Identifying Observables That Can Differentiate Between Impulsive and Footpoint Heating: Time Lags and Intensity Ratios
Observations of solar coronal loops have identified several common loop characteristics, including that loops appear to cool and have higher than expected densities. Two potential heating scenarios have been suggested to explain these observations. One scenario is that the loops are formed by many strands, each heated independently by a series of small-scale impulsive heating events, or nanoflares. Another hypothesis is that the heating is quasi-steady and highly stratified, i.e., “footpoint heating” such heating can drive thermal nonequilibrium in some structures depending on the scale height and magnitude of the energy deposition, and the geometry of the structure. Studies of both types of heating have found that they can qualitatively reproduce the observed loop properties. The goal of this paper is to identify observables that can be used to differentiate between these two heating scenarios. To do this, we use a single loop geometry. For footpoint heating, we vary the heating magnitude and stratification, for impulsive heating, we vary the heating magnitude. We use one-dimensional hydrodynamic codes to calculate the resulting temperature and density evolution. We convolve the temperature and density with the response functions of four EUV channels of the Atmospheric Imaging Assembly and one filter channel of Hinode's X-ray Telescope. We consider two principal diagnostics: the time lag between the appearance of the loop in two different channels, and the ratio of the peak intensities of the loop in the two channels. Based on this limited data set, we find (1) that footpoint heating can predict longer time lags than impulsive heating in some channel pairs, (2) that footpoint heating can predict zero or negative time lags in some channel pairs, (3) that the intensity ratio expected from impulsive heating is confined to a narrow range, while footpoint heating predicts a wider range of intensity ratios, and (4) that the range of temperatures expected in impulsive heating is broader than the range of temperatures expected in footpoint heating. This preliminary study identifies observables that may be useful in discriminating between heating models in future work.