Statistical Correlation Analysis of Field‐Aligned Currents Measured by Swarm

Abstract

We investigate the statistical, dual-spacecraft correlations of field-aligned current (FAC) signatures between two Swarm spacecraft. For the first time, we infer the orientations of the current sheets of FACs by directly using the maximum correlations obtained from sliding data segments. The current sheet orientations are shown to broadly follow the mean shape of the auroral boundary for the lower latitudes and that these are most well ordered on the dusk side. Orientations at higher latitudes are less well ordered. In addition, the maximum correlation coefficients are explored as a function of magnetic local time and in terms of either the time shift (δt) or the shift in longitude (δlon) between Swarms A and C for various filtering levels and choice of auroral region. We find that the low-latitude FACs show the strongest correlations for a broad range of magnetic local time centered on dawn and dusk, with a higher correlation coefficient on the dusk side and lower correlations near noon and midnight. The positions of maximum correlation are sensitive to the level of low-pass filter applied to the data, implying temporal influence in the data. This study clearly reflects the two different domains of FACs: small-scale (some tens of kilometers), which are time variable, and large-scale (>50 km), which are rather stationary. The methodology is deliberately chosen to highlight the locations of small-scale influences that are generally variable in both time and space. We may fortuitously find a potential new way to recognize bursts of irregular pulsations (Pi1B) using low-Earth orbit satellites.

Diagnosing the Role of Alfvén Waves in Global Field‐Aligned Current System Dynamics During Southward IMF: Swarm Observations

“…In our view, this is significant since it validates the electrodynamic importance of the regions between adjacent oppositely directed currents for electromagnetic energy transport from the magnetosphere to the ionosphere. Similar scale-dependent separation between temporally varying wave-like dynamics (~10s km) and more static FACs (>50 km) using Swarm magnetic field cross correlations using mostly Swarm A and C at was also reported by Yang et al (2018). The three Alfvén speeds correspond to the maximum, mean, and minimum Alfvén speeds determined during the crossing period (calculated using Swarm C density measurements since Swarm A density measurements were unavailable for this time period).…”

Multi-spacecraft Current Estimates at Swarm

“…A complementary technique based on the cross-correlation of the single spacecraft estimates of FACs at each Swarm spacecraft has been recently introduced (Lühr et al 2015;Yang et al 2018), which results in estimates of the maximum correlation between sliding data intervals at two-spacecraft. The position of the maximum correlations found can be defined in terms of the required time shift (for example, from spacecraft A-C) and corresponding difference in the local time (this is given by the relative location of the spacecraft on the pair of orbits), and the correlation values can be computed as a function of local time (MLT) of the orbits.…”

“…Conversely, if there are significant small-scale structures or fluctuations present then this will be reflected in unstable orientations and a lack of ordering. Figure 5.9 (adapted from Yang et al 2018) shows the patterns in current sheet orientations found for intervals broadly associated with both region 1 and region 2 currents. Yang et al (2018), for a Northern hemisphere polar map, showing the average FACs for Swarm A and C data from 17th Apr.…”

Two‐Dimensional Maps of In Situ Ionospheric Plasma Flow Data Near Auroral Arcs Using Auroral Imagery

“…During times where both SWARM satellites are operational and cross a stable arc, their measurements can give us an example to validate our conjecture. The SWARM spacecraft are displaced in both time and space from one another and as long as the arc is stable over the period of time between the spacecraft crossings we can compare the data (Yang et al, ).…”

“…In the long run, successful interpretation of image events without the need for in situ measurements can then be applied for events outside the rocket flight, where only ground‐based measurements are present, in order to interpret the fields around auroral arc boundaries, with similar improvement over previous radar data maps not utilizing camera imagery. We show examples utilizing data outside our rocket flight from the SWARM satellites (Yang et al, ) and PFISR data products (Heinselman & Nicolls, ) in section .…”