317 research outputs found
Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind
Solar wind plasma is supposed to be structured in magnetic flux tubes carried
from the solar surface. Tangential velocity discontinuity near the boundaries
of individual tubes may result in Kelvin-Helmholtz instability, which may
contribute into the solar wind turbulence. While the axial magnetic field may
stabilize the instability, a small twist in the magnetic field may allow to
sub-Alfvenic motions to be unstable. We aim to study the Kelvin-Helmholtz
instability of twisted magnetic flux tube in the solar wind with different
configurations of external magnetic field. We use magnetohydrodynamic equations
in the cylindrical geometry and derive the dispersion equations governing the
dynamics of twisted magnetic flux tube moving along its axis in the cases of
untwisted and twisted external fields. Then we solve the dispersion equations
analytically and numerically and found thresholds for Kelvin-Helmholtz
instability in both cases of external field. Both analytical and numerical
solutions show that the Kelvin-Helmholtz instability is suppressed in the
twisted tube by external axial magnetic field for sub-Alfvenic motions.
However, even small twist in the external magnetic field allows the
Kelvin-Helmholtz instability to be developed for any sub-Alfvenic motions. The
unstable harmonics correspond to vortices with high azimuthal mode numbers,
which are carried by the flow. Twisted magnetic flux tubes can be unstable to
Kelvin-Helmholtz instability when they move with small speed relative to main
solar wind stream, then the Kelvin-Helmholtz vortices may significantly
contribute into the solar wind turbulence.Comment: 8 pages, 3 figures, accepted in A&
Az ultraibolya sugárzás szerepe tavak oldott szervesanyagainak transzformációjában és degradációjában = Role of ultraviolet radiation in the transformation and degradation of dissolved organic substances in lakes
SekĂ©ly tavakra (30 magyarországi vĂztest, 140 mĂ©rĂ©s) ultraibolya (UV) Ă©s láthatĂł (PAR) hullámhossz tartományban többváltozĂłs regressziĂłs modelleket fejlesztettĂĽnk ki a fĂ©nyabszorpciĂłs komponensek: algák, algamentes lebegĹ‘anyagok, szĂnes oldott szervesanyagok (CDOM) Ă©s a vertikális extinkciĂłs koefficiensek (Kd) közötti összefĂĽggĂ©s leĂrására. A modellek megadták az egyes komponensek specifikus Kd Ă©rtĂ©keit. Ezek, Ă©s a komponensek koncentráciĂł Ă©rtĂ©kei alapján vĂzalatti mĂ©rĂ©sre alkalmas radiomĂ©ter hiányában in situ fĂ©nymĂ©rĂ©s nĂ©lkĂĽl kiszámĂthatĂłak egy adott vĂztestre jellemzĹ‘ Kd Ă©rtĂ©kek. A vizsgált tavakban a CDOM határozta meg a vĂzalatti UV klĂmát, az algák szerepe alárendelt volt. A PAR klĂmát a legtöbb vĂztestben a lebegĹ‘anyagok határozták meg, de hipertrĂłf vizekben az algák, mĂg barna vizekben a CDOM volt a meghatározĂł. KĂsĂ©rletesen megállapĂtottuk, hogy az UV sugárzás hatására az oldott szervesanyagok bomlottak, az oldott szervesszĂ©n (DOC) koncentráciĂłja csökkent, a hidrogĂ©nperoxidĂ© nĹ‘tt, miközben a szervesanyagok kĂ©miai Ă©s fizikai tulajdonságai változtak, Ăgy barna szĂnĂĽk fakult, fĂ©nyabszorpciĂłjuk Ă©s fluoreszcencia intenzitásuk csökkent, miközben a biolĂłgiailag hozzáfĂ©rhetĹ‘ DOC koncentráciĂł többszörösĂ©re nĹ‘tt. Kimutattuk, hogy az UV sugárzás hatása a bakterioplanktonra komplex, direkt gátlĂł Ă©s indirekt gátlĂł Ă©s serkentĹ‘ hatás egyaránt Ă©rvĂ©nyesĂĽlt. A mikrobiális Ă©lĹ‘lĂ©nyegyĂĽttesek működĂ©se Ă©s a vĂzi szĂ©nforgalom megĂ©rtĂ©se a CDOM fotolĂzisĂ©nek ismerete nĂ©lkĂĽl nem lehetsĂ©ges. | Multiple regression models were developed in the ultraviolet (UV) and visible (PAR) wavelength range to describe the relationship between light-absorbing components: algae, algal-free suspended solids, coloured dissolved organic matter (CDOM) and vertical attenuation coefficients (Kd) in shallow