| 451. |
- Vapirev, A. E., et al.
(author)
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Formation of a transient front structure near reconnection point in 3-D PIC simulations
- 2013
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In: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202 .- 2169-9380. ; 118:4, s. 1435-1449
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Journal article (peer-reviewed)abstract
- Massively parallel numerical simulations of magnetic reconnection are presented in this study. Electromagnetic full-particle implicit code iPIC3D is used to study the dynamics and 3-D evolution of reconnection outflows. Such features as Hall magnetic field, inflow and outflow, and diffusion region formation are very similar to 2-D particle-in-cell (PIC) simulations. In addition, it is well known that instabilities develop in the current flow direction or oblique directions. These modes could provide for anomalous resistivity and diffusive drag and can serve as additional proxies for magnetic reconnection. In our work, the unstable evolution of reconnection transient front structures is studied. Reconnection configuration in the absence of guide field is considered, and it is initialized with a localized perturbation aligned in the cross-tail direction. Our study suggests that the instabilities lead to the development of finger-like density structures on ion-electron hybrid scales. These structures are characterized by a rapid increase of the magnetic field, normal to the current sheet (Bz). A small decrease in the magnetic field component parallel to the reconnection X line and the component perpendicular to the current sheet is observed in the region ahead of the front. The instabilities form due to fact that the density gradient inside the front region is opposite to the direction of the acceleration Lorentz force. Such density structures may possibly further develop into larger-scale earthward flux transfer events during magnetotail reconnection. In addition, oscillations mainly in the magnetic and electric fields and the electron density are observed shortly before the arrival of the main front structure which is consistent with recent THEMIS observations. Key PointsThree dimensional particle-in-cell simulation of reconnection in the magnetotailEvolution of dipolarization front at reconnection and associated plasma flowDevelopment of instabilities in the plasma population
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| 452. |
- Vedin, Jörgen, et al.
(author)
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Electron pressure effects on driven auroral Alfvén waves
- 2005
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In: Journal of Geophysical Research. - Washington : American Geophysical Union.. - 0148-0227 .- 2156-2202. ; 110:A01214, s. 11-
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Journal article (peer-reviewed)abstract
- 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.
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| 453. |
- Vedin, Jörgen, et al.
(author)
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Electrostatic potentials in the downward auroral current region
- 2005
-
In: Journal of Geophysical Research. - Washington : American Geophysical Union. - 0148-0227 .- 2156-2202. ; 110:A08207, s. 1-9
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Journal article (peer-reviewed)abstract
- 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.
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| 454. |
- Vedin, Jörgen, et al.
(author)
-
Particle-fluid simulation of the auroral current circuit
- 2006
-
In: Journal of Geophysical Research. - Washington : American Geophysical Union.. - 0148-0227 .- 2156-2202. ; 111:A12201, s. 12-
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Journal article (peer-reviewed)abstract
- 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.
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| 455. |
- Verdes, C., et al.
(author)
-
Pointing and temperature retrieval from millimeter-submillimeter limb soundings
- 2002
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:16, s. 4299-
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Journal article (peer-reviewed)abstract
- Passive microwave limb sounding instruments like the Millimeter-Wave Atmospheric Sounder (MAS) or the Microwave Limb Sounder (MLS) observe dedicated oxygen lines for the derivation of temperature and pointing information, since these quantities are essential for the quality of the retrieval of the trace gas mixing ratio. Emission lines of oxygen are chosen because the volume mixing ratio (VMR) profile is known. In this paper, we demonstrate the capabilities of a new and innovative method by means of which accurate temperature and pointing information can be gathered from other strong spectral features like ozone lines, without including accurate knowledge of the VMR profile of these species. For this purpose, retrievals from two observation bands with a bandwidth of about 10 GHz each, one including an oxygen line, have been compared. A full error analysis was performed with respect to critical instrument and model parameters, such as uncertainties in the antenna pattern, calibration uncertainties, random pointing error, baseline ripples, baseline discontinuities, and spectroscopic parameters. The applied inversion algorithm was the optimal estimation method. For the selected scenario and instrumental specifications we find that the retrieval of a pointing offset and the atmospheric temperature profile can be achieved with a good accuracy. The retrieval precision of the pointing offset is better than 24 m. The retrieval precision of the temperature profile is better than 2 K for altitudes ranging from 10 to 40 km. Systematic errors (due to model parameter uncertainties) are somewhat larger than these purely statistical errors. Investigations carried out for different atmospheric states or different instrumental specifications show similar results.
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| 456. |
- Volwerk, M, et al.
