A comparison of archaeointensity results from Chinese ceramics using microwave and conventional Thellier's and Shaw's methods

Palaeo- and archaeointensity determinations of the geomagnetic field are limited by mineralogical alteration that may occur when samples are heated in the laboratory to produce a TRM ( Thellier & Thellier 1959 ; Shaw 1974 ). By using microwave excitation of the magnetic grains we have been able to create a TRM without significantly heating the bulk samples, thereby avoiding thermal alteration ( Walton et al . 1993 ; Shaw et al . 1996 ). When applied to Peruvian ceramics in a manner analogous to the standard Thellier method, the microwave technique was found to reduce the scatter dramatically in the archaeointensity results ( Shaw et al . 1996 ). In the present study we apply this microwave technique to a collection of Chinese ceramics covering the time interval 2700–7500  yr BP. These ceramics have already been investigated using adaptations of both Thellier's ( Coe 1967 ) and Shaw's ( Rolph & Shaw 1985 ) palaeointensity methods ( Yang et al . 1993a ). Although an acceptable agreement was found between those two methods, the equivalent virtual axial dipole moments (VADMs) were significantly lower than for the global model of McElhinny & Senanayake (1982 ). The present study, using the microwave technique with cooling-rate correction, has provided more consistent VADMs, and is much closer to the global model.     

小旋转椭球粒子群的微波衰减系数与雷达反射率因子之间的关系

通过模拟及取样导出了小旋转椭球粒子群旋转轴呈 3种不同取向 ,而入射电磁波分别为水平发射水平偏振波及水平发射垂直偏振波时的衰减系数与雷达反射率因子之间的关系 ,获得 3种波长的具体表达式 ,并对结果作了物理分析。所得结果可直接用于雷达定量测量降水时的衰减订正。     

海面微波散射与风生波短波谱

从海面微波散射的物理机制出发 ,在短波平衡范围条件下 ,讨论了风生波短波谱形式 ,通过对不同波段的微波散射数据分析 ,建立起能适应相当宽微波波段 ( 0 .42 8— 34.4GHz)的海面微波散射模型。文中对若干个波段的微波散射系数进行了试算 ,并与实测资料进行了比较 ,结果是令人满意的     

Magnetic field intensity study of the 1960 Kilauea lava flow, Hawaii, using the microwave palaeointensity technique

It is extremely valuable to study historic lava flows where the geomagnetic field at their time of extrusion is well known. In this study, two vertical sections, 16 m apart, have been sampled from the approximately 1 m thick 1960 Kilauea lava flow, Hawaii. Variations are seen in the rock-magnetic and palaeomagnetic properties between and within the two sections, indicating that there are small-scale lateral and vertical variations in the lava flow. The two sections showed different responses to microwave palaeointensity analysis. Section H6001 generally gave ideal linear behaviour on plots of natural remanent magnetization (NRM) lost against microwave-induced thermoremanent magnetization (T_M RM) gained, whilst the majority of samples from H6002 showed anomalous two-slope behaviour. When all plots were interpreted by taking the best-fitting line through all points, the flow mean intensity for H6001 was 31.6 ± 3.6 μT and that for H6002 was 37.1 ± 6.4 μT, compared with the expected intensity of 36 μT. Additional historic flows need to be studied in order to ascertain whether this behaviour is typical of all lava, and whether it is best to always interpret NRM lost/T_M RM gained plots by taking the line of best fit regardless of shape.     

Western disturbances seen with AMSU-B and infrared sensors

Western disturbances (WD) of winter and pre-monsoon seasons are the important sources of rainfall in the Indo-Gangetic plains. WDs are troughs or circulations in the westerly winds modified by the Himalayas. Operationally, WDs are monitored using infrared (IR) and water vapour (WV) images. Advanced Microwave Sounding Unit-B (AMSU-B), flying onboard the NOAA satellites, also allows WDs to be monitored in five microwave frequencies. Two are in water vapour window (89, 150 GHz) and three are absorption channels (centred at 183.31 GHz). Unlike the top of cloud view in IR or WV, AMSU-B radiances show the effect of moisture and hydrometeors in different layers. Two cases of WD (17 April 2001 and 18–19 February 2003) are discussed using the microwave data from AMSU-B and the IR and WV data from Meteosat-5. The aim here is to demonstrate the skill of AMSU-B in delineating structure of WDs. In particular, the cold intrusion and the moist conveyor belts are examined. It was found that the multi-channel view of the AMSU-B permits a better understanding of the moist structures seen in the conveyor belts. The à trous wavelet transform is used to clearly bring out mesoscale features in WDs. AMSU-B brings out intense convection as a large depression of BTs (>50K) at 150/176 GHz, cirrus and moist bands at 180/182 GHz. Mesoscale convection lines within WDs that last short time are shown here for the first time only in the AMSU-B images. Large-scale cirrus features are separated using the à trous wavelet transform. Lastly, it is shown that there is a good likeness in the rain contours in the 3-h rain 3B42 (computed from TRMM and other data) to AMSU-B depressions in BT. Overall, AMSU-B shows better skill in delineating the structure of clouds and rain in WDs.     

