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Browsing Artikler, rapporter og annet (fysikk og teknologi) by Author
Now showing items 1-20 of 94
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Ahluwalia, Balpreet Singh; Mccourt, peter anthony; Huser, Thomas Rolf; Hellesø, Olav Gaute (Journal article; Tidsskriftartikkel; Peer reviewed, 2010)[+][-]
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Akbari, Vahid; Motagh, Mahdi; Rajabi, Mohammad Ali; Yahya, Djamour (Conference object; Konferansebidrag, 2010)[+][-]
Abstract: This study presents the weighted least squares method based on Interferometric Synthetic Aperture Radar (InSAR) images to retrieve spatial-temporal evolution of land subsidence in Mashhad Valley, northeast Iran. Using the analysis of a few interferograms covering the 2003-2005 period, Motagh et al (GJI 2006) presented a preliminary analysis of the subsidence in this area. Here we extend this study and use additional SAR data to retrieve time-dependent deformation in Mashhad Valley We utilize 17 SAR images acquired by the ENVISAT satellite in a descending orbit during Jun. 2004-Nov. 2007, make 53 differential interferograms spanning different long- and short-term intervals, and do a time series analysis to extract deformation signals out of differential interferograms. Time series analysis suggests that the subsidence occurs within a northwest-southeast elongated elliptical shaped bowl along the axis of Mashhad valley. The maximum accumulated subsidence during 1260 days reaches about 86 cm, located northeast of Mashhad City. The comparison between InSAR time series results with continuous GPS station in the city of Tous, northeast of Mashhad, yields comparable results at the level of 1 cm. URI: http://hdl.handle.net/10037/3066 File(s) in this item: 1
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Akbari, Vahid; Eltoft, Torbjørn; Doulgeris, Anthony Paul (Conference object; Konferansebidrag, 2010)[+][-]
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Aslaksen, Torsten; Shiokawa, K.; Ieda, A.; Nakajima, A.; Sakaguchi, K.; Nomura, R.; Greffen, M.; Spanswick, E.; Donovan, E.; Mende, S. B.; McFadden, J.; Glassmeier, K. H.; Angelopoulos, V.; Miyashita, Y. (Journal article; Tidsskriftartikkel; Peer reviewed, 2009)[+][-]
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Behlke, Rico; Clausen, L. B. N.; Yeoman, T. K.; Fear, R. C.; Lucek, E. A.; Engebretson, M. J. (Journal article; Tidsskriftartikkel; Peer reviewed, 2009)[+][-]
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Clausen, L. B. N.; Behlke, Rico; Yeoman, T. K.; Lucek, E. A. (Journal article; Tidsskriftartikkel; Peer reviewed, 2008)[+][-]
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Bekele, Meseret Kassa; Rapp, M.; Hartquist, T W; Havnes, Ove (Journal article; Tidsskriftartikkel; Peer reviewed, 2012)[+][-]
Abstract: We report measurements of dust currents obtained with a small probe and a larger probe during the flight of the ECOMA-4 rocket through the summer polar mesosphere. The payload included two small dust probes behind a larger dust probe located centrally at the front. For certain phases of the payload rotation, the current registered by one of the small dust probes was up to 2 times the current measured with the larger probe, even though the effective collection area of the larger probe was 4 times that of the small one. We analyze the phase dependence of the currents and their difference with a model based on the assumption that the small probe was hit by charged dust fragments produced in collisions of mesospheric dust with the payload body. Our results confirm earlier findings that secondary charge production in the collision of a noctilucent cloud/Polar Summer Mesospheric Echo (NLC/PMSE) dust particle with the payload body must be several orders of magnitude larger than might be expected from laboratory studies of collisions of pure ice particles with a variety of clean surfaces. An important consequence is that for some payload configurations, one should not assume that the current measured with a detector used to study mesospheric dust is simply proportional to the number density of ambient dust particles. The higher secondary charge production may be due to the NLC/PMSE particles containing multiple meteoric smoke particles. URI: http://hdl.handle.net/10037/4964 File(s) in this item: 1