(author)
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Solitary kinetic Alfven waves: A study of the Poynting flux
- 1996
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In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:A6, s. 13335-13343
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Journal article (other academic/artistic)abstract
- Using a particular mode of the FREJA wave experiment (three magnetic and one electric simultaneous measurements), we have investigated the electromagnetic structure of the solitary kinetic Alfven waves observed in the topside ionosphere, It is shown that
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| 457. |
- von Engeln, Axel, et al.
(author)
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One-dimensional variational (1-D Var) retrieval of temperature, water vapor, and a reference pressure from radio occultation measurements : A sensitivity analysis
- 2003
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 108:11, s. 4337-
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Journal article (peer-reviewed)abstract
- A 1-D Var retrieval study of simulated radio occultation measurements is presented. Temperature and a water vapor profile are retrieved along with a reference pressure to generate the pressure profile by applying the hydrostatic equation. High-resolution European Center for Medium-Range Weather Forecasts (ECMWF) atmospheric fields are used by a ray tracing tool to calculate the exact positions of the tangent point. The 1-D atmospheric profiles following the calculated tangent point trajectory in the 3-D ECMWF fields are used to simulate bending angle measurements with a 1-D forward model. Assimilation of these bending angles in a 1-D Var tool employing the same 1-D forward model is performed. We analyze the sensitivity of the retrieval to changes in vertical resolution, horizontal smearing of the tangent point trajectory, and the assumption of hydrostatic equilibrium for a nonvertical atmospheric scan. We find that retrievals calculated without adequate vertical resolution can have significant errors in temperature and water vapor. Errors in the retrieval by assuming hydrostatic equilibrium for a nonvertical scan generally cause only minor errors in the retrieved profiles. A study into the occurrence of rays curving down toward the Earth surface indicates that about 5–10% of the profiles could experience so-called critical refraction at altitudes between 0.5 km and 2 km in case of the applied high-resolution ECMWF data.
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| 458. |
- von Engeln, Axel, et al.
(author)
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Temperature profile determination from microwave oxygen emissions in limb sounding geometry
- 2002
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:19, s. 12-1
-
Journal article (peer-reviewed)abstract
- We present a theoretical investigation of the temperature profile retrieval capabilities of oxygen emission lines in the microwave. The main focus is on two strong lines, both allowing temperature retrieval throughout the mesosphere. One is within the oxygen cluster at 61.15 GHz, the other one is isolated at 118.75 GHz. A thorough comparison of these two lines is presented. Several instrumental parameters such as system noise temperature, antenna beam width, filter width, and coverage of the line are assessed, as well as the possible impact of an error in the spectroscopic parameters. The instrumental setup follows roughly the specifications for the Millimeter Wave Acquisitions for Stratosphere/Troposphere Exchange Research (MASTER) instrument, serving as a basis for a modern passive microwave instrument. The instrumental parameters have also been varied in order to allow comparisons with two instruments that use the 118.75-GHz line for temperature profile determination, the Odin submillimeter radiameter (SMR) and the EOS microwave limb sounder (MLS). Simultaneous retrieval of temperature and pointing bias is performed with the Optimal Estimation Method. We find temperature retrieval errors of
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| 459. |
- Vonk, Jorien, et al.
(author)
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A centennial record of fluvial organic matter input from the discontinuous permafrost catchment of Lake Torneträsk
- 2012
-
In: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 117:G03018, s. 1-11
-
Journal article (peer-reviewed)abstract
- High-latitude regions are underlain by the most organic carbon (OC)-rich soils on earth and currently subject to intense climate warming, potentially increasing remobilization and mineralization of soil OC. Sub-Arctic Scandinavia is located on the 0°C mean annual isotherm and is therefore particularly vulnerable to climate change. This study aimed to establish a baseline for soil OC release over the past century into Lake Torneträsk, the largest lake in sub-Arctic Scandinavia, through bulk geochemical and molecular radiocarbon analyses in chronologically constrained sediment cores. Our results suggest a dominance of peat-derived terrestrial OC inflow. We show that the annual terrestrial OC inflow to the lake is ∼12 times higher than the in-lake produced particulate OC, and consists for a large part (ca. 60%) of old OC from deep reservoirs in the catchment. The sedimentary record shows signs of increasing inflow of more degraded terrestrial matter since ∼1975, as indicated by increasing %TOC concentrations, a lower δ13C value and lower TOC:TN ratios. Based on simultaneous changes in local climate and reported signs of permafrost degradation (e.g., active layer deepening, mire/peat erosion), the observed changes in the sedimentary record of Scandinavia's largest mountain lake likely reflect a climate warming-induced change in terrestrial OC inflow.
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| 460. |
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