Accelerating and Supersonic Density Fluctuations in Coronal Hole Plumes: Signature of Nascent Solar Winds

TL;DR

This study observes accelerating density fluctuations in coronal hole plumes, identifying them as nascent solar wind flows, and demonstrates their potential as seismological probes for understanding solar wind acceleration.

Contribution

It provides the first quantification of the evolution of nascent solar wind in plumes at the low corona using imaging data.

Findings

Density perturbations accelerate to supersonic speeds

Subsonic flows are about 44.1% of the solar wind mass flux

Density fluctuations can serve as seismological probes

Abstract

Slow magnetoacoustic waves in a static background provide a seismological tool to probe the solar atmosphere in the analytic frame. By analyzing the spatiotemporal variation of the electron number density of plume structure in coronal holes above the limb for a given temperature, we find that the density perturbations accelerate with supersonic speeds in the distance range from 1.02 to 1.23 solar radii. We interpret them as slow magnetoacoustic waves propagating at about the sound speed with accelerating subsonic flows. The average sonic height of the subsonic flows is calculated to be 1.27 solar radii. The mass flux of the subsonic flows is estimated to be 44.1$\%$ relative to the global solar wind. Hence, the subsonic flow is likely to be the nascent solar wind. In other words, the evolution of the nascent solar wind in plumes at the low corona is quantified for the first time from imaging observations. Based on the interpretation, propagating density perturbations present in plumes could be used as a seismological probe of the gradually accelerating solar wind.