The Nature of Running Penumbral Waves Revealed

TL;DR

This study demonstrates that running penumbral waves are low-beta slow-mode waves propagating along magnetic fields, clarifying their nature through spectropolarimetric analysis and phase difference measurements.

Contribution

The paper reveals that RP waves are slow-mode waves propagating along magnetic fields, supported by phase difference analysis and wave dispersion modeling.

Findings

RP waves are low-beta slow-mode waves.

Phase difference analysis supports wave propagation along magnetic fields.

Modified acoustic wave dispersion relation fits observations.

Abstract

We seek to clarify the nature of running penumbral (RP) waves: are they chromospheric trans-sunspot waves or a visual pattern of upward-propagating waves? Full Stokes spectropolarimetric time series of the photospheric Si I 10827 \AA line and the chromospheric He I 10830 \AA multiplet were inverted using a Milne-Eddington atmosphere. Spatial pixels were paired between the outer umbral/inner penumbral photosphere and the penumbral chromosphere using inclinations retrieved by the inversion and the dual-height pairings of line-of-sight velocity time series were studied for signatures of wave propagation using a Fourier phase difference analysis. The dispersion relation for radiatively cooling acoustic waves, modified to incorporate an inclined propagation direction, fits well the observed phase differences between the pairs of photospheric and chromospheric pixels. We have thus demonstrated that RP waves are in effect low-beta slow-mode waves propagating along the magnetic field.