FIRST DIRECT MEASUREMENTS OF TRANSVERSE WAVES IN SOLAR POLAR PLUMES USING <i>SDO</i> /AIA

Abstract

There is intense interest in determining the precise contribution of\nAlfv\\'enic waves propagating along solar structures to the problems of coronal\nheating and solar wind acceleration. Since the launch of SDO/AIA, it has been\npossible to resolve transverse oscillations in off-limb solar polar plumes and\nrecently McIntosh et al. (2011, Nature, 475, 477) concluded that such waves are\nenergetic enough to play a role in heating the corona and accelerating the fast\nsolar wind. However, this result is based on comparisons to Monte Carlo\nsimulations and confirmation via direct measurements is still outstanding.\nThus, this letter reports on the first direct measurements of transverse wave\nmotions in solar polar plumes. Over a 4 hour period, we measure the transverse\ndisplacements, periods and velocity amplitudes of 596 distinct oscillations\nobserved in the 171 \\AA channel of SDO/AIA. We find a broad range of\nnon-uniformly distributed parameter values which are well described by\nlog-normal distributions with peaks at $234$ km, $121$ s and $8$ km s$^{-1}$,\nand mean and standard deviations of $407\\pm297$ km, $173\\pm118$ s and $14\\pm10$\nkm s$^{-1}$. Within standard deviations, our direct measurements are broadly\nconsistent with previous results. However, accounting for the whole of our\nobserved non-uniform parameter distribution we calculate an energy flux of\n$9-24$ W m$^{-2}$, which is $4-10$ times below the energy requirement for solar\nwind acceleration. Hence, our results indicate that transverse MHD waves as\nresolved by SDO/AIA cannot be the dominant energy source for fast solar wind\nacceleration in the open-field corona.\n