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Abstract The Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) provides continuous global maps of Birkeland currents, using magnetic field perturbations (dB) obtained by calibrating and detrending data from engineering magnetometers on the 66 polar‐orbiting Iridium satellites in the communications constellation. Here, we provide an assessment of AMPERE dBaccuracy, as compared with magnetic field observations from the Swarm satellite mission. The CHAOS v8.1 model (Finlay et al., 2020,https://doi.org/10.1186/s40623‐020‐01252‐9) was used to remove the main field and other non‐ionospheric contributions from both data sets. In a nearest‐neighbor comparison covering August 2022, AMPERE's calibrated and detrended dBdata from the Iridium NEXT satellites are found to have root‐mean‐square deviations of 31 and 33 nT (for dB_θand dB_φ, respectively) as compared with data from Swarm, while the biases are −7 and −2 nT. For the same interval, AMPERE's fitted maps have root‐mean‐square errors of <40 nT, rising to 109–185 nT in active conditions (defined as Swarm dB > 250 nT). However, there is evidence that small scale (<400‐km along Swarm track direction) dBstructures are not fully resolved. Overall, we find that the AMPERE dBdata and fitted products are unbiased and are typically in excellent agreement with the Swarm data. 

Due to differences in solar illumination, a geomagnetic field line may have one footpoint in a dark ionosphere while the other ionosphere is in daylight. This may happen near the terminator under solstice conditions. In this situation, a resonant wave mode may appear which has a node in the electric field in the sunlit (high conductance) ionosphere and an antinode in the dark (low conductance) ionosphere. Thus, the length of the field line is one quarter of the wavelength of the wave, in contrast with half-wave field line resonances in which both ionospheres are nodes in the electric field. These quarter waves have resonant frequencies that are roughly a factor of 2 lower than the half-wave frequency on the field line. We have simulated these resonances using a fully three-dimensional model of ULF waves in a dipolar magnetosphere. The ionospheric conductance is modeled as a function of the solar zenith angle, and so this model can describe the change in the wave resonance frequency as the ground magnetometer station varies in local time. The results show that the quarter-wave resonances can be excited by a shock-like impulse at the dayside magnetosphere and exhibit many of the properties of the observed waves. In particular, the simulations support the notion that a conductance ratio between day and night footpoints of the field line must be greater than about 5 for the quarter waves to exist.