Radial evolution of a density structure within a solar wind magnetic sector boundary

Abstract

This study focuses on a radial alignment between Parker Solar Probe (PSP) and Solar Orbiter (SolO) on the 29$^{\text{th}}$ of April 2021 (during a solar minimum), when the two spacecraft were respectively located at $\sim 0.075$ and $\sim 0.9$~au from the Sun. A previous study of this alignment allowed the identification of the same density enhancement (with a time scale of $\sim$1.5~h), and substructures ($\sim$20-30~min timescale), passing first by PSP, and then SolO after a $\sim 138$~h propagation time in the inner heliosphere. We show here that this structure belongs to the large scale heliospheric magnetic sector boundary. In this region, the density is dominated by radial gradients, whereas the magnetic field reversal is consistent with longitudinal gradients in the Carrington reference frame. We estimate the density structure radial size to remain of the order L$_R \sim 10^6$~km, while its longitudinal and latitudinal sizes, are estimated to expand from L$_{φ, θ} \sim 10^4$-$10^5$~km in the high solar corona, to L$_{φ, θ} \sim 10^5$-$10^6$~km at PSP, and L$_{φ, θ} \sim 10^6$-$10^7$~km at SolO. This implies a strong evolution of the structure's aspect ratio during the propagation, due to the plasma's nearly spherical expansion. The structure's shape is therefore inferred to evolve from elongated in the radial direction at $\sim$2-3 solar radii (high corona), to sizes of nearly the same order in all directions at PSP, and then becoming elongated in the directions transverse to the radial at SolO. Measurements are not concordant with local reconnection of open solar wind field lines, so we propose that the structure has been generated through interchange reconnection near the tip of a coronal streamer.