lakes (30 Hungarian water bodies, 140 measurements). The models have provided the specific Kd values of the components. Based on these and the concentration of the components, the Kd can be calculated without in situ measurement in the absence of an underwater radiometer. The underwater UV climate was mainly determined by CDOM in the studied lakes, the role of algae was negligible. The PAR climate was determined by suspended solids in most cases, but in hypertrophic waters the algae and in brown colour waters the CDOM was dominant. We have experimentally ascertained that the dissolved organic substances were decomposed by UV radiation: the DOC decreased, the H2O2 increased, while their brown colour faded, their absorbance and fluorescence intensity decreased, while the concentration of the biologically available DOC multiplied. We have found that the effect of UV radiation on bacterioplankton is complex: both the direct inhibitory and indirect inhibitory and stimulating effects were detected. Our research shows that the understanding of the functioning of microbial assemblages and the aquatic carbon cycle is not possible without the thorough knowledge of the photolysis of CDOM
A fitoplankton produktivitása Ă©s diverzitása pikoplankton dominanciájĂş vĂzterekben = Productivity and diversity of phytoplankton in picoplankton dominated water bodies
Feltártuk a bakteriális mĂ©retű algákban (pikoplankton) gazdag turbid vizekben a pikoplankton tömegĂ©nek, összetĂ©telĂ©nek, dinamikájának Ă©s fotoszintĂ©zisĂ©nek alapvetĹ‘ jellemzĹ‘it. A fitoplankton tömege alapján ezek a tavak hipertrĂłfok (a-klorofill>100 micro g/l), ugyanakkor a fĂ©nylimitáciĂł miatt elsĹ‘dleges termelĂ©sĂĽk a vártnál alacsonyabb volt. VizĂĽkben a pikoplankton abundanciája egy-kĂ©t nagyságrenddel nagyobb volt, mint más tavakban, tĂ©len a pikoeukariĂłták nyáron a pikocianobaktĂ©riumok domináltak a Balatonhoz hasonlĂłan. A Balatonban a pikoalgák rĂ©szesedĂ©se az összes produkciĂłbĂłl 23-54% között volt. MegállapĂtottuk, hogy a pikoplankton abundancia folyĂł vizekben egy nagyságrenddel kisebb, mint tavakban, azonos fitoplankton biomassza esetĂ©n. A BalatonbĂłl egy Ăşj pikoeukariĂłta, a szikes tavakbĂłl egy Ăşj pikocianobaktĂ©rium morfotaxont Ărtunk le. A termĂ©szetes pikoplankton egyĂĽttesek molekuláris vizsgálata bizonyĂtotta, hogy a Pannon Biogeográfiai RĂ©giĂł vĂztereinek Synechococcus flĂłrája igen gazdag Ă©s diverz. Az izolált pikocianobaktĂ©rium törzsek genetikai karakterizálása is ezt az eredmĂ©nyt támasztotta alá. A Böddi-szĂ©kbĹ‘l izolált törzsek a pikofitoplankton klád egy Ăşj, eddig tenyĂ©sztĂ©sbe nem vont csoportját kĂ©pviselik. PikoeukariĂłta Ă©s pikocianobaktĂ©rium törzsek ökofiziolĂłgiai vizsgálatával bizonyĂtottuk, hogy a Duna-Tisza közi szikes tavakban (Ă©s minden bizonnyal más tavakban is) a fĂ©ny Ă©s a hĹ‘mĂ©rsĂ©klet változása egyĂĽtt szabályozza a pikoalgák szezonális szukcessziĂłját. | The abundance, composition, dynamics and photosynthesis of the bacterium-sized algae (picoplankton) has been studied in Hungarian turbid waters, where the abundance of picoplankton is one or two order of magnitude higher than in other lakes. Based on the phytoplankton biomass these ponds were hypertrophic (chlorophyll-a > 100 micro g/l), however the primary production was lower than expected due to light limitation. Picoeukaryotes dominate in winter, while picocyanobacteria dominate in summer, similarly to Lake Balaton, where the proportion of the picoplankton in the total primary production was 23 and 54%. It has been stated, that the picoplankton abundance in running waters was one order of magnitude lower, than in shallow lakes having the same trophic state. One new eukaryotic and one cyanobacterial picoalgal morphotaxa has been described. The molecular characterization of phytoplankton assemblages showed that the Synechococcus flora of the Pannon Biogeographic Region is very diverse. The molecular identification of isolated picocyanobacterial strains also confirmed these results. The picocyanobacterial strains isolated from Böddi-szĂ©k pond formed a new, distinct group inside the picophytoplankton clade. The ecophysiological studies of picoeukaryotic and picocyanobacterial strains showed that the light and temperature control together the seasonal succession of the picoplankton in turbid soda lakes in the Danube-Tisza Interfluve (and presumably in other shallow lakes)
