Large variations of streamer belt densities at 0.5- to 3-day timescales

Recent innovations in tomography techniques are opening a new window on the evolution of the slow solar wind in the streamer belts near the Sun. In this work, a new time-dependent technique is applied to COR2 A /Solar Terrestrial Relations Observatory (STEREO) observations from a period near solar minimum (2018/11/11) for heliocentric distances of 4 to 8Rs. For the first time, we find density variations of large amplitude throughout the quiescent streamer belt, ranging between 50-150% 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 CMEs or smaller transients. A comparison of time series at different heights reveals a consistent time lag, so that changes at 4Rs occur later at increasing height, corresponding to an outward propagation speed of around 100km/s. 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 non-moving streamer belt (rigid magnetic field, variable density), or a combination of both?