Swarm Observation of Field‐Aligned Currents Associated With Multiple Auroral Arc Systems

Abstract

Auroral arcs occur in regions of upward field‐aligned currents (FACs); however, the relation is not one to one, since kinetic energy of the current‐carrying electrons is also important in the production of auroral luminosity. Multiple auroral arc systems provide an opportunity to study the relation between FACs and auroral brightness in detail. In this study, we have identified two types of FAC configurations in multiple parallel arc systems using ground‐based optical data from the Time History of Events and Macroscale Interactions during Substorms all‐sky imagers, magnetometers and electric field instruments on board the Swarm satellites. In “unipolar FAC” events, each arc is an intensification within a broad, unipolar current sheet and downward return currents occur outside of this broad sheet. In “multipolar FAC” events, multiple arc systems represent a collection of multiple up/down current pairs. By collecting 17 events with unipolar FAC and 12 events with multipolar FACs, we find that (1) unipolar FAC events occur most frequently between 20 and 21 magnetic local time and multipolar FAC events tend to occur around local midnight and within 1 h after substorm onset. (2) Arcs in unipolar FAC systems have a typical width of 10–20 km and a spacing of 25–50 km. Arcs in multipolar FAC systems are wider and more separated. (3) Upward currents with more arcs embedded have larger intensities and widths. (4) Electric fields are strong and highly structured on the edges of multiple arc system with unipolar FAC. The fact that arcs with unipolar FAC are much more highly structured than the associated currents suggests that arc multiplicity is indicative not of a structured generator deep in the magnetosphere, but rather of the magnetosphere‐ionosphere coupling process.

Leveraging Commercial Cubesat Constellations for Auroral Science: A Case Study

“…This serves as a bound on the raw uncertainty in the magnetometer measurements, which translates to ∼2-μA/m 2 uncertainty in current density (under typical sheet-like current structure assumptions) for the 0.1-Hz sampling rate. While this uncertainty is large compared to those obtained with a scientific grade magnetometer, it is small compared to current densities observed in specific case studies of active aurora (e.g., Stasiewicz & Potemra, 1998;Wu et al, 2017).…”

“…The main progenitors for the present study are the ST‐5 (Slavin et al, ) and SWARM (Gillies et al, ; Ritter & Lühr, ) missions. For both missions, studies involving auroral arc crossings have revealed substantial space‐time variability in FACs, even under quiet conditions (Cumnock et al, ; Wu et al, ). The sensors used in the present study are lower quality “engineering grade” magnetometers, but the sheer number of sensors available in the Planet constellation (presently ∼200) poses interesting questions about trade‐offs between sensor capability, network density, and network coverage.…”

Multiscale estimation of the field-aligned current density

“…For both cases we show the multiscale FAC density and compare the scale integrated FAC density with the input FAC density. The simple case of a pair of FACs resembles the large scale R1/R2 system, as well as, the basic cell of a multiple arc system (Gillies et al, 2014;Wu et al, 2017). In the second case, the embedded smaller scale superposition can be associated to the analysis of the auroral oval with embedded smaller scale FACs, e.g.…”

“…The relation between multiple arc systems and their FAC signatures was addressed recently by (Wu et al, 2017) based on Swarm/THEMIS ASI observations. Wu et al (2017) selected events with clearly identifiable stable arcs and separated the observations in two categories, unipolar (multiple arcs embedded into a single large upward FAC) and multipolar events (a collection of multiple arcs and related pairs of upward and downward FACs). Arcs associated with multipolar FAC events were found to be broader and more separated than those associated with unipolar FAC events.…”

“…Arcs associated with multipolar FAC events were found to be broader and more separated than those associated with unipolar FAC events. In this section we perform MSMVA analysis for an event included in Wu et al (2017).…”