We present observations on 24 April 2023 by the Magnetospheric Multiscale spacecraft at the dayside, mid-latitude magnetopause, when an interplanetary magnetic cloud (MC) with sub-Alfvénic flows and northward and dawnward interplanetary magnetic field components impacted Earth’s magnetosphere. The aim is to reveal the processes of solar wind-magnetosphere interaction under sub-Alfvénic solar wind with northward magnetic field. Our analysis of electron and ion data suggests that magnetopause reconnection occurred near both polar cusps, forming boundary layers on closed magnetic field lines on both the MC and magnetospheric sides of the magnetopause. Grad-Shafranov, electron-magnetohydrodynamics, and polynomial reconstructions of magnetopause current layers show that local (equator-of-the-cusp) reconnection occurred in a sub-ion-scale magnetopause current sheet with a low magnetic shear angle (30°). Interestingly, the local reconnection was observed between the two (MC-side and magnetosphere-side) layers of closed field lines. It indicates that reconnected field lines from double cusp reconnection were interacting to induce another reconnection at the mid-latitude magnetopause. Our results suggest that magnetopause reconnection was more efficient or frequent under sub-Alfvénic solar wind with much lower beta plasma conditions than typical conditions. We will discuss the role of such efficient reconnection in the formation of low-latitude boundary layers.

The near-Earth plasma sheet becomes cold and dense under northward interplanetary magnetic field (IMF) condition, which suggests efficient solar wind plasma entry into the magnetosphere across the magnetopause for northward IMF and a possible contribution of ionospheric oxygen ion outflow. The cold and dense characteristics of the plasma sheet are more evident in the magnetotail flank regions that are the interface between cold solar wind plasma and hot magnetospheric plasma. Several physical mechanisms have been proposed to explain the solar wind plasma entry across the magnetopause and resultant formation of the cold-dense plasma sheet (CDPS) in the tail flank regions. However, the transport path of the cold-dense plasma inside the magnetotail has not been understood yet. Here we present a case study of the CDPS in the dusk magnetotail by Magnetospheric Multiscale (MMS) spacecraft under strongly northward IMF and high-density solar wind conditions. The ion distribution function consists of high- and low-energy components, and the low-energy one intermittently shows energy dispersion in the directions parallel and anti-parallel to the local magnetic field. The time-of-flight analysis of the energy-dispersed low-energy ions suggests that these ions originate in the region farther down the tail, move along the magnetic field toward the ionosphere and then come back to the magnetotail by the mirror reflection. The pitch-angle dispersion analysis gives consistent results on the traveling time and path length of the energy-dispersed ions. Based on these observations, we discuss possible generation mechanisms of the energy-dispersed structure of the low-energy ions during the northward IMF.