Correlations, spectra, and instability of phase-space density fluctuations in open-cluster models

The dynamical evolution of six open star cluster models is analyzed using the correlation and spectral analysis of phase-space density fluctuations. The two-time and mutual correlation functions are computed for the fluctuations of the phase-space density of cluster models. The data for two-time and two-particle correlations are used to determine the correlation time for phase-space density fluctuations ((0.1–1) τ _v.r., where τ _v.r. is the violent relaxation time of the model) and the average phase velocities of the propagation of such fluctuations in cluster models. These velocities are 2–20 times smaller than the root mean square velocities of the stars in the cluster core. The power spectra and dispersion curves of phase-space density fluctuations are computed using the Fourier transform of mutual correlation functions. The results confirm the presence of known unstable phase-space density fluctuations due to homologous fluctuations of the cluster cores. The models are found to exhibit a number of new unstable phase-space density fluctuations (up to 32–41 pairs of fluctuations with different complex conjugate frequencies in each model; the e-folding time of the amplitude growth of such fluctuations is (0.4–10) τ _v.r. and their phases are distributed rather uniformly). Astrophysical applications of the obtained results (irregular structure of open star clusters, formation and decay of quasi-stationary states in such clusters) are discussed.     

Solar-Activity Indices in the 24th Cycle and the Behavior of the Ionospheric F2-Layer

Geomagnetism and Aeronomy - This work is a continuation of the author’s earlier analysis of the relationship between the most often used solar-activity index, F10.7, and solar-activity...     

Time and spatial variations in the ratio of nighttime and daytime critical frequencies of the F 2 layer

The ratio of daytime and nighttime values of the foF2 critical frequency is analyzed on the basis of the data of 28 ionospheric stations in the Eastern Hemisphere. It is found that three types of time variations in this ratio are observed after 1980: an increase with time (a positive trend), a decrease with time (a negative trend), and the absence of pronounced changes (a zero trend). The sign of this trend is shown to be governed by the signs of the magnetic declination D and magnetic inclination I at the given ionospheric station. This fact makes it possible to assume that the above trend is caused by long-term variations in the zonal component V _ny of the horizontal wind in the thermosphere, the latter component contributing into the vertical drift velocity W. The causes of the systematic changes in the thermospheric circulation regime after 1980 are still unknown; however, it is quite probable that they are related to anthropogenic changes in the atmosphere.     

Relation of the F2 region to the parameters of the stratosphere: Comparison of two solar cycles

The results concerning the relation of the stratosphere and ionosphere, obtained on the basis of the data of the 1979–1989 solar cycle, are compared to the data of the following solar cycle. It is shown that all the regularities of the behavior of the correlation coefficient r(h, fo) between the stratospheric parameter h(100) and the critical frequency fo F2 found for the 1980s are also true for the 1990s. Moreover, it turns out that the r(h, fo) dependence on solar activity is similar within both solar cycles.     

Scatter of <Emphasis Type="Italic">hmF</Emphasis>2 values as an indicator of trends in the thermospheric dynamics

The variability degree of the F ₂-layer height, hmF2, from the 1950s–1960s to the 1990s has been analyzed based on the vertical sounding data for a series of midlatitude ionospheric stations. It has been found that the scatter of the hmF2 values (standard deviation) abruptly increases from the earlier decades to the later ones. This increase is more evident in the spring period of the year and is independent of geomagnetic activity. An increase in the scatter of hmF2 apparently indicates systematic changes (trends) in the thermospheric dynamics, the existence of which was suggested in the recent publications of the authors.     

Reindeer in tundra ecosystems: the challenges of understanding system complexity

The resilience of tundra ecosystems is limited, with relatively few key biotic components determining the general pattern of the dynamics of these systems. Sustainable use of reindeer pastures, which are natural tundra ecosystems, should take into account interactions within the whole complex of key components. Among the most important are the small herbivorous rodents. For example, during peak densities lemmings may reduce above-ground plant biomass by 50-70%. At the same time, rapid turnover of nutrients in the form of significant amounts of potassium, phosphorus and nitrogen that lemmings excrete in urine promotes vascular plant growth. It is concluded that these kinds of studies are essential for the sustainable management of Russia's grazing lands: 1) comparative studies of productivity, biological diversity and structure of plant communities under the different pressures exerted by the main groups of herbivorous animals (reindeer and small rodents), as well as under varying levels of industrial development; 2) studies of the direct and indirect effects of herbivorous mammals on biological turnover and energy flow within the system; 3) studies of the role of industrial contamination on the turnover of nutrients in tundra ecosystems.     

On utility of triangular C-grid type discretization for numerical modeling of large-scale ocean flows

Ocean circulation models based on triangular C-grid discretization are frequently employed to simulate coastal ocean dynamics on unstructured meshes. It is shown that on time and space scales dominated by slow geostrophic dynamics, this discretization tends to exhibit checkerboard noise in the field of horizontal velocity divergence and vertical velocity, respectively. The noise is linked to the geometry of triangular C-grid and is amplified in regimes that are close to geostrophic balance through the particular structure of the Coriolis operator. It can be partly suppressed in some cases but remains a problem in a general case and makes the triangular C-grid a suboptimal choice for large-scale ocean modeling.