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  • 1.
    Becher, Marina
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Olid, Carolina
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Klaminder, Jonatan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Buried soil organic inclusions in non-sorted circles fields in northern Sweden: Age and Paleoclimatic context2013Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 118, nr 1, s. 104-111Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Although burial of surface organic soil horizons into deeper mineral soil layers helps drive the long-term buildup of carbon in arctic soils, when and why buried horizons formed as result of cryoturbation in northern Sweden remain unclear. In this study, we used C-14 and Pb-210 dating to assess when organic matter was buried within non-sorted circles fields near Abisko in northern Sweden. In addition, we used aerial photos from 1959 and 2008 to detect eventual trends in cryogenic activities during this period. We found that organic matter from former organic horizons (stratigraphically intact or partly fragmented) corresponds to three major periods: 0-100 A. D., 900-1250 A. D., and 1650-1950 A. D. The latter two periods were indicated by several dated samples, while the extent of the oldest period is more uncertainty (indicated by only one sample). The aerial photos suggest a net overgrowth by shrub vegetation of previously exposed mineral soil surfaces since 1959. This overgrowth trend was seen in most of the studied fields (92 out of 137 analyzed fields), indicating that the cryogenic activity has mainly decreased in studied non-sorted circles fields since the 1950s. This latter interpretation is also supported by the absence of buried organic layers formed during the last decades. We suggest that the organic matter was buried during the transition from longer cold periods to warmer conditions. We believe these climatic shifts could have triggered regional scale burial of soil organic matter and thus affected how these soils sequestered carbon.

  • 2.
    Campbell-Brown, Margaret
    et al.
    University of Western Ontario.
    Kero, Johan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Szasz, Csilla
    Institutet för rymdfysik.
    Pellinen-Wannberg, Asta
    Weryk, Rob
    University of Western Ontario.
    Photometric and ionization masses of meteors with simultaneous EISCAT UHF radar and intensified video observations2012Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, s. A09323-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    There are significant uncertainties in the calculation of photometric and ionization masses of meteors, particularly those derived from meteor head echoes observed by high power, large aperture radars. Simultaneous observations of meteors with the EISCAT UHF tristatic system and narrow field two-station intensified video were conducted in October 2007; 11 hours of data produced four useful meteors observed on all three radar receivers and both cameras. The positions and speeds calculated on the two systems generally agree to within the observational uncertainty. The photometric and ionization masses for each meteor were calculated using several values of luminous efficiency and ionization probability from literature, and all of these masses were found to agree to within the estimated error in the methods. More observations are required to select among the various values of ionization coefficient and luminous efficiency.

  • 3.
    Eliasson, Bengt
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Papadopoulos, K.
    Departments of Physics and Astronomy, University of Maryland, College Park, Maryland, USA.
    Penetration of ELF currents and electromagnetic fields into the Earth's equatorial ionosphere2009Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, s. A10301-A10314Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The penetration of extremely low frequency (ELF) transient electromagnetic fields and associated currents in the Earth's equatorial E-region plasma is studied theoretically and numerically. In the low-frequency regime, the plasma dynamics of the E-region is characterized by helicon waves since the ions are viscously coupled to neutrals while the electrons remain mobile. For typical equatorial E-region parameters, the plasma is magnetically insulated from penetration of very long timescale magnetic fields by a thin diffusive sheath. Wave penetration driven by a vertically incident pulse localized in space and time leads to both vertical penetration and the triggering of ELF helicon/whistler waves that carry currents obliquely to the magnetic field lines. The study presented here may have relevance for ELF wave generation by lightning discharges and seismic activity and can lead to new concepts in ELF/ULF injection in the earth-ionosphere waveguide.

