Highly efficient modeling of dynamic coronal loops

Observational and theoretical evidence suggests that coronal heating is impulsive and occurs on very small cross-field spatial scales. A single coronal loop could contain a hundred or more individual strands that are heated quasi-independently by nanoflares. It is therefore an enormous undertaking to model an entire active region or the global corona. Three-dimensional MHD codes have inadequate spatial resolution, and one-dimensional (1D) hydrodynamic codes are too slow to simulate the many thousands of elemental strands that must be treated in a reasonable representation. Fortunately, thermal conduction and flows tend to smooth out plasma gradients along the magnetic field, so zero-dimensional (0D) models are an acceptable alternative. We have developed a highly efficient model called enthalpy-based thermal evolution of loops'' (EBTEL), which accurately describes the evolution of the average temperature, pressure, and density along a coronal strand. It improves significantly on earlier models of this type-in accuracy, flexibility, and capability. It treats both slowly varying and highly impulsive coronal heating; it provides the time-dependent differential emission measure distribution, DEM(T), at the transition region footpoints; and there are options for heat flux saturation and nonthermal electron beam heating. EBTEL gives excellent agreement with far more sophisticated 1D hydrodynamic simulations despite using 4 orders of magnitude less computing time. It promises to be a powerful new tool for solar and stellar studies.