Determining the Typical Nanoflare Cadence in Active Regions: Modeling Light Curves of Active Regions

Active region coronal loops visible at 1MK are likely composed of many unresolved strands, heated by storms of impulsive nanoflares. Though well-studied, these loops often contribute only a fraction of the total emission in an active region; the degree to which the entire active region is heated in the same manner as loops are is highly debated. Is the majority of coronal active region plasma heated impulsively, or is the majority of the heating quasi-steady? Addressing this question is complicated by the fact that the corona is optically thin: many thousands of strands which are heated completely independently are contributing to the total emission along a given line of sight. Furthermore, certain geometries preclude even the best background subtraction methods from fully isolating the emission from even a single coronal loop. Therefore, a different and necessary approach to analyzing active region heating is to account for emission along the line of sight from all of the contributing strands. We model the active region corona as a line-of-sight integration of many thousands of completely independently heated strands. The emission from these flux tubes may be time dependent, quasi-steady, or a mix of both, depending on the cadence of heat release on each strand. We examine a full range of heat cadences from effectively steady (heat pulse repeat time << plasma cooling time) to fully impulsive (heat pulse repeat time >> plasma cooling time) and model the resulting emission when superposing strands undergoing these differing heat cycles. We demonstrate that despite the superposition of randomly heated strands, different distributions of heat cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from the AIA telescopes on SDO. For this reason, high time cadence spectral information for lines sensitive to the 1-10 MK range will be especially useful in future missions. Using these model predictions, we evaluate the typical cadence of heat release in different active regions and patterns therein, which is a crucial constraint on coronal heating mechanisms.