  • 4.
    Eliasson, Bengt
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Theoretische Physik IV, Ruhr – Universität Bochum, Bochum, Germany.
    Stenflo, Lennart
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Department of Physics, Linköping University, Linköping, Sweden.
    Full-scale simulation study of the initial stage of ionospheric turbulence2008Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, nr A2, artikkel-id A02305Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a full-scale simulation of the nonlinear interaction between an intense electromagnetic wave and the Earth's ionosphere, by means of a generalized Zakharov model. The radio wave propagates from the neutral atmosphere into the ionospheric plasma layer and reaches the turning points of the ordinary and extraordinary wave modes. At the turning point of the ordinary mode, a parametric instability takes place in which the electromagnetic wave decays into an electron plasma wave and an ion acoustic wave with a typical wavelength of one meter. This is followed by collapse and caviton formation and trapping of the intense electron plasma wave. The cavitons lead to an efficient excitation of slow X (or Z) waves that propagate further into the denser ionospheric layer at higher altitudes. We use a realistic ion (oxygen) mass, length scales, and other plasma parameters. This numerical study should be useful for understanding the nonlinear interaction between intense radio waves and the ionosphere.

  • 5. Galli, André
    et al.
    Wurz, Peter
    Kallio, Esa
    Ekenbäck, Andreas
    Institutet för rymdfysik (IRF).
    Holmström, Mats
    Institutet för rymdfysik, Kiruna.
    Barabash, Stas
    Institutet för rymdfysik, Kiruna.
    Gregoriev, Alexander
    Futaana, Yoshifumi
    Institutet för rymdfysik, Kiruna.
    Fok, Mei-Ching
    Gunell, H
    The tailward flow of energetic neutral atoms observed at Mars2008Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, artikkel-id E12012Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The ASPERA-3 experiment on Mars Express provides the first measurements of energetic neutral atoms (ENAs) from Mars. These measurements are used to study the global structure of the interaction of the solar wind with the Martian atmosphere. In this study we describe the tailward ENA flow observed at the nightside of Mars. After characterizing energy spectra of hydrogen ENA signals, we present composite images of the ENA intensities and compare them to theoretical predictions (empirical and MHD models). We find that the tailward flow of hydrogen ENAs is mainly generated by shocked solar wind protons. Despite intensive search, no oxygen ENAs above the instrument threshold are detected. The results challenge existing plasma models and constrain the hydrogen exospheric densities and atmospheric hydrogen and oxygen loss rates at low solar activity.

  • 6.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Andre, M
    Ganguli, G
    Gavrishchaka, VV
    Koepke, ME
    Zintl, MW
    Ivchenko, N
    Karlsson, T
    Clemmons, JH
    Inhomogeneous transverse electric fields and wave generation in the auroral region: a statistical study2001Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 106, nr A6, s. 10803-10816Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We use data from the Freja satellite to investigate the importance of localized transverse DC electric fields for the generation of broadband waves responsible for ion heating in the auroral region. Theoretical models indicate that shear in the plasma Row perpendicular to the geomagnetic field can generate waves in a broad range around the ion gyrofrequency for parallel currents significantly below the threshold of the current-driven electrostatic ion cyclotron instability. We compare in situ data with laboratory measurements and theoretical predictions, and we find that inhomogeneous electric fields might well be important for the generation of waves in the auroral region.

  • 7.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marghitu, O
    Norqvist, Patrik
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Buchert, S
    André, M
    Klecker, B
    Kistler, LM
    Dandouras, I
    The role of the inner tail to midtail plasma sheet in channeling solar wind power to the ionosphere2012Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, nr A6, s. A06310-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this article we use Cluster power density (E . J) data from 2001, 2002, and 2004 to investigate energy conversion and transfer in the plasma sheet. We show that a southward IMF B-z is favorable for plasma sheet energy conversion, and that there is an increased particle and Poynting flux toward the Earth at times when Cluster observes an enhanced energy conversion in the plasma sheet. Conversion from electromagnetic to kinetic energy is increasingly dominant farther down-tail, while the generation of electromagnetic power from kinetic energy becomes important toward the Earth with a maximum at roughly 10 R-E. By linking observations of the key quantity E . J to observations of the solar wind input and earthward energy flux, our results demonstrate the role of the inner tail to midtail plasma sheet as a mediator between the solar wind energy input into the magnetosphere and the auroral dissipation in the ionosphere.

