On the Generation and Evolution of Switchbacks and the Morphology of the Alfvénic Transition: Low Mach-number Boundary Layers

Abstract

We investigate the generation and evolution of switchbacks (SBs), the nature of the sub-Alfvénic wind observed by the Parker Solar Probe (PSP), and the morphology of the Alfvénic transition, all of which are key issues in solar wind research. First we highlight a special structure in the pristine solar wind, termed a low Mach-number boundary layer (LMBL). An increased Alfvén radius and suppressed SBs are observed within an LMBL. A probable source on the Sun for an LMBL is the peripheral region inside a coronal hole with rapidly diverging open fields. The sub-Alfvénic wind detected by PSP is an LMBL flow by nature. The similar origin and similar properties of the sub-Alfvénic intervals favor a wrinkled surface for the morphology of the Alfvénic transition. We find that a larger deflection angle tends to be associated with a higher Alfvén Mach number. The magnetic deflections have an origin well below the Alfvén critical point, and deflection angles larger than 90° seem to occur only when M A ≳ 2. The velocity enhancement in units of the local Alfvén speed generally increases with the deflection angle, which is explained by a simple model. A nonlinearly evolved, saturated state is revealed for SBs, where the local Alfvén speed is roughly an upper bound for the velocity enhancement. In the context of these results, the most promising theory on the origin of SBs is the model of expanding waves and turbulence, and the patchy distribution of SBs is attributed to modulation by reductions in the Alfvén Mach number. Finally, a picture of the generation and evolution of SBs is created based on the results.

Prediction and Verification of Parker Solar Probe Solar Wind Sources at 13.3 R⊙

“…It is not a surprising association since pseudostreamers are generally thought to be a contributing source of the slow solar wind (Wang et al., 2012; Wang & Ko, 2019; Wang & Panasenco, 2019) and do not have a current sheet so maintain a strong B field throughout the structure. It is also a similar association to that highlighted recently by (Liu et al., 2023) who associate earlier sub‐alfvènic crossings of Parker with overexpanded field from the edge of coronal holes, a structure they term a “low Mach‐number boundary layer” (LMBL). We also comment that the radial scans on the inbound and outbound portions of the orbit allow the same portion of the surface to be estimated over an extended period of time.…”

“…Combined with the footpoint mapping, we can associate such structure with underlying magnetic topology. We demonstrated this with the measurements of the Alfvén Mach number for E10 and found that it implies an average surface just below the current closest approach (13.3 R ⊙ ), but also a wrinkled surface with narrow protrusions which cross Parker's trajectory (see also Verscharen et al., 2021; Liu et al., 2023). By associating these protrusions with the well‐validated source mapping, we observed they appear to occur over slow solar wind streams emanating from the tips of pseudostreamers or at least the overexpanded edges of coronal holes, as was reported for the first such crossing (Kasper et al., 2021).…”

“…(2021); Liu et al. (2023), the latter orbits of the Parker mission are likely to reach the average R A value implied by our scalings, it may imminently be possible to contrast differences crossings of protrusions over special solar wind streams with samples from inside the “normal sub‐alfvènic”’ corona.…”

“…(2021); Liu et al. (2023) is the limiting case of physical scaling assuming the radial magnetic field and plasma density vary as 1/ r 2 and a constant solar wind speed (e.g., Weber et al., 1967). In particular, the latter assumption of constant wind speed needs quantitative re‐examination with Parker now measuring substantial solar wind acceleration (Dakeyo et al., 2022; Halekas et al., 2022), but for the purposes of the present manuscript is sufficient to describe when Parker is above or below the Alfvèn surface.…”

“…On top of this, we superimpose a simple construction to create a visual representation of the topology of the Alfvèn surface. Following (Liu et al., 2021, 2023), we take Parker's heliocentric distance as a timeseries and divide it by M A where we smooth the data via a rolling 20 min (4 × 5 min data points) window. Recombining this modified radius coordinate with Parker's Carrington longitude gives us the black curve which necessarily intersects Parker's orbit when M A = 1, is further out than Parker when M A < 1 and closer in that Parker when M A > 1.…”