The Polar Cusp Seen by Cluster

Abstract

The investigation of the magnetospheric polar cusps was one of the main objectives of the Cluster mission. The four satellites have crossed those regions numerous times over the years and, with their suitable instrumentation, favorable orbits, and unique multipoint measurements, many aspects of the cusp have been unveiled. The first of those is its highly dynamic nature. With four satellites, whatever their altitude and thus their configuration, the spatial/temporal ambiguity has been lifted and a completely new insight into the motion of the cusp and evolution of the transient small‐scale structures within it was given. Second, the high altitude or exterior cusp, its properties and dynamics, magnetic configuration, the energetic particles observed within, and its boundary with the magnetosheath were characterized in a number of case studies and on a statistical basis. Third, the Cluster cusp observations brought a fresh eye on the reconnection process at the magnetopause, including its location and spatio‐temporal variability. Finally, the complete payload with particle, field and wave detectors have made it possible to treat cusp as a plasma physics laboratory to progress our understanding of wave‐particle interaction and turbulence within the cusp. We have no choice but to note that Cluster has been extremely successful for all these matters.

Hybrid Simulations of the Cusp and Dayside Magnetosheath Dynamics Under Quasi‐Radial Interplanetary Magnetic Fields

“…The boundary between the cusp and magnetosheath also becomes indented (compare the |v i | contours in Figures 9 and 11). This is consistent with observations which show indentations of the cusp boundary (Pitout & Bogdanova, 2021;Zhang et al, 2007).…”

Distribution and Occurrence Frequency of dB/dt Spikes During Magnetic Storms 1980–2020

“…Some authors suggested that dB/dt spikes in the local morning sector could alternatively be associated with Pc5 pulsations generated by the Kelvin‐Helmholtz instability (KHI; Kataoka et al., 2001; Ngwira et al., 2018; Pulkkinen & Kataoka, 2006; Weigel et al., 2003) or by upstream ion‐cyclotron resonance (Howard & Menk, 2005) on the morning flank of the magnetopause. However, we do not believe this explanation because any processes at the magnetopause, including KHI (independently of their scale), would magnetically map very close to the vicinity of the dayside cusp, that is, not much further than 08–09 MLT into the dawn sector, as the longitudinal variability of the cusp in response to solar wind forcing is not more than three to maximum of 4 hr from local noon (Pitout & Bogdanova, 2021). Thus, it appears very difficult to connect the ionospheric current features caused by the KHI at the flanks with our observations in the morning sector at 05–06 MLT.…”

Distribution and occurrence frequency of dB/dt spikes during magnetic storms 1980-2019

“…Some authors suggested that dB/dt spikes in the local morning sector could alternatively be associated with Pc5 pulsations generated by the Kelvin-Helmholtz instability (KHI; Kataoka et al, 2001;Ngwira et al, 2018;Pulkkinen & Kataoka, 2006;Weigel et al, 2003) or by upstream ion-cyclotron resonance (Howard & Menk, 2005) on the morning flank of the magnetopause. However, we do not believe this explanation because any processes at the magnetopause, including KHI (independently of their scale), would magnetically map very close to the vicinity of the dayside cusp, that is, not much further than 08-09 MLT into the dawn sector, as the longitudinal variability of the cusp in response to solar wind forcing is not more than three to maximum of 4 hr from local noon (Pitout & Bogdanova, 2021). Thus, it appears very difficult to connect the ionospheric current features caused by the KHI at the flanks with our observations in the morning sector at 05-06 MLT.…”