25 research outputs found
Exploring the latitude and depth dependence of solar Rossby waves using ring-diagram analysis
No full textContext. Global-scale equatorial Rossby waves have recently been unambiguously identified on the Sun. Like solar acoustic modes, Rossby waves are probes of the solar interior. Aims. We study the latitude and depth dependence of the Rossby wave eigenfunctions. Methods. By applying helioseismic ring-diagram analysis and granulation tracking to observations by HMI aboard SDO, we computed maps of the radial vorticity of flows in the upper solar convection zone (down to depths of more than 16 Mm). The horizontal sampling of the ring-diagram maps is approximately 90 Mm (∼7.5°) and the temporal sampling is roughly 27 hr. We used a Fourier transform in longitude to separate the different azimuthal orders m in the range 3 ≤ m ≤ 15. At each m we obtained the phase and amplitude of the Rossby waves as functions of depth using the helioseismic data. At each m we also measured the latitude dependence of the eigenfunctions by calculating the covariance between the equator and other latitudes. Results. We conducted a study of the horizontal and radial dependences of the radial vorticity eigenfunctions. The horizontal eigenfunctions are complex. As observed previously, the real part peaks at the equator and switches sign near ±30°, thus the eigenfunctions show significant non-sectoral contributions. The imaginary part is smaller than the real part. The phase of the radial eigenfunctions varies by only ±5° over the top 15 Mm. The amplitude of the radial eigenfunctions decreases by about 10% from the surface down to 8 Mm (the region in which ring-diagram analysis is most reliable, as seen by comparing with the rotation rate measured by global-mode seismology). Conclusions. The radial dependence of the radial vorticity eigenfunctions deduced from ring-diagram analysis is consistent with a power law down to 8 Mm and is unreliable at larger depths. However, the observations provide only weak constraints on the power-law exponents. For the real part, the latitude dependence of the eigenfunctions is consistent with previous work (using granulation tracking). The imaginary part is smaller than the real part but significantly nonzero
Solar convective velocities: Updated helioseismic constraints
No full textModeling heat transport by convection is one of the most challenging aspects of solar and stellar physics. The literature currently provides apparently inconsistent observational estimates of the strength of large-scale convective flows in the upper layers of the solar convection zone. In addition, the large-scale convective flows predicted from numerical simulations are substantially stronger than some of the observational inferences in the literature. The current work aims to provide a consistent presentation of some of the main results in the literature both from observations and simulations. To achieve this aim, we carry out an analysis of published estimates of the strength of solar convection at different spatial scales. In particular, we employ a consistent set of conventions to compute the kinetic energy density in the east-west flows. This establishes a clear baseline for future work. The main conclusion is that there are inconsistencies between different observational results and also differences between observations and simulations. This conclusion is important as it demonstrates a need to determine the sources of the inconsistencies between different observational inferences and also to determine the missing ingredients in simulations of solar subsurface convection.HORIZON EUROPE European Research Council 10.13039/10001918
Global-scale equatorial Rossby waves as an essential component of solar internal dynamics
No full textThe Sun’s complex dynamics is controlled by buoyancy and rotation in the convection zone. Large-scale flows are dominated by vortical motions1 and appear to be weaker than expected in the solar interior2. One possibility is that waves of vorticity due to the Coriolis force, known as Rossby waves3 or r modes4, remove energy from convection at the largest scales5. However, the presence of these waves in the Sun is still debated. Here, we unambiguously discover and characterize retrograde-propagating vorticity waves in the shallow subsurface layers of the Sun at azimuthal wavenumbers below 15, with the dispersion relation of textbook sectoral Rossby waves. The waves have lifetimes of several months, well-defined mode frequencies below twice the solar rotational frequency, and eigenfunctions of vorticity that peak at the equator. Rossby waves have nearly as much vorticity as the convection at the same scales, thus they are an essential component of solar dynamics. We observe a transition from turbulence-like to wave-like dynamics around the Rhines scale6 of angular wavenumber of approximately 20. This transition might provide an explanation for the puzzling deficit of kinetic energy at the largest spatial scales
Solar inertial modes: Observations, identification, and diagnostic promise
No full textThe oscillations of a slowly rotating star have long been classified into spheroidal and toroidal modes. The spheroidal modes include the well-known 5-min acoustic modes used in helioseismology. Here we report observations of the Sun’s toroidal modes, for which the restoring force is the Coriolis force and whose periods are on the order of the solar rotation period. By comparing the observations with the normal modes of a differentially rotating spherical shell, we are able to identify many of the observed modes. These are the high-latitude inertial modes, the critical-latitude inertial modes, and the equatorial Rossby modes. In the model, the high-latitude and critical-latitude modes have maximum kinetic energy density at the base of the convection zone, and the high-latitude modes are baroclinically unstable due to the latitudinal entropy gradient. As a first application of inertial-mode helioseismology, we constrain the superadiabaticity and the turbulent viscosity in the deep convection zone
