Swarm in situ observations of F region polar cap patches created by cusp precipitation

TL;DR

This study uses high-resolution in situ measurements from Swarm spacecraft to investigate the formation and evolution of F region polar cap patches caused by cusp precipitation and plasma transport in the northern Scandinavian dayside cusp.

Contribution

It provides the first in situ observations tracking polar cap patch development from creation to relaxation, highlighting the roles of flow channels and particle impact ionization.

Findings

Cusp particle impact ionization dominates initial plasma structuring.

Density features are twice as dense as the background.

Patches become less structured as they move toward the nightside.

Abstract

High-resolution in situ measurements from the three Swarm spacecraft, in a string-of-pearls configuration, provide new insights about the combined role of flow channel events and particle impact ionization in creating $\textit{F}$ region electron density structures in the northern Scandinavian dayside cusp. We present a case of polar cap patch formation where a reconnection-driven low-density relative westward flow channel is eroding the dayside solar-ionized plasma but where particle impact ionization in the cusp dominates the initial plasma structuring. In the cusp, density features are observed which are twice as dense as the solar-ionized background. These features then follow the polar cap convection and become less structured and lower in amplitude. These are the first in situ observations tracking polar cap patch evolution from creation by plasma transport and enhancement by cusp precipitation, through entrainment in the polar cap flow and relaxation into smooth patches as they approach the nightside auroral oval.