Behavior of parameters of the ionospheric F2 layer at the turn of the Centuries: 1. critical frequency

The available massifs of experimental data on the critical frequency of the ionospheric F2 layer, foF2, covering the first decade of the new century, are considered. On the basis of studying these massifs, a conclusion is drawn that the scatter of foF2 values (measured by the standard deviation (SD)) relative to the dependence on solar activity has grown substantially over recent decades as compared to the period 1958–1979. The possible causes of the SD increase are considered. It is shown that the foF2 values for the period 1998–2010 decreased as compared to the period 1958–1979 by an average of 0.6 MHz which gives an estimate of the foF2 trend of ～-0.03 MHz per year. Linear trends in foF2 for some ionospheric stations are analyzed. It is obtained that, in spite of the scatter in the data, it is possible to obtain statistically significant trends for each considered situation (day and postsunset period in summer and winter). At the same time, the winter negative trends (～-0.052 MHz per year) are approximately a factor of 2 higher than the summer ones (～-0.024 MHz per year). Comparisons with the trends obtained for earlier periods show that the negative trend in foF2 increased substantially towards the first decade of our century.     

Relation between changes in foF2 and hmF2 within various time intervals

The relation between the critical frequency foF2 and F2-layer height hmF2 is considered for ten ionospheric stations in the periods before and after 1980. It is shown that in the earlier period the relation between foF2 and hmF2 is well pronounced. In the later period, a distortion of this relation is observed. The statistical characteristics of the foF2 dependence on hmF2 are spoiled. That shows that due to the cooling and contraction of the upper atmosphere the height distribution of the photochemical parameters governing the equilibrium concentration in the layer maximum changes. A larger contribution to this effect is evidently provided by changes in the atom-to-molecule concentrations ratio.     

Parameters of the ionospheric <Emphasis Type="Italic">F</Emphasis><Subscript>2</Subscript> layer as a source of information on trends in thermospheric dynamics

The problem of determining trends in thermospheric dynamics parameters (horizontal winds) based on analysis of trends in various combinations of ionospheric F ₂-layer parameters is formulated. The previous attempts of the authors in this direction are briefly described. It is shown that all studied parameters lead to the same result: after the “boundary date” (approximately 1980) a systematic change in these parameters (a long-term trend) is observed, this fact manifesting changes in the dynamical regime of the thermosphere because of cooling and contraction of the entire middle and upper atmosphere. The results of a search for trends in the hmF2 height for the moment (T(ss) + 2 h) are described. These trends are found higher than the hmF2 trends obtained earlier by various authors analyzing the hmF2 behavior at fixed moments of local time.     

Long-term ocean simulations in FESOM: evaluation and application in studying the impact of Greenland Ice Sheet melting

The Finite Element Sea-ice Ocean Model (FESOM) is formulated on unstructured meshes and offers geometrical flexibility which is difficult to achieve on traditional structured grids. In this work, the performance of FESOM in the North Atlantic and Arctic Ocean on large time scales is evaluated in a hindcast experiment. A water-hosing experiment is also conducted to study the model sensitivity to increased freshwater input from Greenland Ice Sheet (GrIS) melting in a 0.1-Sv discharge rate scenario. The variability of the Atlantic Meridional Overturning Circulation (AMOC) in the hindcast experiment can be explained by the variability of the thermohaline forcing over deep convection sites. The model also reproduces realistic freshwater content variability and sea ice extent in the Arctic Ocean. The anomalous freshwater in the water-hosing experiment leads to significant changes in the ocean circulation and local dynamical sea level (DSL). The most pronounced DSL rise is in the northwest North Atlantic as shown in previous studies, and also in the Arctic Ocean. The released GrIS freshwater mainly remains in the North Atlantic, Arctic Ocean and the west South Atlantic after 120 model years. The pattern of ocean freshening is similar to that of the GrIS water distribution, but changes in ocean circulation also contribute to the ocean salinity change. The changes in Arctic and sub-Arctic sea level modify exchanges between the Arctic Ocean and subpolar seas, and hence the role of the Arctic Ocean in the global climate. Not only the strength of the AMOC, but also the strength of its decadal variability is notably reduced by the anomalous freshwater input. A comparison of FESOM with results from previous studies shows that FESOM can simulate past ocean state and the impact of increased GrIS melting well.     

Relationship between foF2 trends and geographic and geomagnetic coordinates

The possibility of presenting parameters k(foF2) and ΔfoF2 characterizing the change over time in the critical frequency of the ionospheric F2 layer in recent decades was considered. These parameters were previously obtained by the authors as functions of geographic coordinates, latitude and longitude. It was shown that there is such a possibility and the corresponding statistical significances of the obtained dependencies lie within the limits from ～90% to more than 99%. Simultaneously, the dependence of the linear trend k on magnetic coordinates D and I was studied. This dependence should manifest the relation of the trends in foF2 to dynamical processes (horizontal winds) via the vertical drift induced by them and the corresponding change in the F2-layer height.     

Effects of Short‐term and Long‐term Aluminium Stress on Photosynthesis,Respiration, and Reproductive Capacity in a Unicellular Green Flagellate (Euglena gracilis)

Effects of short‐term (1 h exposure) and long‐term (7 d exposure) aluminium stress on photosynthesis and reproductive capacity have been studied in Euglena gracilis strain Z. Following concentrations of Al_tot (added as AlCl₃) were tested: 0.5 mg L^‐1, 1.0 mg L^‐1, 1.5 mg L^‐1, 2.5 mg L^‐1, 5.0 mg L^‐1, 7.5 mg L^‐1, 10.0 mg L^‐1, and 15.0 mg L^‐1 Al, respectively. Growth rates at different aluminium concentrations did not show significant differences, except at 15.0 mg L^‐1Al. Initial respiration was higher in long‐term than in the short‐term experiments. It is supposed that an energy‐dependent mechanism of excretion of aluminium ions has been active in the stressed cells. Consequently, the cells of E. gracilis after long‐term exposure to aluminium are believed to be more acclimatised to the aluminium stress. Photosynthetic efficiency (PE) has been negatively affected by aluminium in all experiments performed. Differences between control algae and those treated with aluminium were significant in all cases. PE in long‐term experiments was in general significantly higher at all concentrations of aluminium studied, compared to the short‐term experiments. The aluminium concentrations tested led only to a general decrease in PE while the level of decrease was not especially concentration‐dependent. In general, aluminium tolerance of E. gracilis can be estimated as low, especially by short‐term exposure. However, good acclimatisation capacity of this green flagellate to aluminium doses by long‐term exposure can be supposed.     

Relation of foF2 to parameters of the stratosphere: Specification of the dependence on magnetic activity

The dependence of the correlation coefficient r(h, fo) between the stratospheric parameter h(100) and the critical frequency foF2 on geomagnetic activity is studied. In one of the previous publications of the authors, a general conclusion was drawn that if one chooses for the analysis only the days with geomagnetic index Ap < 12, this coefficient grows. In this paper we present a more complicated and statistically substantiated analysis of the effect of the r(h, fo) dependence on the limitations on Ap imposed on the analyzed days. Wide range of limitations on Ap from Ap = 4 to Ap = 40, five stations, and years of high (1980) and low (1987) solar activity are considered.