| 1. |
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| 2. |
- Becher, Marina, et al.
(författare)
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Buried soil organic inclusions in non-sorted circles fields in northern Sweden : Age and Paleoclimatic context
- 2013
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202 .- 2169-8953. ; 118:1, s. 104-111
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Buffam, Ishi, et al.
(författare)
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Landscape-scale variability of acidity and dissolved organic carbon during spring flood in a boreal stream network
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112
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Tidskriftsartikel (refereegranskat)abstract
- Acidity is well known to influence stream biota, but the less well-studied spatial and temporal distributions of acidity are likely to play a larger ecological role than average values. We present data on spatial variability of chemical parameters contributing to acidity during winter baseflow and spring flood periods in Krycklan, a fourth-order boreal stream network in northern Sweden. Fifteen stream sites were monitored in subcatchments spanning 3 orders of magnitude in size and representing a wide range of percent wetland. At baseflow, pH ranged from 3.9 to 6.5 at the different sites. Baseflow dissolved organic carbon (DOC) concentration varied by an order of magnitude and was positively correlated with subcatchment percent wetland, resulting in high spatial variability in dissociated organic acids (OA(-)). During spring flood, DOC and OA(-) increased in forested sites and decreased in wetland sites, resulting in reduced spatial variability in their concentrations. In contrast, base cations and strong acid anions diluted throughout the stream network, resulting in decreased acid neutralizing capacity (ANC) at all sites. The spatial variability of base cations increased slightly with high flow. As a result of the changes in OA(-) and ANC, pH dropped at all but the most acidic site, giving a slightly narrowed pH range during spring flood (4.2-6.1). The transition from winter to spring flood stream chemistry could largely be explained by: (1) a shift from mineral to upper riparian organic soil flow paths in forested catchments and (2) dilution of peat water with snowmelt in wetland catchments.
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| 4. |
- Campbell-Brown, Margaret, et al.
(författare)
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Photometric and ionization masses of meteors with simultaneous EISCAT UHF radar and intensified video observations
- 2012
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 117, s. A09323-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Eliasson, Bengt, et al.
(författare)
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Full-scale simulation study of the initial stage of ionospheric turbulence
- 2008
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 113:0148-0227, s. A02305-1-A02305-9
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Eliasson, Bengt, et al.
(författare)
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Penetration of ELF currents and electromagnetic fields into the Earth's equatorial ionosphere
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114, s. A10301-A10314
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Eliasson, Bengt, et al.
(författare)
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Zakharov simulation study of spectral features of on-demand Langmuir turbulence in an inhomogeneous plasma
- 2008
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 113:A2
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Tidskriftsartikel (refereegranskat)abstract
- [1] We have performed a simulation study of Langmuir turbulence in the Earth's ionosphere by means of a Zakharov model with parameters relevant for the F layer. The model includes dissipative terms to model collisions and Landau damping of the electrons and ions, and a linear density profile, which models the ionospheric plasma inhomogeneity whose length scale is of the order 10-100 km. The injection of energy into the system is modeled by a constant source term in the Zakharov equation. Langmuir turbulence is excited "on-demand" in controlled ionospheric modification experiments where the energy is provided by an HF radio beam injected into the overhead ionospheric plasma. The ensuing turbulence can be studied with radars and in the form of secondary radiation recorded by ground-based receivers. We have analyzed spectral signatures of the turbulence for different sets of parameters and different altitudes relative to the turning point of the linear Langmuir mode where the Langmuir frequency equals the local plasma frequency. By a parametric analysis, we have derived a simple scaling law, which links the spectral width of the turbulent frequency spectrum to the physical parameters in the ionosphere. The scaling law provides a quantitative relation between the physical parameters ( temperatures, electron number density, ionospheric length scale, etc.) and the observed frequency spectrum. This law may be useful for interpreting experimental results.
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| 8. |
- Galli, André, et al.
(författare)
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The tailward flow of energetic neutral atoms observed at Mars
- 2008
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Ingår i: Journal of Geophysical Research. - Washington : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 113
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Gustavsson, Björn, et al.
(författare)
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HF radio wave acceleration of ionospheric electrons: Analysis of HF-induced optical enhancements
- 2008
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH - SPACE PHYSICS. - 0148-0227. ; 113:8, s. A08319-
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Tidskriftsartikel (refereegranskat)abstract
- The shape of the HF-pump modified electron energy distribution has long been a central question in the field of ionospheric high-frequency radio wave modification experiments. Here we present estimates of the enhanced differential electron flux, from 1.9 to 100 eV based on optical multiwavelength (6300, 5577, 8446, and 4278 Å) data and 930 MHz incoherent scatter radar measurements of ion temperature, electron temperature and concentration. According to our estimate, the electron energy distribution is depleted in the energy range of 2 to approximately 4 eV, probably caused by electron excitation of vibrational states in N2. At the higher energies the electron energy distribution has a nonthermal tail. Further we show that the altitude variations of the four optical emissions should differ both in altitude coverage and center altitude.
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| 10. |
- Hamrin, Maria, et al.
(författare)
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A statistical study of wave properties and electron density at 1700 km in the auroral region
- 2002
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:A8, s. SIA 21-1-SIA 21-13
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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