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Browsing Artikler, rapporter og annet (fysikk og teknologi) by Title
Now showing items 1-20 of 94
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Grydeland, Tom; Chau, J.L.; La Hoz, C.; Brekke, Asgeir (Journal article; Peer reviewed, 31-Jan-2005)[+][-]
Abstract: Interferometric imaging (aperture synthesis imaging) is a technique used by radio astronomers to achieve angular resolution that far surpasses what is possible with a single large aperture. A similar technique has been used for radar imaging studies of equatorial ionospheric phenomena at the Jicamarca Radio Observatory. We present plans for adding an interferometric imaging capability to the EISCAT Svalbard Radar (ESR), a capability which will contribute significantly to several areas of active research, including naturally and artificially enhanced ion-acoustic echoes and their detailed relation in space and time to optical phenomena, polar mesospheric summer echoes (PMSE), and meteor studies. Interferometry using the two antennas of the ESR has demonstrated the existence of extremely narrow, fieldaligned scattering structures, but having only a single baseline is a severe limitation for such studies. Building additional IS-class antennas at the ESR is not a trivial task. However, the very high scattering levels in enhanced ion-acoustic echoes and PMSE means that a passive receiver antenna of more modest gain should still be capable of detecting these echoes. In this paper we present simulations of what an imaging interferometer will be capable of observing for different antenna configurations and brightness distributions, under ideal conditions, using two different image inversion algorithms. We also discuss different antenna and receiver technologies. URI: http://hdl.handle.net/10037/548 File(s) in this item: 1
article.pdf (334.7Kb) -
Isham, B.; Rietveld, Michael T.; Hagfors, T.; Belyey, Vasyl; Khudukon, B.; La Hoz, Cesar; Yurik, R.Yu; Brekke, Asgeir; Tereshchenko, E.D.; Grill, M.; Heinselman, C. (Journal article; Peer reviewed, 31-Jan-2005)[+][-]
Abstract: When the Earth's ionosphere is irradiated by a radiofrequency (RF) electromagnetic wave of sufficiently high power density and tuned to match a natural E- or F-region plasma frequency, ionospheric magnetoionic wave modes may be excited and may generate RF electromagnetic sideband waves via nonlinear interactions. These secondary emissions, which may then escape from the ionosphere, have been termed stimulated electromagnetic emission or SEE. The frequency spectra of this radiation has been studied extensively, and a number of characteristic spectral features have been identified and in some cases related to particular plasma processes. The separation in frequency between the RF pump and the harmonics of the local electron gyrofrequency is critical in determining the amount of anomalous absorption suffered by the pump wave and the spectral properties of the stimulated sidebands. The pump can excite electrostatic waves which do not propagate away but can in some cases be observed via radio-wave scattering from the electron density fluctuations associated with them. These enhanced density fluctuations are created by processes commonly referred to as upper-hybrid and Langmuir turbulence. Langmuir turbulence has been the subject of 930-MHz scattering observations with antenna scanning through several pre-selected angles between the geographic and geomagnetic zenith directions, and a preference for pointing angles between the Spitze angle and geomagnetic field-aligned was identified. Other phenomena, such as the generation of enhanced electron temperatures and artificial aurora, have more recently been shown to have special behavior at similar angles, near but apparently not quite at field-aligned. In view of this evidence for angular structure in several pump-induced effects, in light of the rich variety of SEE phenomena strongly dependent on the geomagnetic field via the frequency interval between the pump and the gyrofrequency harmonics, and in view of the not yet understood but complex relationship between electrostatic fluctuations and SEE, it is of interest to investigate experimentally whether a similar angular structure is present in the various spectral features of the SEE signals and to compare the results with radar and other observations of RF-pump-induced effects. To this end we describe a simple two-element radio interferometer designed to search for aspect angle dependence of SEE features. We present an example of the initial data produced by this system, and draw preliminary conclusions based on the example data. URI: http://hdl.handle.net/10037/565 File(s) in this item: 1