Evolution of kinklike fluctuations associated with ion pickup within reconnection outflows in the Earth's magnetotail
Magnetic reconnection (MR) in Earth's magnetotail is usually followed by a
systemwide redistribution of explosively released kinetic and thermal energy.
Recently, multispacecraft observations from the THEMIS mission were used to
study localized explosions associated with MR in the magnetotail so as to
understand subsequent Earthward propagation of MR outbursts during substorms.
Here we investigate plasma and magnetic field fluctuations/structures
associated with MR exhaust and ion-ion kink mode instability during a well
documented MR event. Generation, evolution and fading of kinklike oscillations
are followed over a distance of 70 000 km from the reconnection site in the
midmagnetotail to the more dipolar region near the Earth. We have found that
the kink oscillations driven by different ion populations within the outflow
region can be at least 25 000 km from the reconnection site.Comment: 11 pages, 4 figure
The constitutional landscape after the fourth and fifth amendments of Hungarian Fundamental Law
Study of reconnection-associated multi-scale fluctuations with Cluster and Double Star
The objective of the paper is to asses the specific spectral scaling
properties of magnetic reconnection associated fluctuations/turbulence at the
Earthward and tailward outflow regions observed simultaneously by the Cluster
and Double Star (TC-2) spacecraft on September 26, 2005. Systematic comparisons
of spectral characteristics, including variance anisotropy and scale-dependent
spectral anisotropy features in wave vector space were possible due to the
well-documented reconnection events, occurring between the positions of Cluster
(X = -14--16 ) and TC-2 (X = -6.6 ). Another factor of key importance
is that the magnetometers on the spacecraft are similar. The comparisons
provide further evidence for asymmetry of physical processes in
Earthward/tailward reconnection outflow regions. Variance anisotropy and
spectral anisotropy angles estimated from the multi-scale magnetic fluctuations
in the tailward outflow region show features which are characteristic for
magnetohydrodynamic cascading turbulence in the presence of a local mean
magnetic field. The multi-scale magnetic fluctuations in the Earthward outflow
region are exhibiting more power, lack of variance and scale dependent
anisotropies, but also having larger anisotropy angles. In this region the
magnetic field is more dipolar, the main processes driving turbulence are flow
breaking/mixing, perhaps combined with turbulence ageing and non-cascade
related multi-scale energy sources.Comment: 30 pages, 6 figure
Társadalmi konfliktusokat generálĂł ökolĂłgiai törtĂ©nĂ©sek a Balaton Ă©letĂ©ben az utĂłbbi nĂ©hány Ă©vtizedben - A vĂzszintváltozás hatása a parti öv növĂ©nyvilágára | Social Conflict Generating Ecological Developments in the Life of Lake Balaton in Recent Decades – Effect of water level fluctuation on the plants of the littoral zone
While the Hungarian public was primarily concerned with the water quality of Lake Balaton in the 1980s and 1990s, low water levels and their environmental effects became the focus of public attention in 2002, 2003 as well as 2012.
