Short time scales of magma-mixing processes prior to the 2011 eruption of Shinmoedake volcano, Kirishima volcanic group, Japan

We estimated time scales of magma-mixing processes just prior to the 2011 sub-Plinian eruptions of Shinmoedake volcano to investigate the mechanisms of the triggering processes of these eruptions. The sequence of these eruptions serves as an ideal example to investigate eruption mechanisms because the available geophysical and petrological observations can be combined for interpretation of magmatic processes. The eruptive products were mainly phenocryst-rich (28 vol%) andesitic pumice (SiO₂ 57 wt%) with a small amount of more silicic pumice (SiO₂ 62–63 wt%) and banded pumice. These pumices were formed by mixing of low-temperature mushy silicic magma (dacite) and high-temperature mafic magma (basalt or basaltic andesite). We calculated the time scales on the basis of zoning analysis of magnetite phenocrysts and diffusion calculations, and we compared the derived time scales with those of volcanic inflation/deflation observations. The magnetite data revealed that a significant mixing process (mixing I) occurred 0.4 to 3 days before the eruptions (pre-eruptive mixing) and likely triggered the eruptions. This mixing process was not accompanied by significant crustal deformation, indicating that the process was not accompanied by a significant change in volume of the magma chamber. We propose magmatic overturn or melt accumulation within the magma chamber as a possible process. A subordinate mixing process (mixing II) also occurred only several hours before the eruptions, likely during magma ascent (syn-eruptive mixing). However, we interpret mafic injection to have begun more than several tens of days prior to mixing I, likely occurring with the beginning of the inflation (December 2009). The injection did not instantaneously cause an eruption but could have resulted in stable stratified magma layers to form a hybrid andesitic magma (mobile layer). This hybrid andesite then formed the main eruptive component of the 2011 eruptions of Shinmoedake.     

Phreatomagmatic eruptions associated with the caldera collapse during the Miyakejima 2000 eruption,Japan

The 2000 AD eruption of Miyakejima was characterized by a series of phreatomagmatic eruptions from the subsiding caldera. Six major eruptive events occurred, and they can be divided into the first and second periods separated by a 25-day hiatus. The phreatomagmatic eruptions produced a total of ~ 2 × 10¹⁰ kg of tephra, which mainly comprised fine-grained volcanic ash. The tephra layers could be divided into six fall units corresponding to the six major eruptive events.     

基于降落相机图像的嫦娥三号着陆轨迹恢复

嫦娥三号降落相机在探测器实施软着陆的过程中获取了大量序列月面图像,这些图像不仅为公众提供了可视化的着陆过程,而且为着陆器在卫星遥感影像上的高精度定位提供了数据基础。在分析这些序列图像成像几何特性的基础上,提出一种利用降落序列图像进行嫦娥三号着陆器轨迹及姿态恢复的方法。首先通过序列影像进行自由网平差,建立各降落影像及月面的相对位置和姿态关系;然后通过量测卫星遥感影像及降落影像上月面标志物的尺寸,求解出建立的相对模型的尺度,从而恢复出序列降落影像在着陆过程中的带尺寸的位置和姿态;最后通过降落影像的密集匹配,完成着陆区域月表3维模型的重建。本文充分挖掘了降落相机的应用潜力,为嫦娥三号着陆任务中后续科学探测工作提供了高精度的数据支持。     

Migration of a volcanic front inferred from K–Ar ages of late Miocene to Pliocene volcanic rocks in central Japan

K–Ar ages have been determined for 14 late Miocene to Pliocene volcanic rocks in the north of the Kanto Mountains, Japan, for tracking the location of the volcanic front through the time. These samples were collected from volcanoes located behind the trench–trench–trench (TTT) triple junction of the Pacific, Philippine Sea, and North American plates. This junction is the site of subduction of slabs of the Pacific and the Philippine Sea plates, both of which are thought to have influenced magmatism in this region. The stratigraphy and K–Ar ages of volcanic rocks in the study area indicate that volcanism occurred between the late Miocene and the Pliocene, and ceased before the Pleistocene. Volcanism in adjacent areas of the southern NE Japan and northern Izu–Bonin arcs also occurred during the Pliocene and ceased at around 3 Ma with the westward migration of the volcanic front, as reported previously. Combining our new age data with the existing data shows that before 3 Ma the volcanic front around the TTT junction was located about 50 km east of the preset‐day volcanic front. We suggest that northward subduction of the Philippine Sea Plate slab ended at ～3 Ma as a result of collision between the northern margin of the plate with the surface of the Pacific Plate slab. This collision may have caused a change in the subduction vector of the Philippine Sea Plate from the original north‐directed subduction to the present‐day northwest‐directed subduction. This indicates that the post ～3 Ma westward migration of the volcanic front was a result of this change in plate motion.     

APOD mission status and preliminary results

On September 20 th, 2015, twenty satellites were successfully deployed into a near-polar circular orbit at 520 km altitude by the Chinese CZ-6 test rocket, which was launched from the Tai Yuan Satellite Launch Center. Among these satellites, a set of 4 Cube Sats conform the atmospheric density detection and precise orbit determination(APOD) mission, which is projected for atmospheric density estimation from in-situ detection and precise orbit products. The APOD satellites are manufactured by China Spacesat Co. Ltd. and the payload instruments include an atmospheric density detector(ADD), a dual-frequency dualmode global navigation satellite system(GNSS) receiver(GPS and Beidou), a satellite laser ranging(SLR) reflector, and an S/Xband very long baseline interferometry(VLBI) beacon. In this paper, we compare the GNSS precise orbit products with colocated SLR observations, and the 3 D orbit accuracy shows better than 10 cm RMS. These results reveal the great potential of the onboard micro-electro-mechanical system(MEMS) GNSS receiver. After calibrating ADD density estimates with precise orbit products, the accuracy of our density products can reach about 10% with respect to the background density. Density estimates from APOD are of a great importance for scientific studies on upper atmosphere variations and useful for model data assimilation.     

嫦娥一号绕月探测卫星精密定轨实现

对探月任务精密定轨技术进行了论述,分析了轨道确定过程中的关键技术问题。基于SMART-1探月卫星测轨数据,对精密定轨软件系统进行了测试验证,3 d数据弧段定轨结果精度优于百米。在嫦娥一号任务实施过程中,各轨道段轨道的计算结果准确,卫星成功进入环月使命轨道,特别是原计划三次中途修正仅执行了一次,为卫星节约了宝贵的燃料。与外部星历互差的结果表明,整个任务阶段定轨精度在百米量级,环月段定轨精度约数十米。实施结果表明,该文给出的定轨技术理论正确,关键技术解决有效,完全满足探月任务工程测控和科学研究的需要。     

COSMIC-2掩星反演数据质量分析

为评价COSMIC-2掩星反演数据精度,利用探空站点资料,对比验证了基于COSMIC-2附加相位数据由无线电掩星数据处理软件ROPP计算得到的大气折射率及温湿廓线。实验结果表明:COSMIC-2数据经反演后的大气廓线质量较高,大气折射率相对偏差标准差为1.5%~4.3%,大气温度和相对湿度均方根误差分别为1.3~2.2 K、10%~15%;低纬地区样本精度总体优于中纬地区; 季节性差异不明显,但冬春季样本精度总体略优于夏秋季。反演数据与COSMIC-2二级产品相比,折射率与温度在部分高度层精度更优,相对湿度在总体上精度更优。