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Geomagnetic indices are convenient quantities that distill the complicated physics of some region or aspect of near‐Earth space into a single parameter. Most of the best‐known indices are calculated from ground‐based magnetometer data sets, such as Dst, SYM‐H, Kp, AE, AL, and PC. Many models have been created that predict the values of these indices, often using solar wind measurements upstream from Earth as the input variables to the calculation. This document reviews the current state of models that predict geomagnetic indices and the methods used to assess their ability to reproduce the target index time series. These existing methods are synthesized into a baseline collection of metrics for benchmarking a new or updated geomagnetic index prediction model. These methods fall into two categories: (1) fit performance metrics such as root‐mean‐square error and mean absolute error that are applied to a time series comparison of model output and observations and (2) event detection performance metrics such as Heidke Skill Score and probability of detection that are derived from a contingency table that compares model and observation values exceeding (or not) a threshold value. A few examples of codes being used with this set of metrics are presented, and other aspects of metrics assessment best practices, limitations, and uncertainties are discussed, including several caveats to consider when using geomagnetic indices.

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[1] We analyze nightside measurements of the DEMETER spacecraft related to lightning activity. At the 707 km altitude of DEMETER, we observe 3-D electric and magnetic field waveforms of fractional-hop whistlers. At the same time, the corresponding atmospherics are recorded by a very low frequency (VLF) ground-based station located in Nançay (France). The source lightning strokes are identified by the METEORAGE lightning detection network. We perform multidimensional analysis of the DEMETER measurements and obtain detailed information on wave polarization characteristics and propagation directions. This allows us for the first time to combine these measurements with ray-tracing simulation in order to directly characterize how the radiation penetrates upward through the ionosphere. We find that penetration into the ionosphere occurs at nearly vertical wave vector angles (as was expected from coupling conditions) at distances of 100-900 km from the source lightning. The same distance is traveled by the simultaneously observed atmospherics to the VLF ground station. The measured dispersion of fractional-hop whistlers, combined with the ionosonde measurements at the Ebro observatory in Spain, allows us to derive the density profile in the topside ionosphere.

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