Rossby Waves in Astrophysics
No full textRossby waves are a pervasive feature of the large-scale motions of the Earth's atmosphere and oceans. These waves (also known as planetary waves and r-modes) also play an important role in the large-scale dynamics of different astrophysical objects such as the solar atmosphere and interior, astrophysical discs, rapidly rotating stars, planetary and exoplanetary atmospheres. This paper provides a review of theoretical and observational aspects of Rossby waves on different spatial and temporal scales in various astrophysical settings. The physical role played by Rossby-type waves and associated instabilities is discussed in the context of solar and stellar magnetic activity, angular momentum transport in astrophysical discs, planet formation, and other astrophysical processes. Possible directions of future research in theoretical and observational aspects of astrophysical Rossby waves are outlined
Upgrading electron temperature and electron density diagnostic diagrams of forbidden line emission
No full textContext. Diagnostic diagrams of forbidden lines have been a useful tool
for observers for many decades now. They are used to obtain information on the basic
physical properties of thin gaseous nebulae. Some diagnostic diagrams are in wavelength
domains that were difficult to apply either due to missing wavelength coverage or the low
resolution of older spectrographs. Furthermore, most of the diagrams were calculated using
just the species involved as a single atom gas, although several are affected by
well-known fluorescence mechanisms as well. Additionally, the atomic data have improved up
to the present time.
Aims. The aim of this work is to recalculate well-known, but also
sparsely used, unnoted diagnostics diagrams. The new diagrams provide observers with
modern, easy-to-use recipes for determining electron temperature and densities.
Methods. The new diagnostic diagrams were calculated using large grids
of parameter space in the photoionization code CLOUDY. For a given basic parameter (e.g.,
electron density or temperature), the solutions with cooling-heating-equilibrium were
chosen to derive the diagnostic diagrams. Empirical numerical functions were fitted to
provide formulas usable in, e.g., data reduction pipelines.
Results. The resulting diagrams differ significantly from those used up
to now and will improve thermodynamic calculations. To our knowledge, detailed, directly
applicable fit formulas are given for the first time, leading to the calculation of
electron temperature or density from the line ratios
OH level populations and accuracies of Einstein-A coefficients from hundreds of measured lines
No full textLine emission from hydroxyl (OH) molecules at altitudes of about 90 km strongly contributes to the Earth's night-sky brightness and is therefore used as an important indicator of atmospheric chemistry and dynamics. However, interpreting the measurements can be ambiguous since necessary molecular parameters and the internal state of OH are not well known. Based on high-quality spectral data, we investigated these issues and found solutions for a better understanding of the OH line intensities
Mechanisms for varying non-LTE contributions to OH rotational temperatures from measurements and modelling. I. Climatology
No full textRotational temperatures Trot from OH line intensities are an important approach to study the Earth's mesopause region. However, the interpretation can be complicated as the resulting Trot are effective values weighted for the varying OH emission layer. Moreover, the measured Trot only equal kinetic temperatures Tkin if the rotational level population distribution for the considered OH lines is fully thermalised. In many cases, this basic condition of a local thermodynamic equilibrium (LTE) does not seem to be fulfilled. In order to better understand the non-LTE temperature excesses ΔTNLTE and their variations, we used Trot measurements based on 1526 high-resolution spectra of the UVES spectrograph at the Very Large Telescope at Cerro Paranal in Chile in combination with Tkin weighted for the OH emission layer based on 4496 nighttime temperature and OH emission profiles from the SABER radiometer onboard TIMED taken at a similar location. Both data sets were linked via climatologies consisting of the nighttime and seasonal temperature variations. The study focusses on the non-LTE effects at the vibrational level v=9, which is directly populated by the OH-producing hydrogen–ozone reaction and therefore especially prone to incomplete thermalisation of the rotational level population. In comparison to the less critical v=3, the ΔTNLTE climatology showed clear and strongly variable temperature excesses of several kelvins with minima in the evening around the equinoxes and a reliable maximum in the second half of the night around the turn of the year. The Trot non-LTE contributions are positively correlated with the effective OH emission height and volume mixing ratio of atomic oxygen. A significant anti-correlation is found for the air density. Thus, especially variations in the OH emission layer altitude and shape, which are related to changes in the layer-weighted chemical composition and density, are important for the amount of ΔTNLTE
