Geometric Assumptions in Hydrodynamic Modeling of Coronal and Flaring Loops

Abstract

In coronal loop modeling, it is commonly assumed that the loops are semi-circular with a uniform cross-sectional area. However, observed loops are rarely semi-circular, and extrapolations of the magnetic field show that the field strength decreases with height, implying that the cross-sectional area expands with height. We examine these two assumptions directly to understand how they affect the hydrodynamic and radiative response of short, hot loops to strong, impulsive electron beam heating events. Both the magnitude and rate of area expansion impact the dynamics directly, and an expanding cross-section significantly lengthens the time for a loop to cool and drain, increases upflow durations, and suppresses sound waves. The standard $T\sim n^2$ relation for radiative cooling does not hold with expanding loops, which cool with relatively little draining. An increase in the eccentricity of loops, on the other hand, only increases the draining timescale, and is a minor effect in general. Spectral line intensities are also strongly impacted by the variation in the cross-sectional area since they depend on both the volume of the emitting region as well as the density and ionization state. With a larger expansion, the density is reduced, so the lines at all heights are relatively reduced in intensity and, because of the increase of cooling times, the hottest lines remain bright for significantly longer. Area expansion is critical to accurate modeling of the hydrodynamics and radiation, and observations are needed to constrain the magnitude, rate, and location of the expansion or lack thereof.

Accepted to ApJ

References in corpus (10)

• The Solar Optical Telescope for the Hinode Mission: An Overview
• CHIANTI -- an atomic database for emission lines -- Paper XVI: Version 10, further extensions
• Observing the formation of flare-driven coronal rain
• Electron Beams Cannot Directly Produce Coronal Rain
• Anomalous Cooling of Coronal Loops with Turbulent Suppression of Thermal Conduction
• The role of asymmetries in coronal rain formation during thermal non-equilibrium cycles
• Static and dynamic solar coronal loops with cross-sectional area variations
• On the spectroscopic detection of periodic plasma flows in loops undergoing thermal non-equilibrium
• On the Area Expansion of Magnetic Flux-Tubes in Solar Active Regions
• Solar Flare Irradiance: Observations and Physical Modeling

Cited by in corpus (1)

• Delayed Development of Cool Plasmas in X-ray Flares from kappa1 Ceti