Investigating `dark' energy in the solar corona using forward modeling\n of MHD waves

Abstract

It is now well established that the Alfv\\'enic waves are ubiquitous in the\nsolar corona. However, the Alfv\\'enic wave energy estimated from the Doppler\nvelocity measurements in the corona was found to be four orders of magnitude\nless than that estimated from non-thermal line widths. McIntosh & De Pontieu\n(2012) suggested that this discrepancy in energy might be due to the\nline-of-sight (LOS) superposition of the several oscillating structures, which\ncan lead to an underestimation of the Alfv\\'enic wave amplitudes and energies.\nMcIntosh & De Pontieu (2012) termed this coronal `dark' or `hidden' energy.\nHowever, their simulations required the use of an additional, unknown source of\nAlfv\\'enic wave energy to provide agreement with measurements of the coronal\nnon-thermal line widths. In this study, we investigate the requirement of this\nunknown source of additional `dark' energy in the solar corona using\ngravitationally stratified 3D magnetohydrodynamic (MHD) simulations of\npropagating waves. We excite the transverse MHD waves and generate synthetic\nobservations for the Fe XIII emission line. We establish that the LOS\nsuperposition greatly reduces the Doppler velocity amplitudes and increases the\nnon-thermal line widths. Importantly, our model generates the observed\nwedge-shaped correlation between Doppler velocities and non-thermal line\nwidths. We find that the observed wave energy is only 0.2-1\\% of the true wave\nenergy which explains 2-3 orders of magnitude of the energy discrepancy. We\nconclusively establish that the true wave energies are hidden in the\nnon-thermal line widths. Hence, our results rule out the requirement for an\nadditional `dark' energy in the solar corona.\n