No abstract

In this study, two colloidal dispersions of pure amorphous silicon dioxide particles, acetylated, and heat-treated samples were tested for a possible application as a wood protection agents. The silicon, acetylated, and heat-treated samples were exposed to an accelerated weathering experiment, and their dimensional stability was assessed. The weathering experiment comprised cycles of 2 h UV-light irradiation followed by water spray for 18 min. The surface changes of the weathered samples were characterized by FT-IR spectroscopy and color measurements. According to results, the silicon treatments showed lower color changes than untreated ones. However, acetylated and heat-treated samples provided the lowest color changes. The resistance of the silicon, acetylated, and heat-treated wood to decay was studied by means of brown and white rot fungi in laboratory decay tests. Decay test results revealed that acetylated and heat-treated wood samples showed better decay resistance against P. placenta and C. versicolor than silicon treatments. Samples modified with silicon were exposed in above ground standard lap-joint test in Ultuna, Sweden. The dispersions of pure amorphous silicon dioxide impregnated in wood did not significantly influence its hygroscopic and dimensional behavior. However, the silicon treatment reduced the color changes caused by weathering. The silicon impregnated samples showed a weak fungal discoloration similar to that of chromated copper arsenate impregnated controls in above ground standard lap-joint test.

Continuous optical observations of cusp/cleft auroral activities within ≈ 09‐15 MLT and 70‐76° magnetic latitude are studied in relation to changes in solar wind dynamic pressure and interplanetary magnetic field (IMF) variability. The observed latitudinal movements of the cusp/cleft aurora in response to IMF Bz changes may be explained as an effect of a variable magnetic field intensity in the outer dayside magnetosphere associated with the changing intensity of region 1 field‐aligned currents and associated closure currents. Ground magnetic signatures related to such currents were observed in the present case (January 10, 1993). Strong, isolated enhancements in solar wind dynamic pressure (Δp/p ≥ 0.5) gave rise to equatorward shifts of the cusp/cleft aurora, characteristic auroral transients, and distinct ground magnetic signatures of enhanced convection at cleft latitudes. A sequence of auroral events of ≈ 5‐10 min recurrence time, moving eastward along the poleward boundary of the persistent cusp/cleft aurora in the ≈ 10‐14 MLT sector, during negative IMF Bz and By, conditions, were found to be correlated with brief pulses in solar wind dynamic pressure (0.1 < Δp/p < 0.5). Simultaneous photometer observations from Ny Ålesund, Svalbard, and Danmarkshavn, Greenland, show that the events often appeared on the prenoon side (≈ 10‐12 MLT), before moving into the postnoon sector in the case we study here, when IMF By < 0. In other cases, similar auroral event sequences have been observed to move westward in the prenoon sector, during intervals of positive By. Thus a strong prenoon/postnoon asymmetry of event occurence and motion pattern related to the IMF By polarity is observed. We find that this category of auroral event sequence is stimulated bursts of electron precipitation that originate from magnetosheath plasma that has accessed the dayside magnetosphere in the noon or near‐noon sector, possibly at high latitudes, partly governed by the IMF orientation as well as by solar wind dynamic pressure pulses.

A case study is presented of coordinated ground and space measurements featuring a set of transient, auroral fragments located on the poleward side of a stable cusp/cleft arc. Optical ground data from Ny Alesund (Svalbard) and Heiss Island (Franz Josef Land) were combined with DMSP F9 satellite measurements to examine the characteristics of these auroral features. A stable red arc stretched across most of the dayside auroral zone in a region dominated by westward convection in accordance with the orientation of the IMF. Poleward of the red arc were several, westward moving auroral jets having characteristics similar to midday auroral breakup events. Such events may be ground signatures of transitory magnetic merging at the dayside magnetopause. If so, the driven convective motion of these structures should contribute to the polar cap potential. Within this limited data set we find that although the transitory structures have an inherent potential associated with the motion of the optical signatures the structures on the whole contribute a small fraction of the total polar cap potential.

No abstract