High‐spatial‐resolution velocity measurements derived using Local Divergence‐Free Fitting of SuperDARN observations

Abstract

A new technique for analysis of Super Dual Auroral Radar Network (SuperDARN) line‐of‐sight velocity observations enables resolving plasma convection with unprecedented spatial resolution. The technique, Local Divergence‐Free Fitting (LDFF), can be used to produce maps with a spatial resolution that is determined by the resolution of the observations rather than an arbitrary fit order. Other techniques, which express the potential as a sum of harmonic functions, limit the number of functions in the expansion to the fit order. Doing so imposes a limit on the minimum size of features that will be represented in the results. The LDFF technique is not limited by this constraint. Rather, it is limited by the resolution of the observations and the amount of regularization required by the observed noise level, which generally allows finer‐scale features to be represented. The LDFF technique is described and then applied to a synthetic data set to demonstrate its validity. Then high‐resolution convection maps are presented from an interval during which auroral observations over Alaska showed poleward boundary intensifications (PBIs) and auroral streamers. Overlays of the convection vectors on the auroral images illustrate correspondence between flow features and the auroral luminosity. Detailed comparison between the flows and images showed that the PBIs originated from polar cap boundary arcs that extended away from midnight toward earlier local times. As the arcs extended they were accompanied by enhanced shear flow. The arcs intensified then moved equatorward becoming streamers. As the arcs moved, the region of shear flow followed their motion, indicating a pattern of field‐aligned current associated with the moving arc. The observations are the most comprehensive and detailed known to the authors for such an interval and agree well with the expected plasma flows based upon magnetospheric simulations. Flow vectors generated for the interval by the spherical harmonic fit technique for the most part do not show the direct relationship between the convection and the localized luminosity features nearly as well as the LDFF results. Some of the features found in the LDFF fitting are simply not present in the results from the global fits. The LDFF technique should find application in a variety of studies where high‐spatial‐resolution estimates of plasma flows are required. The example study presented here, which examined the details of flows in the region of auroral arcs, is representative of such problems.

Local time extent of magnetopause reconnection using space–ground coordination

“…The ion distributions in Fig. 2k show a distorted D-shaped distribution similar to the finding of Broll et al (2017). The distortion is due to particles traveling in the field-aligned direction from the reconnection site to a stronger magnetic field region, and Broll et al (2017) estimated the traveling distance to be a few Re for the observed level of distortion.…”

“…The transmitted ions have a cutoff parallel velocity (i.e., de Hoffmann-Teller velocity) below which no ions could enter the magnetosphere. The D-shaped ion distributions are deformed into a crescent shape as ions travel away from the reconnection site (Broll et al, 2017). We require the satellites to operate in the fast survey or burst mode in which ion distributions are available at 3 s resolution.…”

Physical Processes of Meso-Scale, Dynamic Auroral Forms

“…Fast plasma flows (up to 1000s of m/s) in the polar cap have been observed in both coherent and incoherent scatter radar measurements prior to the activation of a PBI Bristow et al 2016). Figure 3 shows and example event from Nishimura et al (2010b).…”

Application of RADAR Imaging Analysis to SuperDARN Observations

“…Initial tests of the local‐divergence‐free‐fitting technique (Bristow et al, ) using the image data illustrate the promise of these observations. Figure shows the estimated vector field for 0102 UT on 26 August 2018, which corresponds to frame “b” of Figure .…”