A.A. Norton¹, R.B. Stutz¹, B.T. Welsch²

1. HEPL, Stanford University, Stanford, CA, USA 94305-4085
2. University of Wisconsin, Green Bay, WI, USA 54311

The study of MHD waves in the photosphere informs us about how magnetic fields respond to and modify motions and waves generated by convection and turbulence. Strong magnetic fields decrease the acoustic wave power outgoing from an active region^[1] 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 in energy transport.

Figure 1| Average Fourier power, plotted on a log scale for the y-axis, for 160 pixels selected within the quiet-Sun, PIL, umbra, and plage using 45 second cadence line-of-sight data. Clockwise from top left: Doppler power (divided by 10⁶), magnetogram power (divided by 10⁴), line width power (divided by 10³), and continuum power (divided by 10⁸). The 4 mHz peak is an artifact corresponding to the “pixel-crossing” time and is seen in many of the curves but no other artifacts are known.

A search for Alfvénic waves using HMI data was suggested to measure Doppler velocities of the plasma in the vicinity of field lines that are perpendicular to the observer’s line of sight, i.e., polarity inversion lines or neutral lines. Detecting oscillations transverse to magnetic field lines indicates the waves have properties of Alfvénic waves. Specifically, within this paper, we report on the amplitudes and phase relations of oscillations in quiet-Sun, plage, umbra, and the polarity inversion line (PIL) of NOAA AR 11158 on 16 February 2011. Waves with 5-minute periods are observed. Figure 1 shows the average power spectra in quiet-Sun, umbra, PIL, and plage. Significant Doppler velocity oscillations are present along the PIL, meaning that plasma motion is perpendicular to the magnetic field lines, a signature of Alvénic waves. The 4 mHz peak is an artifact corresponding to the “pixel-crossing” time and is seen in many of the curves but no other artifacts are known. Surprisingly, a 3-minute wave is observed in select regions of the umbra in the magnetogram data and can be seen in the magnetogram power spectra.

Figure 2| Histograms with 20° bins are created for the phase values for oscillations of velocity and intensity, ϕ(v,I), velocity and magnetic flux, ϕ(v,|M|), velocity and line width, ϕ(v,Lw), intensity and line width, ϕ(I,Lw), and line width and magnetic flux, ϕ(Lw,|M|) for pixels in the quiet-Sun, PIL, umbra, and plage regions. Lines are not plotted for the quiet-Sun and plage data for panels in which the data are very noisy.

Common phase values of ϕ(v,I)=π/2, ϕ(v,B_los)=-π/2. In addition, ϕ(I,B_los)=π in plage are observed (see Figure 2). These phase values are consistent with slow standing or fast standing surface sausage wave modes^[2,3]. The line width variations, and their phase relations with intensity and magnetic oscillations, show different values within the plage and PIL regions, which may offer a way to further differentiate wave mode mechanics.

Figure 3| Time-distance slices are shown for two hours for a slit placed horizontally along the polarity inversion line (left panels), a sunspot umbra (middle) and quiet-Sun (right). The relative amplitude of the oscillations are shown in the Doppler velocity (top) with a range of ±250 m s^-1, magnetic flux (middle) with a range of ±50 Mx cm^-2, and line width (bottom) with a range of ±8 mÅ. Background averages have been subtracted from all the data but no filtering has been applied.

A time-distance diagram for a section across the PIL shows Doppler oscillations progressing eastward at ~2.7 km s^-1, with magnetic oscillation amplitudes increasing as the Doppler amplitudes damp (see Figure 3, left column, top row). The magnetic disturbances then propagate at 2-6 km s^-1 (see left column, middle row). Enhanced line widths are found at the locations where the waves change from being primarily acoustic to primarily magnetic (see left column, bottom row). The umbral and quiet-Sun time-distance data (see middle and right columns) show 3- and 5-minute oscillations but no similar behaviors of acoustic to magnetic conversion, so we assume the geometry of the PIL with the horizontal, confined field lines provides an unusual environment for wave propagation and a clear indication of the possible existence of Alvén modes being generated and propagating in the PIL photosphere.

While the amplitudes of oscillations and phase relations in HMI data reported in the full paper^[4] support the presence of MHD waves in and around the active region, forward modeling of the spectral line dynamics in the presence of realistic MHD modes should be carried out prior to a final confirmation. Future collaboration within the Waves in the Lower Solar Atmosphere (WaLSA) group will help to isolate and characterize the photospheric wave properties here shown to be abundantly contained within the HMI data in order to support higher resolution studies from other instruments.

References

[1] Braun, D.C., & Birch, A.C., 2008, Solar Phys, 251, 267.
[2] Moreels, M.G., and van Doorsselaere, T., 2013, A&A, 551, A137.
[3] Ulrich, R.K., 1996, ApJ, 465, 436.
[4] Norton, A.A., Stutz, R. B., Welsch, B.T., 2021, RSTA, 379, 2190 doi: 10.1098/rsta.2020.0175, https://arxiv.org/abs/2101.01349