云南天文台丽江高美古和昆明的大气视宁度研究

研究云南天文台昆明凤凰山和丽江高美古两地点的大气视宁度。利用云南天文台于1994年建立的3孔较差视宁度监视仪（3孔DIMM），对这两个地点的视宁度对比观测。还利用云南天文台的1米望远镜所得到的视宁度与3孔DIMM所测的视宁度进行对比。1米望远镜和3孔DIMM同时观测了7个晚上，各取得308组数据，它们的视宁度平均分别为0．90〃和0．84〃。还讨论了3孔DIMM采用不同曝光时间对视宁度测量值的影响，从实验得出3孔DIMM采用20ms曝光比采用8ms曝光所测视宁度的值要好15％左右。从1995年5月－1996年12月，对丽江高美古和昆明凤凰山二地进行了视宁度的对比观测。在高美古和昆明分别观测了234夜和256夜，观测结果是两个点的视宁度平均；丽江为0．70〃，昆明为0．95〃，同时也统计了两个点的月平均视宁度和最好夜的视宁度，并讨论了两地视宁度随时间的变化规律。     

长期视宁度监测DIMM系统设计方案

视宁度参数r0是现代天文选址中需要测量的一个重要物理量,而差分像视宁度监测仪(Differential Image Motion Monitor,DIMM)是目前普遍采用的测量视宁度参数r0的天文仪器。详细介绍了可进行长期视宁度监测的DIMM系统的设计方案,重点讨论了目标星指向、自动导星、自动测量和数据保存等部分。长期视宁度监测DIMM系统的部署,有助于分析已选站址或候选站址的本征视宁度,评价一个站址的成像质量。     

三孔较差视宁度仪的对比观测

三孔较差视宁度仪（ＤＩＭＭ）已在丽江高美古和云南天文台投入观测，我们用云南天文台的一米望远镜测得的视宁度与ＤＩＭＭ所测得的视宁度进行对比。１９９５年上半年获得了５个晚上的资料，测得的结果表明ＤＩＭＭ测量的视宁度与一米望远镜测量的视中度是相关的。二套ＤＩＭＭ也进行了相互校正和对比，共用了８个晚上获得１２５组ｒ０，二套ＤＩＭＭ的视宁度测量值是相关的，二套系统的误差小于１％。     

我国天文视宁度Fried参数r0的计算与统计分析

在塔塔尔斯基理论模式下，使用全国81个探空气象站1986—1995年10年探空资料计算了各测站不同高度上的Pried参数r0，统计结果表明，r0年平均值存在明显的地理分布，高值中心在3000m高度上是夏河合作、丽江和上海；在1500m高度上为民勤和贵阳，在900m高度上为汉中；在测站海拔高度上为定日，r0的月平均值的地理分布存在显著的季节变化，这种变化与东亚副热带急流季节性变化相吻合．     

高美古的大气视宁度

报导了2000年和2001年丽江高美古所观测的大气视宁度，文中统计了14个晚上的观测数据，得到的平均视宁度为0.97″，变化在0.73″～1.42″之间，这些视宁度值是用国际上天文台站测大气视宁度通用的DIMM方法，采用8ms单幅采样曝光取得的。本文还统计了丽江高美古1995年以来每年12月份的大气视宁度，可以看到高美古不同年份大气视宁度的部分变化情况。     

丽江高美古的二期选址——3^#点和6^#点的视宁度对比观测

介绍了丽江高美古二期选址在踏勘和筛选工作的基础上，对3^#点和6^#点的视宁度进行了对比观测。通过对1999年2月6日－1999年12月31日2个点共4756组同时段观测数据的统计和处理得到：3^#点的视宁度平均值为11.2623cm，6^#点的视宁度平均值为11.5952cm。经F分布检验和t分布检验假设成立，作出3^#和6^#点的视宁度无显著性差异的推断，与初期用衍射环观测视宁度的结果一致，进一步说明了高美古整个山头的视宁度都是优良的。     

轿子山大气视宁度测量

本文介绍了轿子山大气视宁度观测的组织实施情况，给出了观测的结果。结果表明；在我们所观测的时间内（４月和５月），轿子山的视宁度是比较好的，在国际上也属上等。此外，我们的观测还表明，山顶的视宁度比半山腰好。因条件所限，其它月份没有得到观测资料，因此对轿子山全年的大气视宁度的情况无法下结论。但是，按照云南省的气象特征和过去的选址经验，４、５月是云南视宁度最差的季节，因此可以推断，轿子山的大气视宁度是比较好的。     