  • 8.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marghitu, Octav
    Norqvist, Patrik
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Buchert, Stephan
    Andre, Mats
    Klecker, Berndt
    Kistler, Lynn M
    Dandouras, Iannis
    Energy conversion regions as observed by Cluster in the plasma sheet2011Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, nr A1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this article we present a review of recent studies of observations of localized energy conversion regions (ECRs) observed by Cluster in the plasma sheet at altitudes of 15–20RE. By examining variations in the power density, E · J, where E is the electric field and J is the current density, we show that the plasma sheet exhibits a high level of fine structure. Approximately three times as many concentrated load regions (CLRs) (E · J > 0) as concentrated generator regions (CGRs) (E · J < 0) are identified, confirming the average load character of the plasma sheet. Some ECRs are found to relate to auroral activity. While ECRs are relevant for the energy conversion between the electromagnetic field and the particles, bursty bulk flows (BBFs) play a central role for the energy transfer in the plasma sheet. We show that ECRs and BBFs are likely to be related, although details of this relationship are yet to be explored. The plasma sheet energy conversion increases rather simultaneously with increasing geomagnetic activity in both CLRs and CGRs. Consistent with large-scale magnetotail simulations, most of the observed ECRs appear to be rather stationary in space but varying in time. We estimate that the ECR lifetime and scale size are a few minutes and a few RE, respectively. It is conceivable that ECRs rise and vanish locally in significant regions of the plasma sheet, possibly oscillating between load and generator character, while some energy is transmitted as Poynting flux to the ionosphere.

  • 9.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Norqvist, Patrik
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Andre, M
    Eriksson, AI
    A statistical study of wave properties and electron density at 1700 km in the auroral region2002Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 107, nr A8, s. SIA 21-1-SIA 21-13Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a comprehensive overview of the electron density and six different wave types in the range similar to1 Hz to similar to1 MHz, and we investigate their occurrence, average wave frequency and amplitude as a function of location, Kp index, and solar illumination. Twenty-one months of Freja observations from the Northern Hemisphere obtained at similar to1700 km altitude and invariant latitudes 40degrees-75degrees are used. We find that waves around the lower hybrid frequency occur in one low-latitude dayside band and one high-latitude nightside band. The latter band correlates with precipitating auroral electrons and coexists with electromagnetic ion cyclotron (EMIC) waves. This indicates the importance of energetic electrons for the wave generation. Both broadband ELF waves and broadband high-frequency whistler mode waves are found at high latitudes, but whistler mode emissions are most common in regions of high electron densities on the dayside, while broadband ELF waves are found where the density is reduced on the nightside. Moreover, the average density in the presence of broadband ELF waves is more reduced when the ionosphere is dark than when it is sunlit. However, broadband whistler mode waves, Langmuir waves, and waves with an upper cutoff just below the proton gyrofrequency coincide with density enhancements when the ionosphere is dark. Ion heating correlated with auroral electrons coexists with EMIC waves and the high-latitude band of waves around the lower hybrid frequency. Furthermore, ion heating not correlated with downgoing electrons coexists with broadband ELF waves.

  • 10.
    Karlsson, Jan
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Christensen, Torben R.
    Crill, Patrick
    Förster, Johannes
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Hammarlund, Dan
    Jackowicz-Korczynski, Marcin
    Kokfelt, Ulla
    Roehm, Charlotte
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Rosén, Peter
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Quantifying the relative importance of lake emissions in the carbon budget of a subarctic catchment2010Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, s. G03006-6 PPArtikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate change and thawing of permafrost will likely result in increased decomposition of terrestrial organic carbon and subsequent carbon emissions to the atmosphere from terrestrial and aquatic systems. The quantitative importance of mineralization of terrestrial organic carbon in lakes in relation to terrestrial carbon fluxes is poorly understood and a serious drawback for the understanding of carbon budgets. We studied a subarctic lake in an area of discontinuous permafrost to assess the quantitative importance of lake carbon emission for the catchment carbon balance. Estimates of net ecosystem production and stable carbon-isotope composition of dissolved organic carbon in the lake water suggest substantial input and respiration of terrestrial organic carbon in the lake. The lake was a net source of CO2 and CH4 to the atmosphere at ice breakup in spring and during the whole ice-free period. The carbon emission from the lake was similar in magnitude to the terrestrial net release of carbon to the atmosphere. The results indicate that lakes are important sources of catchment carbon emission, potentially increasing the positive feedback from permafrost thawing on global warming.

