A search for mode coupling in magnetic bright points

Abstract

Context. Magnetic bright points (MBPs) are one of the smallest manifestations of the magnetic field in the solar atmosphere and are observed to extend from the photosphere up to the chromosphere. As such, they represent an excellent feature to use in searches for types of magnetohydrodynamic (MHD) waves and mode coupling in the solar atmosphere. Aims. In this work, we aim to study wave propagation in the lower solar atmosphere by comparing intensity oscillations in the photosphere with the chromosphere via a search for possible mode coupling, in order to establish the importance of these types of waves in the solar atmosphere, and their contribution to heating the chromosphere. Methods. Observations were conducted in July 2011 with the ROSA and the HARDCam instruments at the Dunn Solar Telescope. We used wavelet analysis to identify traveling MHD waves and derive frequencies in the G-band and H$\alpha$wave bands. We isolated a large sample of MBPs using an automated tracking algorithm throughout our observations. Two dozen of the brightest MBPs were selected from the sample for further study. Results. We find oscillations in the G-band MBPs, with frequencies between 1.5 and 3.6 mHz. Corresponding MBPs in the lower solar chromosphere observed in H$\alpha$ show a frequency range of 1.4 to 4.3 mHz. In about 38\% of the MBPs, the ratio of H$\alpha$ to G-band frequencies was near two. Thus, these oscillations show a form of mode coupling where the transverse waves in the photosphere are converted into longitudinal waves in the chromosphere. Conclusions. From simple estimates we find an energy flux of $\approx$45 $\times 10^{3}$ W m$^{-2}$ and show that the energy flowing through MBPs is enough to heat the chromosphere, and mode coupling is important in helping us understand the types of MHD waves in the lower solar atmosphere and the overall energy budget.