Propagating Alfvénic Waves Observed in the Chromosphere around a Small Sunspot: Tales of 3-minute Waves and 10-minute Waves

Abstract

Recent observations provided evidence that the solar chromosphere of sunspot regions is pervaded by Alfvénic waves—transverse magnetohydrodynamic (MHD) waves (Alfvén waves or kink waves). In order to systematically investigate the physical characteristics of Alfvénic waves over a wide range of periods, we analyzed the time series of line-of-sight velocity maps constructed from the Hα spectral data of a small sunspot region taken by the Fast Imaging Solar Spectrograph of the Goode Solar Telescope at Big Bear. We identified each Alfvénic wave packet by examining the cross-correlation of band-filtered velocity between two points that are located a little apart presumably on the same magnetic field line. As result, we detected a total of 279 wave packets in the superpenumbral region around the sunspot and obtained their statistics of period, velocity amplitude, and propagation speed. An important finding of ours is that the detected Alfvénic waves are clearly separated into two groups: 3-minute period (<7 minutes) waves and 10-minute period (>7 minutes) waves. We propose two tales on the origin of Alfvénic waves in the chromosphere; the 3-minute Alfvénic waves are excited by the upward-propagating slow waves in the chromosphere through the slow-to-Alfvénic mode conversion, and the 10-minute Alfvénic waves represent the chromospheric manifestation of the kink waves driven by convective motions in the photosphere.

Spectroscopic Detection of Alfvénic MHD Waves in the Sunspot Chromosphere

“…How about ten-minute wave? Are they related to the cutoff period of Alfvénic wave as we proposed (Chae et al 2022) or not? Do the Alfvénic wave detected with the spectroscopic method much overlap those detected with the imaging method or do they represent distinct kinds of Alvénic waves?…”

“…It was later realized by Chae et al (2022) that the detection of Alfvénic waves of different periods can be done more systematically by decomposing the velocity fluctuations into a few band-filtered oscillations. Fig.…”

“…As far as we know, we are the first who clearly detected Alfvénic waves in chromospheric fibrils with a real spectrograph. In this proceeding, we will briefly review the spectroscopic data and method we have used, and the main findings we have obtained (Chae et al 2021b(Chae et al , 2022.…”

Alfvén Wave Connection between the Chromosphere and the Corona of the Sun: An Analytical Study

“…The amplitudes of Alfvénic velocity fluctuations observed so far in active regions are even smaller than those observed in quiet regions. They include 1 km s −1 for horizontal transverse oscillations of a 2-4 minute period in an active region fibril (Pietarila et al 2011), the mean value of 0.8 km s −1 for the horizontal transverse oscillations of periods around 13 minutes in super-penumbral fibrils (Morton et al 2021), and the mean value of 0.6-1.0 km s −1 for the vertical transverse oscillations of periods around 3 and 10 minutes in super-penumbral fibrils (Chae et al 2021(Chae et al , 2022. These velocity amplitudes are too low for the upward Alfvén waves generated in the middle of the chromosphere or below to explain the Alfvén waves required for the corona.…”

“…Meanwhile, polarized transverse waves were detected from the high-resolution tracking of magnetic elements in a magnetic concentration (Stangalini et al 2017). Recently, transverse MHD waves were also identified from the spectroscopic observations of line-of-sight velocity oscillations in super-penumbral fibrils (Chae et al 2021(Chae et al , 2022 and quiet region fibrils. Torsional Alfvén waves in the chromosphere were also spectroscopically inferred from the time variation of the Hα Doppler width of network features (Cauzzi et al 2009;Jess et al 2009), and from the time variation of the Doppler velocity pattern in type II spicules (De Pontieu et al 2012) and fibrils (Srivastava et al 2017) in quiet regions.…”