  • 11.
    Klaminder, J
    et al.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Ekologi, miljö och geovetenskap.
    Yoo, K
    Giesler, R
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Ekologi, miljö och geovetenskap.
    Soil carbon accumulation in the dry tundra: Important role played by precipitation2009Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, s. G04005-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A positive relationship between the mean annual precipitation (MAP) and soil organic carbon (SOC) is found in most surveys covering the subarctic and boreal region. In this paper we assess mechanisms behind variable SOC pools in dry tundra soils developed along a 50 km long subarctic precipitation (snow) gradient in northern Sweden. Lead 210 is used to infer SOC accumulation rates in the O horizon. Despite an unchanged or even slightly decreasing accumulation rate of SOC in the O horizon (range 0.02–0.06 kg C m−2 yr−1) along with increasing MAP and a relative constant litter input (∼0.04 kg C m−2 yr−1), the SOC pool in the upper 1 m increase significantly with increasing MAP. This trend is mainly due to a progressively buildup of SOC in the mineral soil and argued to be the result of an accelerated vertical translocation of SOC at sites overlain by a thick snowpack. Furthermore, the loss of SOC from the O horizon through wind erosion appears to be more pronounced at snow-poor sites. We estimate that vegetated heath soil may loose >0.02 kg C m−2 yr−1 (∼half of the annual litter fall) due to wind erosion in snow-poor areas. We stress that lateral and vertical translocation processes inherent by precipitation regimes may be of fundamental importance for the long-term SOC accumulation in tundra soil.

  • 12.
    Roehm, Charlotte L
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Giesler, Reiner
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Karlsson, Jan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Bioavailability of terrestrial organic carbon to lake bacteria: the case of a degrading subarctic permafrost mire complex2009Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, s. G03006-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Permafrost degradation can result in the loss of significant amounts of carbon, through release to the atmosphere in the form of carbon dioxide and/or methane and through export downstream to lakes and rivers. The fate of this carbon in lake ecosystems is poorly understood. We investigated the capacity of lake bacteria to utilize carbon from different soils from an adjacent mire. Dark bioassays were undertaken to measure the dynamics of the bioavailability and chemical character of dissolved organic carbon (DOC). The soils ranged from already degraded minerotrophic fens to ombrotrophic active layer and soils from the permafrost layer. Our study shows that soil DOC was rapidly consumed by bacteria collected from lake water, particularly within the first 48 h (about 85% of the total consumed DOC). The mean DOC consumption by lake bacteria was 0.087 mg L−1 d−1 when supplied with lake water DOC and varied between 0.382 mg L−1 d−1 (permafrost soil) and 0.491 mg L−1 d−1 (degraded fen soil) when supplied with terrestrial DOC. Thus, the data suggest that export of DOC from degrading permafrost mires at any stage of degradation can potentially increase rates of respiration by fourfold to sevenfold and can have pronounced effects both on receiving lake ecosystems and on the land-atmosphere carbon balance. In this study we also propose simple predictive models, incorporating weight-averaged molecular weight and specific UV absorption in combination with other simple qualitative parameters for the estimation of potential bioavailability of soil DOC in aquatic ecosystems.

