Extreme, transient bursts of energy in the auroral ionosphere. I. Predictive radar tracking
Three-meter Farley-Buneman irregularities observed by the ICEBEAR VHF radar organize into km-scale clusters whose apparent motion is governed by the electric field mapped from the magnetosphere. Based on experience gained from earlier schemes, we have evolved a more sophisticated method to automatically detect and track the clusters, opening a new window on electric field structures in the auroral region. Each cluster is bounded by an Alpha-shape monitored at every time step. The time-space evolution of individual clusters is recognized through an optimal assignment algorithm adapted from multi-target tracking, which minimizes a cost matrix based on the degree of overlap between consecutive time-frames and a position prediction from the cluster's previous displacement. Births, deaths, splits, and mergers are monitored; each tracked trajectory is reduced to per-segment velocities by piecewise linear regression. The extracted velocities have been validated against in-situ satellite observations. Surprisingly large velocities were extracted during the G5 storm of 10 May 2024, while monitoring closed magnetic field-lines on the day-side near the ionospheric cusp. In particular, we retrieved a five-second cluster moving at 11,240$\pm$660 m/s, implying an electric field strength of ~560 mV/m. This value exceeds both the thermal speeds normally found above the E-region and the most extreme values reported in reputed regions of fast sub-auroral drifts. The detected extreme structures appear as short-lived bursts of unexpectedly strong field variability, with implications for electric-field energy deposition rates in space weather modeling.