Series of Small-scale Low Plasma β Magnetic Flux Ropes Originating from the Same Longitudinal Region: Parker Solar Probe Observations

Abstract

In this study, we report on small-scale magnetic flux ropes (SMFRs) observed as a compact series in a narrow Carrington longitudinal range during three Parker Solar Probe (PSP) encounters. First, during ∼1.5 days of PSP's inbound part of Encounter 4, we identified a series of 11 SMFRs within 1.°4 in longitude over the radial distance of ∼8.4 R ☉ (from ∼44 to 35 R ☉). The identified SMFRs lasted from ∼0.5 to 1.8 hr, and adjacent events were separated mostly by a few hours and up to ∼6.5 hr at the longest, but some events were very closely spaced with intervals of a few ∼tens of minutes or less apart. Most of the identified SMFRs are successfully fitted to the force-free model. The SMFRs are clearly distinguished from the surroundings by a notable reduction in plasma β, which itself was comparably low (less than unity) in the background plasma. Furthermore, the magnetic field and plasma flow within the SMFRs fluctuated significantly less than the more turbulent surroundings. The fluctuations in the surrounding medium exhibited occasional Br polarity reversal (possibly switchbacks) and were Alfvénic to a large extent with far weaker compressional components. The majority of these key features with some differences have also been found in the series of SMFRs and their surroundings identified within 1.°3 or less in longitude during Encounters 1 and 5. We speculate that these SMFRs were repetitively generated by successive reconnection within a very narrow angular zone located close to the Sun but not necessarily at the same radial position.