  • 13.
    Rydberg, Johan
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Rosen, Peter
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Lambertsson, Lars
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Kemiska institutionen.
    De Vleeschouwer, F
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Tomasdotter, S
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Bindler, Richard
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Assessment of the spatial distributions of total- and methyl-mercury and their relationship to sediment geochemistry from a whole-lake perspective2012Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, s. G04005-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The aim of this study was to determine the spatial variability for total-and methylmercury in surface sediments (0-2 cm) across a single whole-lake basin, and to relate this variability to the sediment's geochemical composition. 83 surface sediment samples from Stor-Stromsjon - a lake with multiple sub-basins located in northern Sweden - were analyzed for geochemical composition as well as total-mercury (total-Hg) and methylmercury (methyl-Hg; 35 samples) concentrations. Our results indicate that variations in fine-grained mineral matter (36%) and organic matter (34%) explain an equal amount of the total-Hg variation, but that their relative importance varies between different parts of the lake. Total-Hg concentrations were similar in locations controlled by organic matter or fine-grained mineral matter (average 109 ng g(-1)); however, total-Hg inventories (mass per unit area) were significantly higher in the latter (35 and 53 mu g m(-2), respectively). Methyl-Hg concentrations are largely (55% of variance) controlled by water depth and sulfur concentration, which supports the importance of within lake methylation reported from other studies. Both for concentrations and inventories the spatial distribution for methyl-Hg in surface sediments is patchy, and interestingly the highest methyl-Hg inventory (1.4 mu g m(-2)) was found in a shallow location with coarse-grained minerogenic sediment (very low organic matter). A large spatial variability, even within a single lake, is something that needs to be recognized, e.g., when studying processes affecting mercury cycling, mercury loadings and when using lake sediments to reconstruct historic mercury deposition.

  • 14.
    Rönnmark, Kjell
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Auroral Electron acceleration by Alfven waves and electrostatic fields2000Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 105, nr A11, s. 25333-25344Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a two-dimensional numerical model for the formation of discrete auroral arcs. This model describes the evolution of shear Alfven waves generated by a growing force near the equatorial plane, and the transition to electrostatic fields when the force becomes stationary. The parallel electric fields on auroral field lines may be regarded as shear Alfven waves driven by a magnetospheric generator at zero frequency. In our collisionless model, precipitating auroral electrons are accelerated to an energy of 350 eV when the upward current is 3.1 mu Am-2. We also find that the electrostatic potential drop is proportional to the square of the current density.

  • 15. Shukla, Padma Kant
    et al.
    Eliasson, Bengt
    Stenflo, Lennart
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Stimulated scattering of intense radio waves in partially ionized space dusty plasmas2004Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 109, nr A3, artikkel-id A03301Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The nonlinear interaction between intense ordinary mode (O-mode) radio waves and modified magnetoacoustic waves in partially ionized space dusty plasmas is considered, including the combined action of the radio wave pressure and the electron Joule heating caused by the O-mode electric field. A set of equations containing the wave equation for the radio waves and the electron density and temperature fluctuations associated with the modified magnetoacoustic waves is derived. The set is then used to investigate decay and modulational instabilities of a constant amplitude O-mode radio wave in the Earth's ionosphere and mesosphere.

  • 16.
    Vedin, Jörgen
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Rönnmark, Kjell
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Electron pressure effects on driven auroral Alfvén waves2005Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 110, nr A01214, s. 11-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Fluid models for the auroral electron acceleration processes have almost exclusively been derived by assuming cold or isothermal electrons. The consequences of these assumptions have never been thoroughly analyzed. In this study we compare results from an isothermal simulation with those obtained when the pressure is calculated from a double adiabatic approximation and from stationary kinetic theory. We find that the reflection of shear Alfvén waves, as well as the current-voltage relation, is very sensitive to the description of the electron pressure variations. Using pressures calculated from steady-state kinetic theory, we find that driven shear Alfvén waves can build up auroral currents and fields that are consistent with a linear current-voltage relation.

     

  • 17.
    Vedin, Jörgen
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Rönnmark, Kjell
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Electrostatic potentials in the downward auroral current region2005Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 110, nr A08207, s. 1-9Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Assuming a fixed ion density, adiabatic electron motion, and quasi-neutrality, we use the stationary Vlasov equation to derive the self-consistent potential in an auroral flux tube that carries downward current. Our model predicts downward electric fields ∼5 mV/m at an altitude near 2000 km, and around 4000 km the potential reaches ∼2.5 kV. A weak upward electric field at high altitudes reduces the potential, and the potential difference between the ionosphere and magnetosphere is much smaller.

