Magneto-acoustic waves in a gravitationally stratified magnetized plasma: eigen-solutions and their applications to the solar atmosphere

Magneto-acoustic gravity (MAG) waves have been studied intensively in the context of astrophysical plasmas. There are three popular choices of analytic modeling using a Cartesian coordinate system: a magnetic field parallel, perpendicular, or at an angle to the gravitational field. Here, we study a gravitationally stratified plasma embedded in a parallel, so called vertical, magnetic field. We find a governing equation for the auxiliary quantity Θ = p 1/ρ 0, and find solutions in terms of hypergeometric functions. With the convenient relationship between Θ and the vertical velocity component, v z , we derive the solution for v z . We show that the four linearly independent functions for v z can also be cast as single hypergeometric functions, rather than the Frobenius series derived by Leroy & Schwartz. We are then able to analyze a case of approximation for a one-layer solution, taking the small wavelength limit. Motivated by solar atmospheric applications, we finally commence study of the eigenmodes of perturbations for a two-layer model using our solutions, solving the dispersion relation numerically. We show that, for a transition between a photospheric and chromospheric plasma embedded in a vertical magnetic field, modes exist that are between the observationally widely investigated three and five minute oscillation periods, interpreted as solar global oscillations in the lower solar atmosphere. It is also shown that, when the density contrast between the layers is large (e.g., applied to photosphere/chromosphere-corona), the global eigenmodes are practically a superposition of the same as in each of the separate one-layer systems

The Frequency-dependent Damping of Slow Magnetoacoustic Waves in a Sunspot Umbral Atmosphere

High spatial and temporal resolution images of a sunspot, obtained simultaneously in multiple optical and UV wavelengths, are employed to study the propagation and damping characteristics of slow magnetoacoustic waves up to transition region heights. Power spectra are generated from intensity oscillations in sunspot umbra, across multiple atmospheric heights, for frequencies up to a few hundred mHz. It is observed that the power spectra display a power-law dependence over the entire frequency range, with a significant enhancement around 5.5 mHz found for the chromospheric channels. The phase-difference spectra reveal a cutoff frequency near 3 mHz, up to which the oscillations are evanescent, while those with higher frequencies propagate upwards. The power-law index appears to increase with atmospheric height. Also, shorter damping lengths are observed for oscillations with higher frequencies suggesting frequency-dependent damping. Using the relative amplitudes of the 5.5 mHz (3 minute) oscillations, we estimate the energy flux at different heights, which seems to decay gradually from the photosphere, in agreement with recent numerical simulations. Furthermore, a comparison of power spectra across the umbral radius highlights an enhancement of high-frequency waves near the umbral center, which does not seem to be related to magnetic field inclination angle effects

Analysis of Gaia radial-velocity standards: stability and new substellar companion candidates

peer reviewedOur main aim is to test the non-variability of the radial velocity (RV) of a sample of 2351 standard stars used for wavelength calibration of the Radial Velocity Spectrometer (RVS) instrument onboard Gaia. In this paper, we present the spectroscopic analysis of these stars with the determination of their physical parameters by matching observed and synthetic spectra. We estimate the offset between different instruments after determining the shift between measured and archived RVs since the instrument pipelines use various numerical masks. Through the confirmation of the stability of the target RVs, we find 68 stars with a long-term variation having an acceleration that exceeds $10 \, \rm {m\, s^{-1}\,yr^{-1}}$. This suggests a barycentric reflex motion caused by a companion. As activity phenomena may be the source of periodic and trend-like RV variations in stars with putative planetary companions, we analysed various activity indicators in order to check their correlations to the RV changes. Among the trend stars, 18 have a trend model scatter greater than $100 \, \rm {m\, s^{-1}}$ over a time span from 10 to 12 yr. We also confirm that six stars with known substellar companions have a total model scatter, 3σ, exceeding the threshold set by Gaia, that is, $300 \, \rm {m\, s^{-1}}$. In addition, TYC8963-01543-1, an SB2 star, has data scatter $\sigma = 176.6\, \rm {m\, s^{-1}}$. Four more other stars are revealed to be variable after combining data from different instruments. Despite the presence of low-amplitude changes, a very large fraction of our sample (98.8 per cent) appears suitable as RV calibrators for Gaia RVS