Abstract
Abstract Several fast solar wind streams and stream interaction regions (SIRs) were observed by the Parker Solar Probe (PSP) during its first orbit (2018 September–2019 January). During this time, several recurring SIRs were also seen at 1 au at both L1 (Advanced Composition Explorer (ACE) and Wind) and the location of the Solar Terrestrial Relations Observatory-Ahead (STEREO-A). In this paper, we compare four fast streams observed by PSP at different radial distances during its first orbit. For three of these fast stream events, measurements from L1 (ACE and Wind) and STEREO-A indicated that the fast streams were observed by both PSP and at least one of the 1 au monitors. Our associations are supported by simulations made by the ENLIL model driven by GONG-(ADAPT-)WSA, which allows us to contextualize the inner heliospheric conditions during the first orbit of PSP. Additionally, we determine which of these fast streams are associated with an SIR and characterize the SIR properties for these events. From these comparisons, we find that the compression region associated with the fast-speed streams overtaking the preceding solar wind can form at various radial distances from the Sun in the inner heliosphere inside 0.5 au, with the suprathermal ion population (energies between 30 and 586 keV) observed as isolated enhancements suggesting localized acceleration near the SIR stream interface at ∼0.3 au, which is unlike those seen at 1 au, where the suprathermal enhancements extend throughout and behind the SIR. This suprathermal enhancement extends further into the fast stream with increasing distance from the Sun.
Highlights
As the faster solar wind stream overtakes preceding slower streams, a density “pileup” of compressed plasma occurs that eventually forms into a stream interaction region (SIR; e.g., Belcher & Davis 1971; Pizzo 1978; Richardson 2018)
The simulations capture general large-scale changes in the solar wind; the timing of the velocity enhancements is generally off by a few days. From both the Parker Solar Probe (PSP) observations and ENLIL simulations, there were likely two SIRs at PSP in early October, but data gaps in the PSP data sets prevent us from analyzing these events in any detail
PSP did observe four regions between slow and fast streams later during the first orbit at solar distances ranging from ∼65 to ∼155 Rs, which are further analyzed and presented in this paper
Summary
As the faster solar wind stream overtakes preceding slower streams, a density “pileup” of compressed plasma occurs that eventually forms into a stream interaction region (SIR; e.g., Belcher & Davis 1971; Pizzo 1978; Richardson 2018). The stream interface is often associated with an increase in density prior to the transition to low-density fast wind, a peak in the magnitude of the magnetic field along with total pressure at the transition (often used to denote the interface; e.g., Jian et al 2006), and an increase in both temperature and entropy when moving from the slow wind into the fast stream. Through a survey of SIRs observed by the Pioneer Venus Orbiter, Jian (2008) found that the rate of SIRs with sharp stream interfaces increased from 9% at 0.7 au to
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