  • 18.
    Vedin, Jörgen
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Rönnmark, Kjell
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Particle-fluid simulation of the auroral current circuit2006Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 111, nr A12201, s. 12-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The incompatibility between stationary kinetic and dynamic fluid descriptions of auroral electron acceleration has been an outstanding problem in space physics for decades. In this study we introduce a new numerical simulation model that provides a unified picture by including electron temperature variations consistent with collisionless kinetic theory in the fluid description. We demonstrate that this new particle-fluid model can describe the partial reflection of Alfvén waves from the acceleration region, as well as the formation of a field-aligned potential drop proportional to the upward current. This study also suggests that for example ion dynamics and high-frequency waves must be added to the model before it properly can describe the return current region. Simulations based on the particle-fluid concept can be applied to various processes in space physics and astrophysics where strong currents flowing along an inhomogeneous magnetic field will cause temperature increases and field-aligned electric fields.

  • 19.
    Yoo, Kyungsoo
    et al.
    Department of Soil, Water, and Climate, University of Minnesota–Twin Cities, St. Paul, Minnesota, USA.
    Ji, Junling
    Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware, USA.
    Aufdenkampe, Anthony
    Stroud Water Research Center, Avondale, Pennsylvania, USA.
    Klaminder, Jonatan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Rates of soil mixing and associated carbon fluxes in a forest vs. tilled agricultural field:  Implications for modeling the soil carbon cycle2011Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, nr G01014Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In natural ecosystems, bioturbation is an essential component of soil formation, whereas tillage drives soil mixing in agricultural soils. Yet soil mixing is commonly neglected in modeling soil organic carbon (SOC) as it responds to land use changes. Here, in order to determine mixing-driven carbon fluxes, we combine a mass balance model with measurements of 210Pb activities and SOC contents. Soil mixing rates by tillage decrease from 3.4 ± 2.3 cm yr−1 at the surface to 0.8 ± 0.2 cm yr−1 at a depth of ∼20 cm, causing the SOC stored in the upper 25 cm of the soil to be physically turned over via mixing annually. In contrast, the bioturbation-driven soil mixing velocity at the forest increases from 0.6 ± 0.1 cm yr−1 at the surface to 2.7 ± 0.5 cm yr−1 at a depth of ∼10 cm, which results in physically turning over SOC in the A horizon via mixing on years to decadal time scales. Therefore, SOC fractions with different susceptibilities to decomposition may have significantly different physical trajectories within the soils over their lifespans, and thus the assumption of C-cycling models that all SOC fractions experience identical environmental conditions is unlikely to be realistic. Carbon sinks, excesses of plant carbon inputs over decomposition carbon losses, are found within the top portion of the A horizons. These carbon excesses are transferred, via mixing, to the lower portion of the A horizon, where they are decomposed. By quantifying mixing-derived SOC fluxes, this study shows a previously unrecognized complexity in understanding SOC dynamics associated with land use changes.

