Investigating “Dark” Energy in the Solar Corona Using Forward Modeling of MHD Waves

Abstract

It is now well established that Alfvénic waves are ubiquitous in the solar corona. However, the Alfvénic wave energy estimated from Doppler velocity measurements in the corona was found to be four orders of magnitude less than that estimated from nonthermal line widths. McIntosh & De Pontieu suggested that this discrepancy in energy might be due to the line-of-sight (LOS) superposition of several oscillating structures, which can lead to an underestimation of the Alfvénic wave amplitudes and energies. McIntosh & De Pontieu termed this coronal “dark” or “hidden” energy. However, their simulations required the use of an additional, unknown source of Alfvénic wave energy to obtain agreement with measurements of the coronal nonthermal line widths. In this study, we investigate the requirement of this unknown source of additional “dark” energy in the solar corona using gravitationally stratified 3D magnetohydrodynamic (MHD) simulations of propagating waves. We excite the transverse MHD waves and generate synthetic observations of the Fe xiii emission line. We establish that LOS superposition greatly reduces the Doppler velocity amplitudes and increases the nonthermal line widths. Importantly, our model generates the observed wedge-shape correlation between Doppler velocities and nonthermal line widths. We find that the observed wave energy is only 0.2%–1% of the true wave energy, which explains the 2–3 order-of-magnitude energy discrepancy. We conclusively establish that true wave energies are hidden in nonthermal line widths. Hence, our results rule out the requirement for an additional “dark” energy in the solar corona.