Author: A.A. Norton, R.B. Stutz & B.T. Welsch
Jan 29, 2021
The study of MHD waves in the photosphere informs us how magnetic fields respond to and modify motions generated by convection and turbulence. Strong magnetic fields decrease the outgoing acoustic wave power and can be explained by acoustic waves converting into MHD waves. As MHD waves are thought to contribute to shock heating of the chromosphere, they are an important mechanism. Using HMI data, we report on the amplitudes and phases of oscillations in the umbra, plage, quiet-Sun and polarity inversion line (PIL) of active region NOAA #11158. Waves with 5–minute periods are observed in PIL and plage with common phase values consistent with slow standing or fast standing surface sausage wave modes. A 3-minute wave is seen in select regions of the umbral magnetograms. Significant Doppler velocity oscillations are present along the PIL, meaning that plasma motion is perpendicular to the magnetic field lines, a signature of Alfvénic waves. A time-distance diagram along a section of the PIL, see figure, shows Eastward propagating Doppler oscillations converting into magnetic oscillations with the propagation speeds ranging between 2-6 km/s. Enhanced line widths are found at the locations where the waves change from being primarily acoustic to primarily magnetic. The geometry of the PIL with the horizontal, confined field lines provides an unusual environment for wave propagation and a clear indication of Alfvén modes.
Norton, A.A., Stutz, R. B. & Welsch, B.T (2021), 'Oscillations observed in umbra, plage, quiet-Sun and the polarity inversion line of active region 11158 using HMI/SDO', Philos. T. R. Soc. A. 379:20200175 DOI:10.1098/rsta.2020.0175.