Accurate and reliable monthly and yearly data about the water balance of Lake Balaton has been available since 1921. The amounts of water getting into the lake from the catchment area have been considerably less than average in nine percent of the period between 2001 and 2012; the water in the catchment area became scarce and the water levels of the lake notably decreased during the drier than average years of 2001-2004 and 2012.
Cladophora glomerata (filamentous green alga) appeared in large quantities in the shallow waters of the south shore in the summer of 2002, 2003 as well as 2012. It is not a new phenomenon: the first documented Cladophora explosion in Lake Bala-ton was in 1934. Scientists at the Balaton Limnological Institute (Center for Ecologi-cal Research of the Hungarian Academy of Sciences) studied the Cladophora vegetation of the south shore in July 2003. They found that excessive Cladophora growth—harming tourism with its gooey green mats and rotting muck—appeared where the water depth was lower than 30 cm. This explosion, however, is not a consequence of water pollution but an unavoidable consequence of the low water levels.
The results show that the photosynthetical apparatus of the main submerged macrophyte species have adapted to the semi-dark waters of Lake Balaton. The photosynthetical parameters of submerged macrophytes indicate that they can survive and disperse in parts of the lake where the water is no deeper than 2.2-2.3 m. It means that the area suitable for the establishment of submerged macrophytes increases by 60 percent with a 50 cm decrease in the water levels, while a 100 cm decrease means a threefold increase in the size of the area favourable for submerged macrophytes.
The decrease in water levels contribute considerably to the spread of reed. This is due the increase in drier areas suitable for sexual reproduction on the one hand and to the oxidation of sediment on the other. The emerging of a reed-covered area out of the water—even if for a short period—contributes considerably to the prevention of reed decline. The artificial stabilization of the water levels in the past fifty years has greatly contributed to the slow degradation of the flora in the littoral zone. At the same time, the low water levels of the past decade have shown that low water levels do not damage the macrophyte populations of Lake Balaton
Unusual behaviour of phototrophic picoplankton in turbid waters
Autotrophic picoplankton (APP) abundance and contribution to phytoplankton biomass was studied in Hungarian shallow lakes to test the effect of inorganic turbidity determining the size distribution of the phytoplankton. The studied lakes displayed wide turbidity (TSS: 4–2250 mg l-1) and phytoplankton biomass (chlorophyll a: 1–460 μg l-1) range, as well as APP abundance (0 and 100 million cells ml-1) and contribution (0–100%) to total phytoplankton biomass. Inorganic turbidity had a significant effect on the abundance and contribution of APP, resulting in higher values compared to other freshwater lakes with the same phytoplankton biomass. Our analysis has provided empirical evidence for a switching point (50 mg l-1 inorganic turbidity), above which turbidity is the key factor causing APP predominance regardless of phytoplankton biomass in shallow turbid lakes. Our results have shown that turbid shallow lakes are unique waters, where the formerly and widely accepted model (decreasing APP contribution with increasing phytoplankton biomass) is not applicable. We hypothesize that this unusual behaviour of APP in turbid waters is a result of either diminished underwater light intensity or a reduced grazing pressure due to high inorganic turbidity
Multiple extreme environmental conditions of intermittent soda pans in the Carpathian Basin (Central Europe)
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