Geometric Assumptions in Hydrodynamic Modeling of Coronal and Flaring Loops

TL;DR

This paper investigates how assumptions about loop geometry, especially cross-sectional expansion, affect hydrodynamic and radiative modeling of coronal and flaring loops, highlighting the importance of accurate geometric considerations.

Contribution

It provides a direct analysis of how cross-sectional expansion and loop eccentricity influence loop dynamics, cooling, and spectral emissions, challenging standard assumptions.

Findings

Expanding cross-section lengthens cooling and draining times.

Standard radiative cooling relation T ~ n^2 does not hold for expanding loops.

Spectral line intensities decrease with larger expansion, affecting observational diagnostics.

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.