Single and Multiple Solar Flare Loops: Hydrodynamics and Ca xix Resonance Line Emission

Abstract

Studies made so far with one-dimensional hydrodynamic simulations have shown that it is difficult to reproduce the soft X-ray spectral line profile observed in the early phase of solar flares. Simulated line profiles predict a dominant emission from a large blueshifted component, while observations show persistently strong stationary components. We resolve these discrepancies by utilizing a multiple-loop system instead of just a single loop for conductively heated flare simulations. Under a fixed heat input rate, we examine how the heating duration tau heat affects the Ca XIX resonance (w) line emission from single and multiple flare loops. In the multiple-loop model, the flare energy is released into individual loops with a specified time delay, which implicitly mimics the successive formation of flare loops due to continuous reconnection. We find that whether or not tau heat is longer than tau c affects the hydrodynamic response in an individual flare loop, where tau c corresponds to the time when the loop is filled with evaporated plasma. The Ca XIX spectral line shape is characterized by an intensity ratio of emission from evaporated plasma to emission from accumulated plasma after evaporation. This ratio is mainly determined by the parameter tau heat/ tau c. Our findings suggest that the following scenario can naturally explain the observed spectral line features. Flare energy is injected into a bundle of loops successively in two steps: in the preflare stage, tau heat tau c for the outer loops. A large initial coronal density is not necessary in this scenario.