article.pdf (5.513Mb) -
Moser, Gabriele; Akbari, Vahid; Eltoft, Torbjørn; Doulgeris, Anthony Paul; Anfinsen, Stian Normann; Sebastian, Serpico (Conference object; Konferansebidrag, 2011)[+][-]
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Kleimann, Jens; Kopp, A.; Fichtner, H.; Grauer, R. (Journal article; Tidsskriftartikkel; Peer reviewed, 2009)[+][-]
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Westman, A.; Grydeland, Tom; Belyey, Vasyl; La Hoz, Cesar (Journal article; Peer reviewed, 31-Jan-2005)[+][-]
Abstract: The real and imaginary parts of baseband signals are obtained from a real narrow–band signal by quadrature mixing, i.e. by mixing with cosine and sine signals at the narrow band’s selected center frequency. We address the consequences of a delay between the outputs of the quadrature mixer, which arise when digital samples of the quadrature baseband signals are not synchronised, i.e. when the real and imaginary components have been shifted by one or more samples with respect to each other. Through analytical considerations and simulations of such an error on different synthetic signals, we show how this error can be expected to afflict different measurements. In addition, we show the effect of the error on actual incoherent scatter radar data obtained by two different digital receiver systems used in parallel at the EISCAT Svalbard Radar (ESR). The analytical considerations indicate a procedure to correct the error, albeit with some limitations due to a small singular region. We demonstrate the correction procedure on actually afflicted data and compare the results to simultaneously acquired unafflicted data. We also discuss the possible data analysis strategies, including some that avoid dealing directly with the singular region mentioned above. URI: http://hdl.handle.net/10037/547 File(s) in this item: 1
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Jenssen, Robert; Kloft, Marius; Zien, Alexander; Sonnenburg, Soeren; Müller, Klaus R. (Journal article; Tidsskriftartikkel; Peer reviewed, 2012)[+][-]
Abstract: We provide a novel interpretation of the dual of support vector machines (SVMs) in terms of scatter with respect to class prototypes and their mean. As a key contribution, we extend this framework to multiple classes, providing a new joint Scatter SVM algorithm, at the level of its binary counterpart in the number of optimization variables. This enables us to implement computationally efficient solvers based on sequential minimal and chunking optimization. As a further contribution, the primal problem formulation is developed in terms of regularized risk minimization and the hinge loss, revealing the score function to be used in the actual classification of test patterns. We investigate Scatter SVM properties related to generalization ability, computational efficiency, sparsity and sensitivity maps, and report promising results. URI: http://hdl.handle.net/10037/4963 File(s) in this item: 1
article.pdf (1.187Mb) -
Rypdal, Martin; Rypdal, Kristoffer (Working paper; Arbeidsnotat, 22-Jul-2008)[+][-]
Abstract: A stochastic theory for the toppling activity in sandpile models is developed, based on a simple mean-field assumption about the toppling process. The theory describes the process as an anti-persistent Gaussian walk, where the diffusion coefficient is proportional to the activity. It is formulated as a generalization of the Itˆo stochastic differential equation with an anti-persistent fractional Gaussian noise source. An essential element of the theory is re-scaling to obtain a proper thermodynamic limit, and it captures all temporal features of the toppling process obtained by numerical simulation of the Bak-Tang-Wiesenfeld sandpile in this limit URI: http://hdl.handle.net/10037/1860 File(s) in this item: 1
article.pdf (332.2Kb) -
Rypdal, Martin; Rypdal, Kristoffer (Journal article; Tidsskriftartikkel; Peer reviewed, 2008)[+][-]
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Sergienko, T; Gustavsson, Björn Johan; Sandahl, Ingrid; Andersson, Laila; Brändström, B. U. E.; Steen, Å (Journal article; Tidsskriftartikkel; Peer reviewed, 2008)[+][-]