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Kosch, M.J.; Blixt, Erik Mårten; Semeter, J. (Journal article, 27-Jul-2005)[+][-]
Abstract: Black auroras are recognized as spatially well-defined regions within uniform diffuse aurora where the optical emission is significantly reduced. Although a well studied phenomenon, there is no generally accepted theory for black auroras. One theory suggests that black regions are formed when energetic magnetospheric electrons no longer have access to the loss cone. If this blocking mechanism drifts with the source electron population in the magnetosphere, black auroras in the ionosphere should drift eastward with a velocity that increases with the energy of the precipitating electrons in the surrounding aurora, since the gradient-B curvature drift is energy dependent. It is the purpose of this paper to test this hypothesis. To do so we have used simultaneous measurements by the European Incoherent Scatter (EISCAT) radar and an auroral TV camera at Tromsø, Norway. We have analyzed 8 periods in which a black aurora occurred frequently to determine their relative drift with respect to the ionospheric plasma. The black aurora was found to drift eastward with a velocity of 1.5–4km/s, which is in accordance with earlier observations. However, one case was found where a black patch was moving westward, this being the first report of such behaviour in the literature. In general, the drift was parallel to the ionospheric flow but at a much higher velocity. This suggests that the generating mechanism is not of ionospheric origin. The characteristic energy of the precipitating electron population was estimated through inversion of E-region plasma density profiles. We show that the drift speed of the black patches increased with the energy of the precipitating electrons in a way consistent with the gradient-B curvature drift, suggesting a magnetospheric mechanism for the black aurora. As expected, a comparison of the drift speeds with a rudimentary dipole field model of the gradient-B curvature drift speed only yields order-of-magnitude agreement, which most likely is due to the nightside disturbed magnetosphere being significantly stretched. URI: http://hdl.handle.net/10037/581 File(s) in this item: 1
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Brekke, Asgeir (Chronicle; Kronikk, 2011)[+][-]
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Brekke, Asgeir (Research report; Forskningsrapport, 2008)[+][-]
Abstract: Den polare atmosfæren har fått mer oppmerksomhet i den senere tid enn den vanligvis har vært forunt. Dette henger sammen med tanken om at den er så ren og upåvirket av eventuelle forurensninger som vi pumper ut i atmosfæren i de mer urbane strøk. Dersom atmosfæren tar skade av denne aktiviteten, forventer en å se skadevirkningene først og klarest i den polare atmosfæren. Svekkelsen av ozonlaget over Antarktis kan være et slikt sykdomstegn som har brakt miljøpolitikere verden over til å reagere. Vi ønsker ut fra de fysiske kunnskaper om atmosfæren å kaste lys over denne situasjonen. URI: http://hdl.handle.net/10037/2275 File(s) in this item: 1
article.pdf (3.179Mb) -
Brekke, Asgeir; Blagoveshchenskaya, N. F.; Carlson, H. C.; Kornienko, V. A.; Borisova, T. D.; Rietveld, M. T.; Yeoman, T. K. (Journal article; Tidsskriftartikkel; Peer reviewed, 08-Jan-2009)[+][-]
Abstract: Multi-instrument observational data from an experiment on 13 October 2006 at the EISCAT/HEATING facility at Tromsø, Norway are analysed. The experiment was carried out in the evening hours when the electron density in the F-region dropped, and the HF pump frequency fH was near and then above the critical frequency of the F2 layer. The distinctive feature of this experiment is that the pump frequency was just below the third electron gyro harmonic frequency, while both the HF pump beam and UHF radar beam were directed towards the magnetic zenith (MZ). The HF pump-induced phenomena were diagnosed with several instruments: the bi-static HF radio scatter on the London- Tromsø-St. Petersburg path, the CUTLASS radar in Hankasalmi (Finland), the European Incoherent Scatter (EISCAT) UHF radar at Tromsø and