  • 20.
    Yurova, Alla
    et al.
    Lund Univ, Dept Phys Geog & Ecosyst Anal, Solvegatan 12, SE-22362 Lund, Sweden..
    Wolf, Annett
    Umeå universitet. Lund Univ, Dept Phys Geog & Ecosyst Anal, Solvegatan 12, SE-22362 Lund, Sweden..
    Sagerfors, Jorgen
    Nilsson, Mats
    Variations in net ecosystem exchange of carbon dioxide in a boreal mire: Modeling mechanisms linked to water table position2007Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, nr G2Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    [1] In mires, which occupy large areas of the boreal region, net ecosystem CO2 exchange ( NEE) rates vary significantly over various timescales. In order to examine the effect of one of the most influencing variables, the water table depth, on NEE the general ecosystem model GUESS-ROMUL was modified to predict mire daily CO2 exchange rates. A simulation was conducted for a lawn, the most common microtopographical feature of boreal oligotrophic minerotrophic mires. The results were validated against eddy covariance CO2 flux measurements from Degero Stormyr, northern Sweden, obtained during the period 2001 - 2003. Both measurements and model simulations revealed that CO2 uptake was clearly controlled by interactions between water table depth and temperature. Maximum uptake occurred when the water table level was between 10 and 20 cm and the air temperature was above 15 degrees C. When the water table was higher, the CO2 uptake rate was lower, owing to reduced rates of photosynthetic carbon fixation. When the water table was lower, NEE decreased owing to the increased rate of decomposition of organic matter. When the water table level was between 10 and 20 cm, the NEE was quite stable and relatively insensitive to both changes within this range and any air temperature changes above + 15 degrees C. The optimal water table level range for NEE corresponds to that characteristic of mire lawn plant communities, indicating that the annual NEE will not change dramatically if climatic conditions remain within the optimal range for the current plant community.

  • 21.
    Åberg, Jan
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Jansson, Mats
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Jonsson, Anders
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Importance of water temperature and thermal stratification dynamics for temporal variation of surface water CO2 in a boreal lake2010Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, nr G02024, s. 10PP-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Variation of the surface water CO2 concentration is likely to be the result of biological activity and physical processes as water mixing and gas exchange with the atmosphere. Here we have studied the variations in surface water CO2 during the ice-free period in the humic Lake Merasjärvi in northern Sweden. Meteorological, hydrological and limnological data were collected using data logging equipment permitting high time-resolution. The surface water of the lake was supersaturated with respect to CO2 throughout the study period. There were, however, considerable diurnal and longer-term temporal variations of the surface water CO2 concentrations. Partial least squares (PLS) models were used to link the logged CO2 data to the multivariate dataset. On the longer-term time scale (analyzed with 24h means of the logged data) high concentrations of surface water CO2 were best related to the depth and temperature of the upper warmer layer (epilimnion), and to erosion of the underlying colder layer (hypolimnion). The diurnal variation (analyzed with 30 minute means of the logged data) was best related to the thermal dynamics within the epilimnion, which regulated the surface water access to CO2 stores within this layer. Variables related to CO2 emission and photosynthesis (wind and PAR), showed only weak correlations to variations of the surface water CO2 concentration. Accordingly, the CO2 flux, measured with the eddy-covariance technique, was not correlated to the surface water CO2 concentration.

  • 22.
    Ågren, Annelie
    et al.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Ekologi, miljö och geovetenskap.
    Buffam, I
    Skogsekologi, Sveriges lantbruksuniversitet, Umeå.
    Jansson, Mats
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Ekologi, miljö och geovetenskap.
    Laudon, Hjalmar
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Ekologi, miljö och geovetenskap.
    Importance of seasonality and small streams for the landscape regulation of dissolved organic carbon export2007Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, s. Art. No. G03003-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The regulation of the spatial and seasonal variation in terrestrial dissolved organic carbon (DOC) exports was studied in a 68 km2 boreal stream system in northern Sweden. A total of 1213 DOC samples were collected in 15 subcatchments over a 3 year period (2003–2005). The mean annual DOC exports from the 15 subcatchments (0.03–21.72 km2) ranged from 14.8 to 99.1 kg ha−1 yr−1. Many catchment characteristics determined the spatial variation in DOC exports. The relative importance of the different catchment characteristics varied greatly between seasons because of differing hydrological conditions. During winter base flow the spatial variation was linked to patterns in wetland coverage. During snowmelt in spring the spatial variation was connected to characteristics describing size and location, i.e., median stream size, silty sediment distribution, stream order, altitude, and proportion of catchment above highest postglacial coastline (HC). During the snow-free season the spatial variation in DOC exports was regulated by the amount of wetlands and forests, particularly forests made up of Norway spruce (Picea abies). Median stream size also influenced the exports during this season. A striking result in this study was the effect of size implying that small headwaters may be the largest contributor to the terrestrial DOC export, per unit area.

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