First direct measurements of transverse waves in solar polar plumes using SDO/AIA (Invited)

Abstract:

Currently, there is intense interest in determining the precise contribution of Alfven waves propagating along solar structures to the problems of coronal heating and solar wind acceleration. Since the launch of SDO/AIA, it has been possible to resolve transverse oscillations in off-limb solar polar plumes and recently McIntosh et al. (2011, Nature, 475, 477) concluded that such waves are energetic enough to play a role in heating the corona and accelerating the fast solar wind. However, this result is based on comparisons to Monte Carlo simulations and confirmation via direct measurements is still outstanding. Here we report on the first direct measurements of transverse wave motions in solar polar plumes. Over a 4 hour period, we measure the transverse displacements, periods and velocity amplitudes of 596 distinct oscillations observed in the 171 Angstrom channel and find a broad range of parameter values (64 - 2558 km, 61 - 2097 s and 1 - 88 km/s respectively). The parameters are non-uniformly distributed with a significant positive skew and are well described by log-normal distributions with peaks at 234 km, 121 s and 8 km/s, and mean and standard deviations of the parameters are 407±297 km, 173±±118 s and 14±10 km/s. Within standard deviations, our direct measurements are broadly consistent with previous results. However, accounting for the whole of our observed non-uniform parameter distribution we calculate a time averaged energy flux of 9 - 24 W/m², indicating that transverse MHD waves carry a much less significant energy flux in the open-field corona than previously thought.