Daily Variations of Plasma Density in the Solar Streamer Belt

Improved space weather diagnostics depend critically on improving our understanding of the evolution of the slow solar wind in the streamer belts near the Sun. Recent innovations in tomography techniques are opening a new window on this complex environment. In this work, a new time-dependent technique is applied to COR2A/Solar Terrestrial Relations Observatory observations from a period near solar minimum (2018 November 11) for heliocentric distances of 4-8 R (circle dot). For the first time, we find density variations of large amplitude throughout the quiescent streamer belt, ranging between 50% and 150% of the mean density, on timescales of tens of hours to days. Good agreement is found with Parker Solar Probe measurements at perihelion; thus, the variations revealed by tomography must form a major component of the slow solar wind variability, distinct from coronal mass ejections or smaller transients. A comparison of time series at different heights reveals a consistent time lag, so that changes at 4 R (circle dot) occur later at increasing height, corresponding to an outward propagation speed of around 100 km s(-1). This speed may correspond to either the plasma sound speed or the bulk outflow speed depending on an important question: are the density variations caused by the spatial movement of a narrow streamer belt (moving magnetic field, constant plasma density), or changes in plasma density within a nonmoving streamer belt (rigid magnetic field, variable density), or a combination of both?

Automated summary

This study focuses on using innovative techniques to observe density variations in the slow solar wind within the streamer belts, finding large amplitude fluctuations over timescales of hours to days. The results show good agreement with measurements from the Parker Solar Probe at perihelion, indicating that these variations likely play a significant role in the slow solar wind variability.