Modeling of GOES Soft X-ray Light Curves: Importance of Multi-threaded Nature and Various Timescales

Previous investigation on various flare events shows the statistical trend that the flare duration measured from the GOES soft X-ray light curves is correlated with the separation of two flare ribbons, or equivalently with the length of the post flare loops. This linear relationship between the flare duration and the loop length may be easily explained by assuming that the duration is controlled by the timescale of magnetic reconnection, which is theoretically proportional to the loop length. However, this discussion totally ignores the thermal processes and, to over come this issue, we examine this relation by performing loop simulations with the HYDRAD code, which includes all thermal processes such as thermal conduction and radiative cooling. We first simulate the GOES light curves of individual loops, which we then use to reproduce the composite light curve assuming a bundle of loops with increasing lengths as the flare reconnection proceeds (pseudo-2D model). As a result, we find that the timescale of light curves (FWHM duration and e-folding decay time) linearly correlates with the size of the loop bundles (represented by ribbon separation), which is in good agreement with the observation. This is because the heating in individual loops is relatively rapid and thus the timescale of composite light curves (i.e. flare duration) reflects the continuation of successive magnetic reconnection. Therefore, in order to model the loop evolution and GOES light curve, we need to take into account both multi-threaded nature of the flare loops and all different timescales (magnetic reconnection and thermal processes). The detail of this work is available in Reep & Toriumi (2017, ApJ, 851, 4).