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We present FARR (Finite-difference time-domain ARRay), an open source, high-performance, finite-difference time-domain (FDTD) code. FARR is specifically designed for modeling radio wave propagation in collisional, magnetized plasmas like those found in the Earth’s ionosphere. The FDTD method directly solves Maxwell’s equations and captures all features of electromagnetic propagation, including the effects of polarization and finite-bandwidth wave packets. By solving for all vector field quantities, the code can work in regimes where geometric optics is not applicable. FARR is able to model the complex interaction of electromagnetic waves with multi-scale ionospheric irregularities, capturing the effects of scintillation caused by both refractive and diffractive processes. In this paper, we provide a thorough description of the design and features of FARR. We also highlight specific use cases for future work, including coupling to external models for ionospheric densities, quantifying HF/VHF scintillation, and simulating radar backscatter. The code is validated by comparing the simulated wave amplitudes in a slowly changing, magnetized plasma to the predicted amplitudes using the WKB approximation. This test shows good agreement between FARR and the cold plasma dispersion relations for O, X, R, and L modes, while also highlighting key differences from working in the time-domain. Finally, we conclude by comparing the propagation path of an HF pulse reflecting from the bottomside ionosphere. This path compares well to ray tracing simulations, and demonstrates the code’s ability to address realistic ionospheric propagation problems. 

ABSTRACT Avian irruptions are facultative, often periodic, migrations of thousands of birds outside of their resident range. Irruptive movements produce regional anomalies of abundance that oscillate over time, forming ecological dipoles (geographically disjunct regions of low and high abundance) at continental scales. Potential drivers of irruptions include climate and food variability, but these relationships are rarely tested over broad geographic scales. We used community science data on winter bird abundance (1989–2021) to identify spatiotemporal patterns of irruption for nine boreal birds across the United States and Canada and compared them to time series of winter climate and annual tree seed production. We hypothesized that, during irruption, bird abundance would decrease in regions experiencing colder winter climates (climate variability hypothesis) or low seed production resulting from the boom‐and‐bust of widespread mast‐seeding patterns (resource variability hypothesis). Across all species, we detected latitudinal or longitudinal irruption modes, or both, demonstrating north–south and east–west migration dynamics across the northern United States and southern Canada. Seven of nine species displayed associations consistent with the climate variability hypothesis and six with the resource variability hypothesis. While irruption dynamics are likely entrained by multiple environmental drivers, future climate change could alter the spatial and temporal characteristics of avian irruption. 

Abstract Since the 1950s, high frequency and very high frequency radars near the magnetic equator have frequently detected strong echoes caused ultimately by the Farley‐Buneman instability (FBI) and the gradient drift instability (GDI). In the 1980s, coordinated rocket and radar campaigns made the astonishing observation of flat‐topped electric fields coincident with both meter‐scale irregularities and the passage of kilometer‐scale waves. The GDI in the daytimeEregion produces kilometer‐scale primary waves with polarization electric fields large enough to drive meter‐scale secondary FBI waves. The meter‐scale waves propagate nearly vertically along the large‐scale troughs and crests and act as VHF tracers for the large‐scale dynamics. This work presents a set of hybrid numerical simulations of secondary FBIs, driven by a primary kilometer‐scale GDI‐like wave. Meter‐scale density irregularities develop in the crest and trough of the kilometer‐scale wave, where the total electric field exceeds the FBI threshold, and propagate at an angle near the direction of total Hall drift determined by the combined electric fields. The meter‐scale irregularities transport plasma across the magnetic field, producing flat‐topped electric fields similar to those observed in rocket data and reducing the large‐scale wave electric field to just above the FBI threshold value. The self‐consistent reduction in driving electric field helps explain why echoes from the FBI propagate near the plasma acoustic speed.