the Tromsø ionosonde (dynasonde). The results show thermal electron excitation of the HF-induced striations seen simultaneously from HF bi-static scatter and CUTLASS radar observations, accompanied by increases of electron temperature when the heater frequency was near and then above the critical frequency of the F2 layer by up to 0.4 MHz. An increase of the electron density up to 25% accompanied by strong HF-induced electron heating was observed, only when the heater frequency was near the critical frequency and just below the third electron gyro harmonic frequency. It is concluded that the combined effect of upper hybrid resonance and gyro resonance at the same altitude gives rise to strong electron heating, the excitation ofstriations, HF ray trapping and extension of HF waves to altitudes where they can excite Langmuir turbulence and fluxes of electrons accelerated to energies that produce ionization. Description: This is the publisher’s version/PDF (published in Annales Geophysicae, an open access journal of the European Geosciences Union) URI: http://hdl.handle.net/10037/2384 File(s) in this item: 1
angeo-27-131-2009.pdf (1.432Mb) -
Oyama, S; Brekke, Asgeir; Watkins, B. J.; Maeda, S.; Shinagawa, H.; Nozawa, S.; Ogawa, Y.; Lathuillere, C.; Kofman, W. (Journal article; Tidsskriftartikkel; Peer reviewed, 11-Jun-2008)[+][-]
Abstract: Lower-thermospheric winds at high latitudes during moderately-disturbed geomagnetic conditions were studied using data obtained with the European Incoherent Scatter (EISCAT) Kiruna-Sodankylä-Tromsø (KST) ultrahigh frequency (UHF) radar system on 9–10 September 2004. The antenna-beam configuration was newly designed to minimize the estimated measurement error of the vertical neutralwind speed in the lower thermosphere. This method was also available to estimate the meridional and zonal components. The vertical neutral-wind speed at 109 km, 114 km, and 120 km heights showed large upward motions in excess of 30ms−1 in association with an ionospheric heating event. Large downward speeds in excess of −30ms−1 were also observed before and after the heating event. The meridional neutral-wind speed suddenly changed its direction from equatorward to poleward when the heating event began, and then returned equatorward coinciding with a decrease in the heating event. The magnetometer data from northern Scandinavia suggested that the center of the heated region was located about 80 km equatorward of Tromsø. The pressure gradient caused the lower-thermospheric wind to accelerate obliquely upward over Tromsø in the poleward direction. Acceleration of the neutral wind flowing on a vertically tilted isobar produced vertical wind speeds larger by more than two orders of magnitude than previously predicted, but still an order of magnitude smaller than observed speeds. 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/10037/2383 File(s) in this item: 1
article.pdf (2.843Mb) -
Fredriksen, Åshild; Mishra, Lekha Nath; Gulbrandsen, Njål; Miloch, Wojciech Jacek (Journal article; Tidsskriftartikkel; Peer reviewed, 2010)[+][-]
Abstract: Plasma parameters and the subsonic flow from a capacitively coupled, cylindrical plasma source of the Njord helicon device are investigated by means of a Mach probe and a retarding field energy analyzer (RFEA). 13.56 MHz and 600 W RF power is inserted into the argon working gas under low-pressure conditions and moderate magnetic field. By means of a downstream field coil, the magnetic field is shaped from a purely expanding field to a configuration with more parallel field lines. It is shown that the downstream plasma density along the outer rim of the source increases significantly and there is a sudden increase by nearly 20 V in the plasma potential already after a moderate increase in the downstream magnetic field. The investigation of the flow indicates that current ratios derived from the Mach probe result in an apparent flow in the direction towards the source, while the current bratios derived from the RFEA indicate a flow in the direction away from the source. PIC simulations demonstrate that the acceptance angle of the probes, being nearly 180o for the Mach probe, and about 45o for the RFEA, can critically affect the current ratios and hence the subsonic flow measured by the probes in the weakly magnetized plasma in our device. The first section in your paper URI: http://hdl.handle.net/10037/3560 File(s) in this item: 1
