22 research outputs found
Observations and Modeling of Potassium Emission in the Terrestrial Nightglow
The ablation of cosmic dust entering the atmosphere causes the formation of an atomic potassium (K) layer in the mesopause region. It can be studied via resonance fluorescence from the K(D1) line at 769.9 nm, stimulated by sunlight or a laser. In addition, the faint emission from a chemiluminescent cycle involving ozone and oxygen atoms has been observed with a nocturnal mean intensity of about 1 Rayleigh. In this study, the K nightglow is investigated in much greater detail, using 2,299 highâresolution spectra taken with the astronomical echelle spectrograph Ultraviolet and Visual Echelle Spectrograph at Cerro Paranal in Chile (24.6°S) between 2000 and 2014. The seasonal variation is dominated by a maximum in June. During the night, the highest intensities are found close to sunrise. Moreover, there is a clear negative correlation with solar activity. These variations are very different from those of the wellâstudied sodium (Na) nightglow. The K nightglow at Cerro Paranal was also simulated with the Whole Atmosphere Community Climate Model including K chemistry. The observed and modeled climatologies do not match well, largely because of unreliable Whole Atmosphere Community Climate Model ozone densities. Satelliteâbased profile retrievals for ozone and temperature from Sounding of the Atmosphere using Broadband Emission Radiometry and K from Optical Spectrograph and Infrared Imaging System were then used to simulate the K nightglow and to derive the quantum yield of the K(D) emission with respect to the reaction of K with ozone. Considering that the obscured K(D2) line is expected on theoretical grounds to be 1.67 times brighter than K(D1), we find about 30% for this quantum yield, which is much higher than for Na(D) emission
Upgrading electron temperature and electron density diagnostic diagrams of forbidden line emission
Context. 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
Mechanisms for varying non-LTE contributions to OH rotational temperatures from measurements and modelling. I. Climatology
Rotational 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 ÎTNLTE 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 hydrogenâozone reaction and therefore especially prone to incomplete thermalisation of the rotational level population. In comparison to the less critical v=3, the ÎTNLTE 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 ÎTNLTE
Exploring the latitude and depth dependence of solar Rossby waves using ring-diagram analysis
Context. 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 16âMm). The horizontal sampling of the ring-diagram maps is approximately 90âMm (âŒ7.5°) 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 3ââ€âmââ€â15. 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 15âMm. The amplitude of the radial eigenfunctions decreases by about 10% from the surface down to 8âMm (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 8âMm 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