Seismological Determination of the Alfvén Speed and Plasma Beta in Solar Photospheric Bright Points

The Alfvén speed and plasma beta in photospheric bright points (BPs) observed by the Broadband Filter Imager (BFI) of the Solar Optical Telescope on board the Hinode satellite are estimated seismologically. The diagnostics is based on the theory of slow magnetoacoustic waves in a non-isothermally stratified photosphere with a uniform vertical magnetic field. We identify and track BPs in a G-band movie by using the 3D region growing method, and align them with blue continuum images to derive their brightness temperatures. From the Fourier power spectra of 118 continuum light curves made in the BPs, we find that light curves of 91 BPs have oscillations with properties that are significantly different from oscillation in quiet regions, with the periods ranging 2.2–16.2 minutes. We find that the model gives a moderate value of the plasma beta when γ lies at around 5/3. The calculated Alfvén speed is 9.68 ± 2.02 km s−1, ranging in 6.3–17.4 km s−1. The plasma beta is estimated to be of 0.93 ± 0.36, ranging in 0.2–1.9.