Auroral Acceleration at the Northern Magnetic Pole During Sub-Alfvénic Solar Wind Flow at Earth

Between 23 and 25 May 2002 the solar wind, due to very low plasma density, became sub-Alfvénic for enough time to promote the establishment of Alfvén wings that can limit typical solar wind-magnetosphere coupling. During this interval, the interplanetary magnetic field (IMF) was oriented northward and duskward, with a slightly dominant $B_Y$ component; driving of the magnetosphere was expected to be low. Many signatures are used to assess solar wind-magnetosphere-ionosphere coupling, including ultraviolet (UV) observations of the auroral zone to infer monoenergetic electron precipitation and radio observations of auroral kilometric radiation (AKR) to infer the development of the auroral acceleration region. Observing these signatures with the IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) and Wind spacecraft, we find evidence of auroral acceleration that allowed amplification of AKR to similar intensities as during super-Alfvénic coupling. This coincides with polar electron aurora around $8°$ square in latitude and at magnetic latitudes greater than 88$°$. The multipoint radio observations imply sources are generated along a constrained flux tube. Given the primary coincidence of AKR and the electron polar spot $\sim$3 hr following the incidence of minimally sub-Alfvénic $\left(M_A\sim 0.4\right)$ solar wind at Earth, this acceleration occurs while the Alfvén wings are most complete. Given the IMF conditions, auroral morphology of the polar spot and the inference of an upward field-aligned current, the magnetospheric dynamics are most related to those of the high-latitude dayside aurora (HiLDA). These observations are the first to show AKR amplification from HiLDA and during a sub-Alfvénic magnetosphere, highlighting the possibility of strong localized coupling under quiet geomagnetic conditions.