Spectroscopic Detection of Alfvénic Waves in the Chromosphere of Sunspot Regions

Abstract

Transverse magnetohydrodynamic waves often called Alfvénic (or kink) waves have been often theoretically put forward to solve the outstanding problems of the solar corona like coronal heating, solar wind acceleration, and chemical abundance enhancement. Here we report the first spectroscopic detection of Alfvénic waves around a sunspot at chromospheric heights. By analyzing the spectra of the Hα line and Ca II 854.2 nm line, we determined line-of-sight velocity and temperature as functions of position and time. As a result, we identified transverse magnetohydrodynamic waves pervading the superpenumbral fibrils. These waves are characterized by the periods of 2.5 to 4.5 minutes, and the propagation direction parallel to the fibrils, the supersonic propagation speeds of 45 to 145 km s −1 , and the close association with umbral oscillations and running penumbral waves in sunspots. Our results support the notion that the chromosphere around sunspots abounds with Alfvénic waves excited by the mode conversion of the upward-propagating slow magnetoacoustic waves.

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

“…If this is the case, the period and propagation speed of the waves should be approximately equal between the two methods. As a matter of fact, we find that the periods and propagation speeds obtained by Chae et al (2021b) are in good agreement with those found by Pietarila et al (2011). The Alfvénic waves reported by Morton et al (2021), however, are apparently different; they have much longer periods and much lower propagation speeds.…”

“…In the same manner, it is found that the period P of 10-minute waves is 12 minutes on average and ranges from 6.6 to 22 minutes. The period range of the 3-minute waves includes all the periods from 2.5 to 4.5 minutes reported by Chae et al (2021b) based on the spectroscopic detection and the period of 2.3 minutes reported respectively, indicating that c of 3-minute waves is 141 km s −1 on average and ranges from 32 to 620 km s −1 . In contrast, the propagation speed of the 10-minute waves is 63 km s −1 on average and ranges from 12 to 330 km s −1 .…”

“…In contrast, the propagation speed of the 10-minute waves is 63 km s −1 on average and ranges from 12 to 330 km s −1 . The above speed range of the 3-minute waves includes all the speeds from 40 to 150 km s −1 reported by Chae et al (2021b). The speed range of the 10-minute waves practically includes all the speeds from 10 to 70 km s −1 reported by Morton et al (2021).…”

“…This result can be regarded as indirect evidence for the assumed presence of Alfvénic waves but is irrelevant to direct detection of Alfvénic waves. In contrast, the work of Chae et al (2021b) was the first clear detection of Alfvénic waves in the chromosphere in that it detected transverse and incompressive velocity oscillations propagating at Alfvén speeds in the direction of magnetic fields.…”

“…It was employed by Tomczyk et al (2007) to detect Alfvénic waves in the corona. The first spectroscopic detection of Alfvénic waves in the chromosphere was done recently by Chae et al (2021b). They reported LOS velocity oscillations and their propagation that are compatible with Alfvénic waves in the solar chromosphere.…”