Abstract: We present results of an investigation of the fine structure of the night sector diffuse auroral zone, observed simultaneously with optical instruments (ALIS) from the ground and the FAST electron spectrometer from space 16 February 1997. Both the optical and particle data show that the diffuse auroral zone consisted of two regions. The equatorward part of the diffuse aurora was occupied by a pattern of regular, parallel auroral stripes. The auroral stripes were significantly brighter than the background luminosity, had widths of approximately 5 km and moved southward with a velocity of about 100 m/s. The second region, located between the region with auroral stripes and the discrete auroral arcs to the north, was filled with weak and almost homogeneous luminosity, against which short-lived auroral rays and small patches appeared chaotically. From analysis of the electron differential fluxes corresponding to the different regions of the diffuse aurora and based on existing theories of the scattering process we conclude the following: Strong pitch angle diffusion by electron cyclotron harmonic waves (ECH) of plasma sheet electrons in the energy range from a few hundred eV to 3–4 keV was responsible for the electron precipitation, that produced the background luminosity within the whole diffuse zone. The fine structure, represented by the auroral stripes, was created by precipitation of electrons above 3–4 keV as a result of pitch angle diffusion into the loss cone by whistler mode waves. A so called "internal gravity wave" (Safargaleev and Maltsev, 1986) may explain the formation of the regular spatial pattern formed by the auroral stripes in the equatorward part of the diffuse auroral zone. URI: http://hdl.handle.net/10037/3147 File(s) in this item: 1
article.pdf (4.188Mb) -
Kube, Ralph; Garcia, Odd Erik; LaBombard, Brian; Madsen, Jens; Terry, James L.; Zweben, Stewart (Conference object; Konferansebidrag, 2012)[+][-]
Abstract: A new blob-tracking algorithm for the GPI diagnostic installed in the outboard-midplane of Alcator C-Mod is developed. I t tracks large-amplitude fluctuations propagating through the scrape-off layer and calculates blob sizes and velocities. We compare the results of this method to a blob velocity scaling from a simple blob-model for sheath-connected blobs. We further present initial results from a fully three-dimensional blob model that features plasma resistivity as a free parameter. URI: http://hdl.handle.net/10037/4947 File(s) in this item: 1
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Rypdal, Kristoffer; Živkovic, Tatjana; Østvand, Lene (Working paper; Arbeidsnotat, 19-Apr-2009)[+][-]
Abstract: In a toroidal plasma confined by a purely toroidal magnetic field the plasma transport is governed by electrostatic turbulence driven by the flute interchange instability on the low-field side of the torus cross section. In this paper we revisit experimental data obtained from the Blaamann torus at the University of Tromso. On time-scales shorter than the poloidal rotation time, the time series of potential and electron density fluctuations measured on stationary Langmuir probes essentially reflect the spatial poloidal structure of the turbulent field (Taylor hypothesis). On these time scales the signals reveals an intermittent character exposed via analysis of probability density functions and computation of multifractal dimension spectra in different regimes of time scales. This intermittency is associated with the shape and distribution of pronounced spikes in the signal. On time scales much longer than the rotation period there are strong global fluctuations in the plasma potential which are shown to to be the result of low-dimensional chaotic dynamics. URI: http://hdl.handle.net/10037/2127 File(s) in this item: 1
article.pdf (309.0Kb) -
Capillary suction model as pipes of different sizes: flow conditions and comparison with experimentsNguyen, Hung Thanh; Jacobsen, Stefan; Melandsø, Frank (Conference object; Konferansebidrag, 2012)[+][-]
Abstract: ABSTRACT Absorption of water in concrete is often described by the simple linear water uptake vs. square-root-of-time law. However, a deviation from this behaviour is frequently seen depending on factors such as initial water content, water/binder ratio and specimen thickness. The deviation increases with thickness (typically from 25 to 100 mm) and is seen even for very dry specimens with capillary pores. We have applied Laplaces law for suction created under a curved meniscus between air and water to series of pipes with different lengths and radii. The resulting analytical model was first compared with numerical simulations at abrupt reduction or increase of pipe radius showing good agreement. Then a complete second order equation describing the relation between the capillary suction and the suction time was developed. Varying geometries of the pipes of the capillary system were investigated including the effects on the flow rate of varying combination of lengths, radii and sequence. The results showed that largest flow reductions occurred with very narrow sections causing a blocking, reducing the capillary flow rate vs. square root of time in the same manner as in concrete. The often observed phenomenon of reduced flow below the straight line water uptake vs. square root of time could be simulated with the multiple diameter pipe models, as seen by comparing simulations with experiments with varying concrete qualities and sample thicknesses in simple capillary absorption tests. Key-words: capillary suction, pipe geometry, model URI: http://hdl.handle.net/10037/5114 File(s) in this item: 1
