Energetic Particles from Quasi-Separatrix Layers and Current Sheets at the Sun

TL;DR

This work addresses the origin of solar energetic particle seeds and broad longitudinal SEP events by proposing that quasi-separatrix layers (QSLs) and current sheets near the Sun act as efficient accelerators. It develops an analytic framework showing second-order Fermi (magnetic pumping) acceleration in time-varying magnetic-field structures, embedded in a Parker-like transport equation with a momentum-diffusion term $D_{pp}$, and couples this to global MHD-SEP simulations via the STAT framework (MAS for MHD and EPREM for transport). The study demonstrates, through relaxation runs and PSP-informed seed spectra, that QSLs can generate seed populations enriched in heavy ions and $^3$He and produce broad, long-lasting SEP fluxes when coupled with CME-driven or quasi-relaxation flows. These results offer a first global model for SEP acceleration at the Sun and provide a basis for improved predictive models of solar energetic particle events and seed populations.

Abstract

Quasi-separatrix layers (QSLs) at the Sun are created from regions where channels of open magnetic flux have footpoints near regions of large-scale closed magnetic flux. These regions are particularly prone to magnetic reconnection at the Sun. In recent simulations of coronal mass ejections (CMEs) with the Magnetohydrodynamic Algorithm outside a Sphere (MAS) model coupled to the Energetic Particle Radiation Environment Module (EPREM) model, common sources of energetic particles were discovered over broad longitudinal distributions in the background solar wind, far from the sites of particle acceleration driven by compressions and shocks in front of CMEs. Further investigation revealed these to be accelerated energetic particles from the QSLs and current sheets. The energy released from magnetic reconnection near the QSL drives reconnection exhausts and field-aligned flows, which in turn accelerate energetic particles. The reconnection process also releases material previously contained within closed magnetic field structures, which are often rich in heavy ions and $^3$He ions, as corroborated by recent PSP observations. Therefore, the seed populations produced by QSLs are expected to be rich in $^3$He and heavy ions. Thus, we present the first global model of energetic particles accelerated from QSLs and above current sheets from the Sun. Our results provide a plausible source for seed populations near the Sun, which likely have $^3$He and heavy ion enhancements. These results aid in the development of predictive solar energetic particle models.

Figures (10)

• Figure 1: Schematic diagram of interchange reconnection and acceleration of energetic particles near the reconnection exhaust termination. An open magnetic field and a closed magnetic field line approach one another (Panel A). The simple closed topology shown here may actually be more complex, as discussed by Gosling:1995. If the closed and open magnetic flux reconnect (Panel B), there are two outflow exhausts from the reconnection site. One exhaust is directed into the closed field region and the other exhaust is directed into the open field. The energetic particles accelerated within the exhaust that forms on the open magnetic field line (the lower side of panel B) are free to move throughout the inner heliosphere, and may provide seed populations for particles accelerated at the bow shocks of coronal mass ejections. Panel C shows the reconfiguration of the open and closed magnetic flux away from the reconnection exhaust. The inset in Panel B shows an idealized reconnection exhaust created by Alfvén wings as fast-mode waves propagate out and away from the reconnection site. The reconnection exhaust forms where the cusp-shaped structures relax into a more potential state.
• Figure 2: Schematic diagram of particle acceleration near an idealized pseudo-streamer, a form of a QSL. The energetic particles accelerated near the QSL experience strong gradients in the magnetic field strength on the open magnetic field lines. Particles moving through this structure with plasma flow undergo particle acceleration as they are scattered through the magnitude changes in field strength. Particles both gain and lose energy through the interaction, and the presence of scattering introduces a randomization of the events causing diffusion in momentum space.
• Figure 3: Distribution of energetic particle fluxes at $2~R_{\odot}$ from the 11/29/2020 relaxation run (top panel). The middle panel shows the log of squashing factor $Q$ associated with QSLs (the S-web) with the red and blue coloring indicating the opposing polarity of the coronal magnetic field---data are displayed in signed log Q format, defined as S-log Q $\equiv \mathrm{sign}(B_{r})\,\mathrm{log}[Q/2 + (Q^{2}/4 - 1)^{1/2} ]$Titov:2011. The heliospheric current sheet is seen as the thin boundary between the red and blue regions. The bottom panel overlays these images to show the similarity between the spatial structures recovered. Streams 2551, near a QSL, and 3181, near a current sheet are labeled. These streamlines are used as representative cases to describe the effects of the separatrix layer on particle acceleration.
• Figure 4: Integrated flux greater than 10 MeV of energetic particles at 10 $R_{\odot}$ 12 hr into the simulation period. The simulation includes the background solar wind configuration used from the November 29, 2020 event. Node locations of streamlines 2551 and 3181 are labeled. The EPREM simulations follow nodes out through evolving plasma flow. An individual line of nodes follows a specific streamline in the simulation. The two streamlines chosen are representative cases: streamline 2551 passes through a QSL without a current sheet, whereas streamline 3181 passes through the current sheet. There is some variation along individual streamlines, but the general behavior observed at the streamlines identified highlights the physical mechanisms responsible for particle acceleration.
• Figure 5: The rate of change for quantities that drive acceleration in equation (\ref{['eq:focused_transport']}) (third panel), the differential flux at 10 MeV/nuc (first panel), and the radial distance from the Sun (bottom panel) of a node along streamline 3181 (shown in Figure \ref{['fig:DASL']}). Also shown is the rate associated with stochastic acceleration (second panel) and the scattering mean free path used in the simulation (fourth panel). The dominant term driving acceleration is the rate of change of the magnetic field flux. As the node moves out approximately 3--8 hr into the simulation and traverses a region ${\sim}2$--3.8 $R_{\odot}$, we observe several large reductions and then increases in the field strength associated with a current near the QSL boundary.