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Lyu Chuangxin Nishimura Satoshi Amiri Seyed Ali Ghoreishian Zhu Feng Eiksund Gudmund Reidar Grimstad Gustav 《Acta Geotechnica》2021,16(12):3831-3847
Acta Geotechnica - A systematical testing program on frozen Onsøy clay under isotropic loading and undrained shearing at different temperatures... 相似文献
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Asta Pellinen-Wannberg Edmond Murad Gudmund Wannberg Assar Westman 《Earth, Moon, and Planets》2004,95(1-4):627-632
High Power Large Aperture (HPLA) radars generally observe very high meteor velocities averaging over 50 km s−1. There are only a few events recorded around 30 km s−1, while meteors at 20 km s−1 or slower are very rare. This is a clear and debated contradiction to specular meteor radar results. A high plasma density
condition contributes, but the dominating phenomenon is the hyperthermal ionization mechanism due to chemical dynamics of
the ionization process. The observed high velocities can be explained in terms of high hyperthermal ionization cross-sections
for collisions between ablated meteoroid metal atoms such as Na and/or Fe and atmospheric species. 相似文献
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Incoherent scatter radars (ISR) are versatile instruments for continuous monitoring of ionisation processes in the Earths atmosphere. EISCAT, The European Incoherent Scatter facility has proven effective also in meteor studies. The time resolution of the radar can be reduced to a few milliseconds, sufficient to resolve the passage of individual meteors through the narrow ISR beam. Methods for group and phase velocity determination of the meteoroids and the discrepancy between the results related to the target behaviour are presented. The radar cross sections of echoes associated with moving meteoroids (–meteor head echoes) are very small and increase with decreasing wavelength. The parent meteoroids are found to have visual magnitudes far below the detection limit of most optical observations. The equivalent visual magnitude limit of the smallest objects observed by EISCAT in the current experiments has been estimated by two different methods, both from the cross-section measurements and from the measured event rates. Both methods give a limit value of +10 for the smallest objects while the upper limit is +4. The lower limit of the visual magnitude for the collocated optical measurement system is +4. Thus the two detection systems observe two different meteor size ranges, with the radar almost reaching micrometeorite population. Meteor fluxes estimated from the event rates and the radar system parameters agree well with previous extrapolated values for this size range. 相似文献
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Csilla Szasz Johan Kero Asta Pellinen-Wannberg David D. Meisel Gudmund Wannberg Assar Westman 《Earth, Moon, and Planets》2008,102(1-4):373-378
We have investigated the conditions for simultaneous meteor observations with the EISCAT UHF radar system and telescopic optical
devices. The observed characteristics of 410 meteors detected by all three UHF receivers are compared with model simulations
and their luminosity is calculated as a part of a meteoroid ablation model using a fifth order Runge–Kutta numerical integration
technique. The estimated absolute visual magnitudes are in the range of +9 to +5. The meteors should therefore be observable
using intensified CCD or EMCCD (Electron Multiplying CCD) cameras with telephoto lenses. A possible setup of a coordinated
radar and optical campaign is suggested. 相似文献
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Amato Giorgia Andò Edward Lyu Chuangxin Viggiani Gioacchino Eiksund Gudmund Reinar 《Acta Geotechnica》2022,17(1):327-338
Acta Geotechnica - This short paper proposes time-series x-ray tomography as a valuable tool for quantifying freezing processes in a remoulded clay. As an example, three simple closed system... 相似文献
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Johan Kero Csilla Szasz Asta Pellinen-Wannberg Gudmund Wannberg Assar Westman 《Earth, Moon, and Planets》2004,95(1-4):633-638
We present observations and preliminary results from a meteor experiment carried out with the 224 MHz EISCAT VHF radar in Tromsø, Norway, which was run for 6 h on November 26, 2003. The data set contains echoes with peculiar pulsations in received power in the frequency range 20–200 Hz, limited by instrumental parameters. The process causing the echo power pulsations has not yet been identified. Plasma effects are the most likely cause, a possible mechanism is for instance asymmetrical dust grains in rotation causing a modulation of the ionization rate. 相似文献
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C. Szasz J. Kero D. D. Meisel A. Pellinen-Wannberg G. Wannberg A. Westman 《Monthly notices of the Royal Astronomical Society》2008,388(1):15-25
The tristatic EISCAT 930-MHz UHF system is used to determine the absolute geocentric velocities of meteors detected with all three receivers simultaneously at 96 km, the height of the common radar volume. The data used in this study were taken between 2002 and 2005, during four 24-h runs at summer/winter solstice and vernal/autumnal equinox to observe the largest seasonal difference. The observed velocities of 410 tristatic meteors are integrated back through the Earth atmosphere to find their atmospheric entry velocities using an ablation model. Orbit calculations are performed by taking zenith attraction, Earth rotation as well as obliquity of the ecliptic into account. The results are presented in the form of different orbital characteristics. None of the observed meteors appears to be of extrasolar or asteroidal origin; comets, particularly short-period (<200 yr) ones, may be the dominant source for the particles observed. About 40 per cent of the radiants can be associated with the north apex sporadic meteor source and 58 per cent of the orbits are retrograde. There is evidence of resonance gaps at semimajor axis values corresponding to commensurabilities with Jupiter, which may be the first convincing evidence of Jupiter's gravitational influence on the population of small sporadic meteoroids surveyed by radar. The geocentric velocity distribution is bimodal with a prograde population centred around 38 km s−1 and a retrograde population peaking at 59 km s−1 . The EISCAT radar system is located close to the Arctic Circle, which means that the North Ecliptic Pole (NEP) is near zenith once every 24 h, i.e. during each observational period. In this particular geometry, the local horizon coincides with the ecliptic plane. The meteoroid influx should therefore be directly comparable throughout the year. 相似文献
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