article.pdf (394.7Kb) -
Cannon, P. S.; Hall, Chris; Brekke, Asgeir (Journal article; Tidsskriftartikkel; Peer reviewed, 29-Nov-2007)[+][-]
Abstract: We have examined the long time series of observations of E-region virtual height (1948–2006) and critical frequency (1935-2006) hitherto made by the Tromsø ionosonde at 70 degrees N, 19 degrees E. Combining a simplistic trend analysis with a rigorous treatment of errors we identify a negative trend in critical frequency. While a similar analysis of the virtual height h'E also suggests a negative trend, a closer examination reveals a possible weak positive trend prior to ~1975 and a strong negative trend from ~1975 to present. These two metrics of essentially the same feature of the ionosphere do not exhibit the same signature since critical frequency is controlled by photochemistry within the E-layer while height is controlled by pressure level. We further find that the trend in critical frequency is a daylight/summer phenomenon, no significant trend being evident in the winter subset of the data. On the other hand, the trends in virtual height are independent of season/daylight. Description: This is the publishers version/PDF (published in Annales Geophysicae, an open access journal of the European Geosciences Union) URI: http://hdl.handle.net/10037/2385 File(s) in this item: 1
article.pdf (481.8Kb) -
Rietveld, Michael T.; Tereshchenko, E.D.; Brekke, Asgeir; Khudukon, B.Z.; Kozlova, M.O.; Evstafiev, O.V.; Nygren, T. (Journal article; Peer reviewed, 2000)[+][-]
Abstract: Results are shown from an experimental campaign where satellite scintillation was observed at three sites at high latitudes and, simultaneously, the F region plasma flow was measured by the nearby EISCAT incoherent scatter radar. The anisotropy parameters of field-aligned irregularities are determined from amplitude scintillation using a method based on the variance of the relative logarithmic amplitude. The orientation of the anisotropy in a plane perpendicular to the geomagnetic field is compared with the direction of F region plasma flow. The results indicate that in most cases a good agreement between the two directions is obtained. URI: http://hdl.handle.net/10037/559 File(s) in this item: 1
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La Hoz, Cesar (Journal article; Tidsskriftartikkel; Peer reviewed, 2011)[+][-]
Abstract: A number of flux transfer events (FTEs) were observed between 09:00 and 12:00UT on 11 February 2004, during southward and dawnward IMF, while the Cluster spacecraft array moved outbound through the northern, highaltitude cusp and dayside high-latitude boundary layer, and the Double Star TC-1 spacecraft was crossing the dayside low-latitude magnetopause into the magnetosheath south of the ecliptic plane. The Cluster array grazed the equatorial cusp boundary, observing reconnection-like mixing of magnetosheath and magnetospheric plasma populations. In an adjacent interval, TC-1 sampled a series of sometimes none standard FTEs, but also with mixed magnetosheath and magnetospheric plasma populations, near the magnetopause crossing and later showed additional (possibly turbulent) activity not characteristic of FTEs when it was situated deeper in the magnetosheath. The motion of these FTEs are analyzed in some detail to compare to simultaneous, poleward-moving plasma concentration enhancements recorded by EISCAT Svalbard Radar (ESR) and “polewardmoving radar auroral forms” (PMRAFs) on the CUTLASS Finland and Kerguelen Super Dual Auroral Radar Network (SuperDARN) radar measurements. Conjugate SuperDARN observations show a predominantly two-cell convection pattern in the Northern and Southern Hemispheres. The results are consistent with the expected motion of reconnected magnetic flux tubes, arising from a predominantly sub-solar reconnection site. Here, we are able to track north and south in closely adjacent intervals as well as to map to the corresponding ionospheric footprints of the implied flux tubes and demonstrate these are temporally correlated with clear iono-Correspondence to: Q.-H. Zhang (zhangqinghe@pric.gov.cn) spheric velocity enhancements, having northward (southward) and eastward (westward) convected flow components in the Northern (Southern) Hemisphere. The durations of these enhancements might imply that the evolution time of the FTEs is about 18–22 min from their origin on magnetopause (at reconnection site) to their addition to the magnetotail lobe. However, the ionospheric response time in the Northern Hemisphere is about 2–4 min longer than the response time in the Southern Hemisphere URI: http://hdl.handle.net/10037/3842 File(s) in this item: 1
