The Equilibrium of Coronal Loops Near Separatrices

TL;DR

This study models asymmetric coronal loops near null points using HYDRAD, revealing siphon flows and Doppler shifts consistent with observations, which helps infer magnetic topology.

Contribution

It introduces detailed numerical simulations of asymmetric coronal loops near null points, linking plasma flows with magnetic topology through observational comparison.

Findings

Siphon flows develop within 4 hours in the model.

Simulated Doppler shifts match Hinode EIS observations.

Flow directions and magnitudes correlate with magnetic topology.

Abstract

We present numerical models from the field-aligned Hydrodynamics and Radiation Code (HYDRAD) of a highly asymmetric closed coronal loop with near-singular expansion factor. This loop was chosen to simulate a coronal magnetic flux tube that passes close to a null point, as in the last set of closed loops under the fan surface of a coronal jet or a pseudostreamer. The loop has a very large cross-section localized near the coronal null. The coronal heating was assumed to be uniform and steady. A siphon flow establishes itself within 4 hours of simulation time, flowing from the smaller-area footpoint to the larger-area footpoint, with high initial speeds dropping rapidly as the plasma approaches the null region. Observationally, this would translate to strong upflows on the order of 10 km s$^{-1}$ from the footpoint rooted in the localized minority polarity, and weak downflows from the fan-surface footpoint on the order of a few km s$^{-1}$, along with near stationary plasma near the null region. We present the model results for two heating rates. In addition, we analyzed analogous Hinode EIS observations of null-point topologies, which show associated Doppler shifts in the plasma that correlate well with the simulation results in both direction and magnitude of the bulk velocity. We discuss the implications of our results for determining observationally the topology of the coronal magnetic field.