Multi-Instrument Ground–Space Conjugate Observations of the Northern Magnetospheric Cusp

The polar cusps are a key region of magnetospheric research due to their role in coupling the solar wind and magnetosphere via dayside reconnection. However, discrepancies persist between the spatial extent of the in situ magnetospheric cusp measured by spacecraft and the ionospheric cusp footprint inferred from ground-based observations. Ground-based signatures of reconnection, such as pulsed ionospheric flows (PIFs) and their optical counterparts, poleward-moving auroral forms (PMAFs), commonly span several hours in magnetic local time (MLT), whereas statistical determinations of the cusp extent from in situ observations typically indicate an extent of 1-2 hours of MLT.

We present an event study of cusp extent on 16 December 2017, during which the Cluster spacecraft made a dusk-dawn pass through the northern cusp, while the EISCAT Svalbard Radar operated in a conjugate measurement campaign. This configuration enabled simultaneous ground- and space-based observations of the northern cusp, effectively ‘pinching’ the cusp between a dusk-sector ground-based measurement and a dawn-sector in situ measurement. These observations are supplemented with optical data from all-sky imagers and ionospheric convection data from SuperDARN radars. While individual observations from each instrument are consistent with literature values of 2–4 h MLT, the instantaneous conjugate measurements reveal significantly wider cusp extents of 4.6 h and 5.4 h MLT. Over the duration of the event, the combined observations span 7.2 h MLT, based on measurements separated by 57 minutes, representing an unusually large cusp extent under non-storm solar wind conditions.

Although cusp dynamics are highly variable and responsive to changes in the solar wind, the observed behaviour in this event does not fully account for the anomalously large extent. Instead, these results suggest that conjugate, instantaneous measurements can reveal broader cusp structures or discontinuities that may be underestimated or go unnoticed by single-point or time-averaged observations.