Escape of Flare-accelerated Particles in Solar Eruptive Events (1909.13578v1)

Abstract: Impulsive solar energetic particle events are widely believed to be due to the prompt escape into the interplanetary medium of flare-accelerated particles produced by solar eruptive events. According to the standard model for such events, however, particles accelerated by the flare reconnection should remain trapped in the flux rope comprising the coronal mass ejection. The particles should reach the Earth only much later, along with the bulk ejecta. To resolve this paradox, we have extended our previous axisymmetric model for the escape of flare-accelerated particles to fully three-dimensional (3D) geometries. We report the results of magnetohydrodynamic simulations of a coronal system that consists of a bipolar active region embedded in a background global dipole field structured by solar wind. Our simulations show that multiple magnetic reconnection episodes occur prior to and during the CME eruption and its interplanetary propagation. In addition to the episodes that build up the flux rope, reconnection between the open field and the CME couples the closed corona to the open interplanetary field. Flare-accelerated particles initially trapped in the CME thereby gain access to the open interplanetary field along a trail blazed by magnetic reconnection. A key difference between these 3D results and our previous calculations is that the interchange reconnection allows accelerated particles to escape from deep within the CME flux-rope. We estimate the spatial extent of the particle-escape channels. The relative timings between flare acceleration and release of the energetic particles through CME/open-field coupling are also determined. All our results compare favourably with observations.

• The paper presents 3D magnetohydrodynamic simulations showing complex, multiple magnetic reconnection sites allow flare-accelerated particles to escape from evolving solar eruptions.
• Simulations reveal a temporal delay, about 33 minutes, between particle acceleration in the flare and their escape into the interplanetary medium, consistent with observed SEP event timing.
• The study indicates that the coupling of the CME with open magnetic field lines creates broad particle escape channels, influencing the longitudinal extent of impulsive solar energetic particle events.

Escape of Flare-Accelerated Particles in Solar Eruptive Events

This paper presents an exploration into the mechanisms by which flare-accelerated particles escape into the interplanetary medium during solar eruptive events. It extends previous axisymmetric models to fully three-dimensional (3D) simulations, considering the complexity of the solar magnetic environment and its evolution over time. The authors utilize magnetohydrodynamic (MHD) simulations to investigate how flare-accelerated particles, initially trapped within the coronal mass ejection (CME) flux rope, gain access to the open interplanetary magnetic field (IMF).

Summary of Model and Methodology

The paper employs the Adaptively Refined Magnetohydrodynamics Solver (ARMS) to solve the ideal MHD equations in spherical coordinates, simulating a coronal system with a bipolar active region embedded within a global dipole field structured by the solar wind. The computational domain extends from the photosphere to 50 solar radii to capture the dynamics from CME initiation through propagation into the solar wind. The model includes complex boundary conditions and adaptive mesh refinement to ensure high resolution in critical areas.

By applying slow photospheric shearing flows to the magnetic arcades in the active region, the model builds a filament channel, leading to CME eruption. The simulation carefully reproduces the magnetic topology, embedding a null point within the magnetic configuration and allowing for realistic CME initiation and dynamics.

Key Findings

1. 3D Reconnection Geometry: The simulations reveal that magnetic reconnection does not happen in a simple, planar manner as in 2-dimensional models. Multiple reconnection sites, such as at the hyperbolic flux tube (HFT) and along the fan structure, allow particles to be accelerated at different stages. These reconnection sites offer new pathways for energetic particles, initially trapped, to escape as the CME evolves.
2. Temporal and Spatial Dynamics: The sequential order of reconnection episodes suggests a time delay between particle acceleration in the flare region and their escape as the CME reconnects with open interplanetary field lines. This delay, simulated as approximately 33 minutes, can explain observed SEP event characteristics where flare emissions and particle arrivals are not simultaneous.
3. Survival of CME Structure: Unlike in previous 2.5D analyses where interchange reconnection disrupted the flux rope, in this 3D scenario, parts of the flux rope survive while others reconnect with the open interplanetary field. This finding is consistent with observations that show CMEs associated with impulsive SEPs maintaining their flux rope structure upon reaching the Earth.
4. Particle Escape and Implications for SEP Observations: The coupling of the CME with the open field lines allows for a broad geographic dispersal of flare-accelerated particles. The longitudinal extent of particle escape channels is roughly equivalent to the CME's width, implying that wide SEP events could be influenced by the initial solar conditions and the extent of magnetic reconnection achieved during CME propagation.

Implications and Future Directions

The paper suggests that 3D simulations provide more accurate and detailed insights into the dynamics of particle escape in solar eruptive events. The results indicate that magnetic reconnection processes far more complex than previously appreciated can enable the escape of flare-accelerated particles, challenging the traditional models that primarily attribute SEP events to CME-driven shock acceleration.

Future developments should involve validating these simulations with observational data and further clarifying the precise locations and timings of reconnection events. Doing so could enhance our understanding of SEP variability and help refine space weather forecasting models. The paper opens the door for investigating how changes in the heliospheric magnetic field affect the propagation and diffusion of energetic particles, potentially improving our predictions of cosmic weather events' impact on Earth's environment.