(News & views on the paper by Ding Yuan, RAA, 2015, vol.15, 1449-1454)
News and Views on "Signature of high-order azimuthal MHD body modes in sunspot's low atmosphere"
(School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China)
There are some mysterious problems in the field of solar physics, such as coronal heating, solar wind acceleration, and mechanisms for solar flares and coronal mass ejections. The basic difficulties in understanding these problems lie in the unknown physical parameters associated with the solar interior and atmosphere. Combining magnetohydrodynamic (MHD) wave theories, multi-wavelength observations and solar magnetoseismology provides a powerful tool to detect some essentially important physical parameters that are difficult to determine with other methods (e.g., Nakariakov & Verwichte 2005; Ruderman & Erdelyi 2009).
MHD waves are extremely abundant because of the anisotropy of the magnetic field as well as gradients in other thermodynamic parameters. By considering a simple but insightful two-component model, Edwin & Roberts (1983) and Roberts et al. (1984) found the dispersion relation for MHD waves in a magnetic flux tube. Based on their characteristic speeds and azimuthal wave numbers, non-leaky MHD body waves can be divided into different categories, such as slow magneto-acoustic, Alfvenic and fast magneto-acoustic waves based on their speeds, as well as sausage, kink, fluting waves and so on based on their azimuthal wave numbers.
MHD wave modes with higher azimuthal wave numbers are difficult to resolve with modern instruments. Yuan (2015) provides observational evidence for high-order azimuthal MHD body waves in the low atmosphere above a sunspot. Such an observation provides the first evidence of resolvable higher azimuthal modes in multiple cross sections of the magnetic waveguide (a sunspot). The higher azimuthal modes provide further constraints on local magnetic and thermal parameters, and therefore extends a technique used in MHD seismology. However, more firm detection is required to solidly confirm the existence and seismological roles of higher order modes.