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Pecseli, Hans L.; Fredriksen, Åshild; Fredriksen, A.A.; Trulsen, Jan (Journal article; Tidsskriftartikkel; Peer reviewed, 2008)[+][-]
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Riccardi, Claudia; Fredriksen, Åshild; Magni, Simone (Conference object; Konferansebidrag, Oct-2004)[+][-]
Abstract: We report results from experiments performed to study how a DC bias with respect to the plasma potential is affecting the plasma states in the toroidal geometry of the Blaamann device in Tromso. In the experiments discussed here, a ring with smaller diameter than the limiter was centered inside the bulk of the plasma and its bias varied with respect to the plasma potential. In the electron saturation current regime of the ring, a significant reduction of the fluctuation levels was observed, and a shear in the poloidal velocity occurred at the lowfield side of the ring. This shear was positioned at the same radial position as the maximum of the radial transport when the ring was left floating or biased in the ion saturation current regime. For the latter ring biases, the poloidal velocities had no shear on the low-field side of the ring. Without the velocity shear, the radial transport was similar to previous experiments without ring. With the velocity shear, the radial transport was destroyed. This plasma state, with very low radial transport and fluctuation levels as well as plasma densities around 1017 m-3, is believed to provide a suitable plasma for wave propagation studies in a magnetized plasma with curved magnetic field lines without end effects. URI: http://hdl.handle.net/10037/1846 File(s) in this item: 1
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Fynbo, J.U.; Østensen, R.; Gorosabe, J.; Solheim, Jan Erik; Dall, T.H.; Hjorth, J.; Pedersen, H.; Andersen, M.I.; Møller, P.; Holland, S.; Smail, I.; Kobayashi, N.; Rol, E.; Vreeswijk, P.; Vrba, F.; Burud, I.; Jensen, B.L.; Henden, A.; Thomsen, B.; Canzian, B.; Castro Cer´on, J.M.; Castro-Tirado, A.J.; Cline, T.; Goto, M.; Greiner, J.; Hanski, M.T.; Hurley, K.; Pursimo, T.; Lund, N.; Tanvir, N.; Terada, H. (Journal article; Tidsskriftartikkel, 11-Apr-2001)[+][-]
Abstract: We present the discovery of the Optical Transient (OT) of the long–duration gamma-ray burst GRB 000926. The optical transient was detected independently with the Nordic Optical Telescope and at Calar Alto 22.2 hours after the burst. At this time the magnitude of the transient was R = 19.36. The transient faded with a decay slope of about 1.7 during the first two days after which the slope increased abruptly (within a few hours) to about 2.4. The light-curve started to flatten off after about a week indicating the presence of an underlying extended object. This object was detected in a deep image obtained one month after the GRB at R=23.87±0.15 and consists of several compact knots within about 5 arcsec. One of the knots is spatially coincident with the position of the OT and hence most likely belongs to the host galaxy. Higher resolution imaging is needed to resolve whether all the compact knots belong to the host galaxy or to several independent objects. In a separate paper we present a discussion of the optical spectrum of the OT, and its inferred redshift (Møller et al. in prep.). Description: This is the authors' manuscript URI: http://hdl.handle.net/10037/2039 File(s) in this item: 1
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Grujic, Katarina; Hole, JP; Wilkinson, JS; Hellesø, Olav Gaute (Journal article; Tidsskriftartikkel; Peer reviewed, 2005)[+][-]
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Grujic, Katarina; Hole, JP; Wilkinson, JS; Hellesø, Olav Gaute (Journal article; Tidsskriftartikkel; Peer reviewed, 2005)[+][-]
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Grujic, Katarina; Hellesø, Olav Gaute (Journal article; Tidsskriftartikkel; Peer reviewed, 2007)[+][-]
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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
article.pdf (238.4Kb)
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