Presentation #122.03 in the session Coronal Heating: Present Understanding and Future Progress — Poster Session.
We have found evidence for the parametric decay instability (PDI) in the lower solar atmosphere, near the transition region. PDI is an instability of Alfven waves, in which a forward-propagating pump Alfven wave decays into forward and backward secondary Alfven waves and a forward sound wave. Models have suggested PDI could be an important source of the counter-propagating Alfven waves that are needed for turbulent heating. We studied Si IV spectra from the Interface Region Imaging Spectrometer (IRIS). These data show that the power spectrum of density fluctuations resembles that of the velocity fluctuations, but with the frequency axis scaled up by about a factor of two. Based on the propagation speeds, the density fluctuations are consistent with sound waves and the velocity fluctuations with Alfven waves. The frequency scaling factor matches the frequency relationship predicted by the theory of the PDI for the plasma conditions of the observed region.