article.pdf (6.486Mb) -
Pecseli, Hans L.; Fredriksen, Åshild; Fredriksen, A.A.; Trulsen, Jan (Journal article; Tidsskriftartikkel; Peer reviewed, 2008)[+][-]
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Haugland, Ole Anton (book; bok, 2007)[+][-]
Description: Kompendium for bruk av regneark og verktøy for formelredigering og enkel tegning i naturfag. Kompendiet tar for seg Excel 2003, men vil også passe for tidligere versjoner bakover til Excel 97. En nyere versjon (januar 2012) av dette kompendiet er tilgjengelig i Munin med følgende adresse: http://hdl.handle.net10037/3805 URI: http://hdl.handle.net/10037/2286 File(s) in this item: 1
book.pdf (510.2Kb) -
Grujic, Katarina; Hellesø, Olav Gaute (Journal article; Tidsskriftartikkel; Peer reviewed, 2007)[+][-]
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Tereshchenko, E.D.; Rietveld, Michael T.; Yurik, R.Yu; Belyey, Vasyl; Khudukon, B.Z.; Brekke, Asgeir; Isham, B.; Hagfors, T.; Grill, M. (Journal article; Peer reviewed, 09-Aug-2006)[+][-]
Abstract: A high frequency (HF) ionospheric modification experiment was carried out between 25 September and 8 October 2004, using the EISCAT HF transmitter located near Tromsø, Norway. During this experiment the spectra of the stimulated HF sideband waves (stimulated electromagnetic emission or SEE) induced by the HF pump were observed using an interferometer consisting of three spaced receiving antennas with baselines both along and perpendicular to the meridian, and a multi-channel coherent receiver, installed in the vicinity of the HF facility. The transmitter operated at 4040kHz and its antenna beam was scanned to angles of 0°, 7°, 14°, and 21° south from vertical, pausing 4min at each position. This paper focuses on features of the downshifted peak (DP) emission, which has not been as thoroughly studied as many of the other SEE spectral features observable within the EISCAT pump frequency range. It was found that the signal-weighted direction of the DP source region remained within 5° of magnetic zenith as the HF beam was tilted between 0 and 21° south of vertical. URI: http://hdl.handle.net/10037/588 File(s) in this item: 1
article.pdf (450.9Kb) -
Havnes, Ove; La Hoz, Cesar; Rietveld, Michael T; Kassa, Meseret; Baroni, Giovanni; Biebricher, Alexander (Journal article; Tidsskriftartikkel; Peer reviewed, 2011)[+][-]
Abstract: We present an analysis of relatively strong Polar Mesospheric Winter Echoes (PMWE) under artificial electron heating that changes the PMWE intensity. A major purpose is to find reliable estimates of the relaxation time of the heater modified PMWE to their undisturbed state during the heater switch-off phase; the implications regarding charge/discharge mechanisms; and to exploit the diagnostic potential of artificial electron heating. The relaxation time is between 60 to 70 s for the regions with strong PMWE layers and substantial electron heating. This short relaxation time, related to the variation of charges on the nanometer dust which most likely is present in PMWE, rules out ion attachment as the mechanism to bring the dust charges to their equilibrium state. Neutral winds, sweeping the heated electrons out of the radar beam, are unlikely to be the cause of the observed relaxation, since this requires winds of around 100 m s−1. The most probable cause is photo detachment by which negatively charged dust can lose excess electrons by photon absorption with energies less than the dust material's work function. By comparing the observed heating with heating model profiles, the electron density at 65 km height must have been of the order of 3 × 109 m−3. This agrees with PMWE occurring mainly during disturbed conditions with high electron densities. Our results also indicate that in the strongest PMWE layers, electron bite-outs exist consistent with the role of charged dust particles in the mechanism of PMWE and implying larger dust densities. URI: http://hdl.handle.net/10037/3972 File(s) in this item: 1
article.pdf (1.256Mb)
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