A new formula on the Fresnel reflectance and its application in microwave remote sensing

As an important physical parameter in the elec-tromagnetic (E-M) theory, the Fresnel reflectance isabsolutely a necessary variable in oceanic remotesensing. For example, the thermal-infrared emissivityof sea surface temperature (SST), and the microwavetheory of passive remote sensing of SST and sea sur-face salinity (SSS) are both connected with it[1—7]. Themicrowave theory of active remote sensing of sea sur-face wind speed is also related to the Fresnel reflec-tance[8—15]. The Fresne…     

Palaeointensity results for historic lavas from Mt Etna using microwave demagnetization/remagnetization in a modifiedThellier-type experiment

The bane of conventional Thellier-type palaeointensity experiments is the thermal alteration of the samples during experimentation. However, high-frequency microwaves can be used to stimulate the magnetic minerals directly, thus eliminating the need for the bulk sample to be heated ( Walton et al . 1993 ). The microwave technique has been successfully applied to ceramics ( Shaw et al . 1996 , 1999 ), and here we present results for historic lavas from Mt Etna, Sicily. 20 samples were randomly selected from 10 different historic flows dating from 1843 to 1983. Hysteresis parameters were monitored as a check for alteration during experimentation and minimal alteration was found. Rock-magnetic analysis and AF plus microwave demagnetizations were carried out on all samples, and microwave intensity analysis was carried out on those samples that were stable to microwave demagnetization (16 in total). With one exception, all samples gave high-quality intensity results. Two or three subsamples from each sample were investigated and a weighted average calculated to give a single estimate of the field. When the intensity values were compared with the SV model based on direct observatory measurements ( Bloxham & Gubbins 1985 ), it was found that those samples that contained a high multidomain (MD) component gave intensity values around 20 per cent lower than expected. This can be explained by the cooling rate effect ( Dodson & McClelland-Brown 1980 ) and/or the presence of an MD component ( McClelland et al. 1996 ). Those samples with the lowest numbers of MD grains gave field values which, within error, were the same as those for the model, or slightly too high: this is compatible with the cooling rate effect.     

Evaluation of SMOS soil moisture retrievals over the central United States for hydro-meteorological application

Soil moisture has been widely recognized as a key variable in hydro-meteorological processes and plays an important role in hydrological modelling. Remote sensing techniques have improved the availability of soil moisture data, however, most previous studies have only focused on the evaluation of retrieved data against point-based observations using only one overpass (i.e., the ascending orbit). Recently, the global Level-3 soil moisture dataset generated from Soil Moisture and Ocean Salinity (SMOS) observations was released by the Barcelona Expert Center. To address the aforementioned issues, this study is particularly focused on a basin scale evaluation in which the soil moisture deficit is derived from a three-layer Xinanjiang model used as a hydrological benchmark for all comparisons. In addition, both ascending and descending overpasses were analyzed for a more comprehensive comparison. It was interesting to find that the SMOS soil moisture accuracy did not improve with time as we would have expected. Furthermore, none of the overpasses provided reliable soil moisture estimates during the frozen season, especially for the ascending orbit. When frozen periods were removed, both overpasses showed significant improvements (i.e., the correlations increased from r = −0.53 to r = −0.65 and from r = −0.62 to r = −0.70 for the ascending and descending overpasses, respectively). In addition, it was noted that the SMOS retrievals from the descending overpass consistently were approximately 11.7% wetter than the ascending retrievals by volume. The overall assessment demonstrated that the descending orbit outperformed the ascending orbit, which was unexpected and enriched our knowledge in this area. Finally, the potential reasons were discussed.