (EISCAT) radars, (e.g., Dahlgren et al., 2011, 2016; Lanchester et al., 2009; Reidy et al., 2017; Whiter et al., 2010).…”

“…The ASK instrument consists of three cameras with a 6° field of view centered on magnetic zenith (Dahlgren et al., 2008, 2016). Each camera is fitted with a different filter corresponding to specific auroral emissions used in the study of small scale aurora (known as ASK1, ASK2, and ASK3).…”

Aurora in the Polar Cap: A Review

“…Most ground-based auroral observations of polar cap arcs are based on all-sky imager observations, as discussed above, but another type of observation can be made by examining auroral emissions on a very fine spatial scale. The Auroral Structure and Kinetics (ASK) instrument is a multi-spectral high-resolution imager located on Svalbard (Ashrafi 2007;Dahlgren et al 2016). Due to its high-latitude location, ASK is perfectly situated for the study of polar cap aurora.…”

Multi‐instrument observation of simultaneous polar cap auroras on open and closed magnetic field lines

“…The ASK instrument has been used to study auroral features at small spatial scale, for example, boundary undulations or “ruffs” [ Dahlgren et al , ] and at high temporal resolution to study flickering aurora [ Whiter et al , ]. The two main strengths of ASK are the ability to estimate the energy of precipitation and to trace plasma flows, which can in turn be used to estimate electric field strength and direction to very good resolution; these methods are outlined in a review paper by Dahlgren et al [].…”