Constraints on the variable nature of the slow solar wind with the Wide-Field Imager on board the Parker Solar Probe

Abstract

Context. The formation of the slow solar wind remains unclear as we lack a complete understanding of its transient outflows. Aims. In a previous work we analysed the white-light coronal brightness as a function of elongation and time from Wide-Field Imager (WISPR) observations on board the Parker Solar Probe (PSP) mission when PSP reached a minimum heliocentric distance of ≈28 R⊙. We found 4–5 transient outflows per day over a narrow wedge in the PSP orbital plane, which is close to the solar equatorial plane. However, the elongation versus time map (J-map) analysis supplied only lower limits on the number of released density structures due to the small spatial-scales of the transient outflows and line-of-sight integration effects. In this work we place constraints on the properties of slow solar wind transient mass release from the entire solar equatorial plane. Methods. We simulated the release and propagation of transient density structures in the solar equatorial plane for four scenarios: (1) periodic release in time and longitude with random speeds; (2) corotating release in longitude, periodic release in time with random speeds; (3) random release in longitude, periodic release in time and speed; and (4) random release in longitude, time, and speed. Results. The simulations were used in the construction of synthetic J-maps, which are similar to the observed J-map. The scenarios with periodic spatial and temporal releases are consistent with the observations for periods spanning 3°–45°longitude and 1–25 h. The four considered scenarios have similar ranges (35–45 for the minimum values and 96–127 for the maximum values) of released density structures per day from the solar equatorial plane and consequently from the streamer belt, given its proximity to the solar equatorial plane during the WISPR observation. Our results also predict that density structures with sizes in the range 2–8 R⊙ covering 1–20% of the perihelion could have been detectable by PSP in situ observations during that interval. Conclusions. Our estimates of the release rates of density structures from the streamer belt represent a first major step towards assessing their contribution to the slow solar wind mass budget and their potential connection with in situ detections of density structures by PSP.