T?p chí Khoa h?c và Phát tri?n,&T. 6, S. 5 (2008)
?nh h??ng c?a stress nhi?t ??n l??ng n??c u?ng, th?c ?n thu nh?n và n?ng su?t s?a c?a ?àn bò lai h??ng s?a nu?i t?i huy?n Ngh?a ?àn, t?nh Ngh? An trong mùa hè
??ng Thái H?i
Theo d&i ???c ti?n h&nh tr&n 6 b& lai F1 (50% m&u HF) v& 6 b& F2 (75% m&u HF) nu&i trong n&ng h? t?i Ngh?a ?&n, Ngh? An trong m&a h& nh?m x&c ??nh ?nh h??ng c?a stress nhi?t ??n l??ng th?c ?n thu nh?n, n??c u?ng v& n?ng su?t s?a. K?t qu? cho th?y khi THI t?ng, l??ng n??c u?ng c?ng t?ng, song l??ng th?c ?n thu nh?n v& n?ng su?t s?a gi?m. L??ng th?c ?n thu nh?n v& n?ng su?t s?a c& t??ng quan &m, c&n l??ng n??c u?ng c& t??ng quan d??ng v?i THI. So v?i F1, b& F2 b? ?nh h??ng nhi?u h?n
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KH? N?NG D? B?O foF2 ?I?N LY X?CH ??O T? VI?T NAM V? ?NG D?NG CHO TRUY?N S?NG V? TUY?N HF
Hoàng Thái Lan, V?nh Hào, D??ng V?n Vinh, ?ào Ng?c H?nh T?m
Predictions foF2 of equatorial ionosphere and applications for HF communication in VietnamThis paper presents the characteristics of the equatorial ionospheric F2 layer over Vietnam based on continuousmonitoring data for 11 years from ionospheric station in Ho Chi Minh city (10.50’N, 106.33’E). These results determinethe diurnal and seasonal variations with solar activity of the F2 layer. The ionospheric parameters were taken fromionograms are often used as the control elements in forecasting. The index representing levels of solar activity is thesunspot number R considered the basic indicators for modeling and forecasting the ionosphere. An important parameteris the maximum plasma frequency, because this is what determines which vertical transmissions will pass through andwhich will be reflected back. This maximum plasma frequency is called the critical frequency, fo. Because the plasmafrequency varies for a given time and location, the critical frequency also varies. However the critical frequency is alwayswithin the HF range. This is what gives HF transmissions their unique characteristics. The highest plasma frequency is inthe F layer (or more specifically the F2 layer). This critical frequency may be referred to using the symbol foF2.Once we know the critical frequency, we can determine the proper operating frequency. The theory behind theselection is straight-forward. We need to select the frequency that is the highest possible without passing through the F2-layer. There are three concepts to recognize. First, the critical frequency for the F2-layer is foF2. This is the highestfrequency that will be reflected back to earth from the F2-layer when hitting the F2-layer at a perpendicular angle, i.e. theangle of incidence is 0 degrees. Go higher in frequency and the signal will pass thorough the F2-layer into space and islost for communications. Second, MUF, or maximum useable frequency, the highest frequency we can use and stillachieve good reflection/refraction off the F2-layer. For any circuit there is a Maximum Usable Frequency (MUF) which isdetermined by the state of the ionosphere in the vicinity of the refraction areas and the length of the circuit. The MUF isrefracted from the area of maximum electron density of a region. Therefore, frequencies higher than the MUF for aparticular region will penetrate that region. The third concept is the F region MUF in particular varies during the day,seasonally and with the solar cycle. A range of F region MUF is provided in the predictions and this range extends fromthe lower decile MUF (called the Optimum Working Frequency, OWF) to the upper decile MUF. Based on actualmonitoring data, the empirical formulas for prediction foF2 in quiet solar conditions have been built. Collected datadisplays the full range of these variations for the location and can give MUF for every hour of the day (24 values), foreach month of the year. The paper presents empirical formulas prediction foF2 calculated for 08 LT (morning’s maximumof ionization), 12 LT (noon) and 16 LT ( afternoon’s maximum of ionization). These formulas are also been modified forsolstice months (June - July and December - January). The calculated foF2 values were comparing with the observedand IRI values. The IRI model was initiated by the Committee on Space Research (COSPAR) and the InternationalUnion of Radio Science (URSI) in the 1969 with the goal of establishing an international standard for the basicparameters of the ionosphere, based on ground monitoring of data in many parts of the world and the satellite data. TheIRI model has been steadily improved with newer data and with better mathematical descriptions of global and temporalvariation patterns. The IRI model has become widely accepted recently so that the first task of an ionospheric science ina new location (as well as stations in Vietnam) is often comparing the observed data with IRI. The result showed that thecalculated foF2 values by empirical formulas better than by IRI, particularly for 16 LT. The application of these predictionformulas for HF communication for the social-economic development in Vietnam has also been discussed.
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??C ?I?M H?NH TH?I V? TH?NH PH?N H?A H?C C?A ZIRCON TRONG GRANITOID KH?I N?NG PHAN SI PAN: ? NGH?A C?A CH?NG TRONG VI?C X?C ??NH NGU?N G?C ?? V?
L?A CH?N CHO PH?N T?CH ??NG V?
Ph?m Th? Dung, Tr?n Tr?ng Hòa, Tr?n Tu?n Anh, Ng? Th? Ph??ng, Nguy?n Vi?t ?, Hoàng Th? Vi?t H?ng, V? Th? Th??ng, V? Hoàng Ly
Morphology and chemistry of zircon from Phanerozoic granitoids in Fansipan uplift: their significance in determination of rock type and select for isotopic analysis Based on morphological characteristics and chemical composition of zircon in the Phanerozoic granitoids in Fansipan uplift, identificable signatures of primary zircon inigneous rocks are initially established. On morphological characteristics: zircon crystals intact crystal shape with two pointed edges (tetragonal prism or pyramid crystals), is quite clear, usually colorless, yellow-brown, brown-pink color,.. Chemical characteristics of zircon: HfO2 &2%, the Zr/Hf ratio in zircon from 17 to 380, these values prove the magmatic origin and correlate with the high total alkaline rocks: syenite, nepheline syenite, granosyenite,.. These characteristics have to be noted in the selection process for zircon isotopic analysis. Based on geochemical characteristics, zircon of two rock types are initially distinguished: 1/ Zircon in I-type granite (consist of granosyenite, alkaline granosyenite, quartz syenitodiorite, syenite,....), characterized by: HfO2 content & 2 wt.%, the Zr/Hf ratio is high (&100), the Hf/(Hf + Zr) ratio &0.025; and 2/ Zircon in A-type granite (afvedsonite bearing granite), characterized by: HfO2 content & 2.5 wt.%, low Zr/Hf ratio (&20), Hf/(Hf + Zr) ratio &0.025, and ThO2 and UO2 content tends to be higher than the first type.
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Vietnam Journal of Earth Sciences, ISSN: 0866 - 7187Contact address:tcckhvtd@vjs.ac. ; Index: ; ; ; ; (C) Vietnam Academy of Science and Technology (VAST)