Spectroscopic Detection of Alfvnic Waves in the Chromospheric Fibrils of a Solar-quiet Region

We report the detection of transverse magnetohydrodynamic waves, also known as Alfvenic waves, in the chromospheric fibrils of a solar-quiet region. Unlike previous studies that measured transversal displacements of fibrils in imaging data, we investigate the line-of-sight (LOS) velocity oscillations of the fibrils in spectral data. The observations were carried out with the Fast Imaging Solar Spectrograph of the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory. By applying spectral inversion to the H alpha and Ca ii 8542 angstrom line profiles, we determine various physical parameters, including the LOS velocity in the chromosphere of the quiet Sun. In the H alpha data, we select two adjacent points along the fibrils and analyze the LOS velocities at those points. For the time series of the velocities that show high cross-correlation between the two points and do not exhibit any correlation with intensity, we interpret them as propagating Alfvenic wave packets. We identify a total of 385 Alfvenic wave packets in the quiet-Sun fibrils. The mean values of the period, velocity amplitude, and propagation speed are 7.5 minutes, 1.33 km s-1, and 123 km s-1, respectively. We find that the detected waves are classified into three groups based on their periods, namely, 3, 5, and 10 minute bands. Each group of waves exhibits distinct wave properties, indicating a possible connection to their generation mechanism. Based on our results, we expect that the identification of Alfvenic waves in various regions will provide clues to their origin and the underlying physical processes in the solar atmosphere.