差分像运动视宁度优化监测法

大气视宁度是衡量台址大气光学品质的重要指标。差分像运动视宁度监测仪(Differential Image Motion Monitor,DIMM)被广泛应用于国内外天文选址的视宁度测量作业。介绍了一种优良的视宁度测量方法——差分像运动视宁度优化监测(Improved to Differential Image Motion Monitor,I-DIMM)法。首先对I-DIMM的结构设计和视宁度计算方法进行了详细描述;随后通过设置0. 36 m和0. 12 m两种口径望远镜进行视宁度的模拟测量,将I-DIMM测量结果与传统的DIMM测量的结果进行对比,均证明I-DIMM视宁度计算方法比DIMM更为精确;最后对模拟结果进行了分析,证明了I-DIMM相比于DIMM的优势。     

丽江高美古2号点的大气视宁度观测

介绍了2007年11月至2008年1月15日在丽江高美古2号选址点的大气视宁度及其它天文气象参数观测结果.数据分析显示:观测时间段内高美古2号点平均r 0为8.74cm(FWHM 1.27″), 观测到的r0的极大值为20.47cm(FWHM 0.54″).     

基于振动测量的视宁度测量方法研究

根据像运动法测视宁度的原理,研究了一种新的视宁度测量方法,可以用口径为10 cm左右的望远镜测视宁度,并能排除人为因素和仪器光学质量的影响。原理是用加速度传感器测量望远镜的振动,并从单个星像的抖动中扣除镜筒振动的影响。为此设计了一套测量振动装置,进行了观测,并与差分像运动视宁度监测仪(Differential Image Motion Monitor,DIMM)作了比较,发现该方法在一定程度上消除了望远镜抖动对像运动法测视宁度的影响,但不能完全消除。     

光学天文台址的高度探讨

本文对全球现有天文台址的海拔高度、大望远镜所在地的高度和近十年来的光学天文台的选址在海拔高度上的考虑进行了综述和分析,并对云南境内候选点的高度提出看法。     

Measurements of the magnetic field and the gradient of temperature in the solar atmosphere above a flocculus using radio observations

Simultaneous observations made at several wavelengths in microwave range using the high spatial resolution of radiotelescope RATAN-600 make it possible to develop methods of measuring the magnetic fields in the solar corona and the chromosphere. In this paper we develop a method of measuring the magnetic fields from thermal bremsstrahlung and demonstrate it, using observations of a flocculus (plage) during August 1–3, 1977. The observations show that the flocculus under investigation possessed bipolar magnetic structure with peak to peak amplitude of magnetic field strength of about 40 G at the level of the upper chromosphere and the transition region (with a r.m.s. error of 5.7 G for favourable conditions). The radio astronomical map of the magnetic field is in agreement with the Mt. Wilson magnetic field map to within the experimental error. It follows that the average longitudinal magnetic field above the flocculus does not drop significantly with height above the photosphere up to the CCTR (chromosphere-corona transition region). An analysis of the spectra of polarized radio emission also gives an opportunity to determine the temperature gradient in the CCTR (which proved to amount to about 1000 K km^-1 and to follow their variation with height.     

Annual and semi-annual zonal wind components and corresponding temperature and density variations, 60–130 km

From rocket and radar-meteor wind observations, annual and semi-annual components of the zonal flow are derived for latitudes N at heights between 60 and 130 km. Height regions of maximum and minimum amplitude are described with reference to changes in phase. The annual components decrease with height throughout the mesosphere and, after a reversal of phase, enhance to 25 m/sec at 100 ± 5 km. The semi-annual components have maximum amplitudes of 25 m/sec over a wide range of latitude in two height regions at 90 and 120 km and in a limited range of latitude (near 50°) at 65 km.

Calculated temperatures and log densities are discussed in terms of amplitude and phase as functions of height and latitude. Below 100 km a comparison is made with temperature amplitudes derived from independent temperature data. Above 100 km the annual temperature variation maximizes at 115 km and is particularly large at high latitudes (exceeding 50°K). On the other hand, the semi-annual component increases rapidly with height between 110 and 120 km at all latitudes maximizing at the 120 km level, where amplitudes exceed 25°K at high and low latitudes and 10°K at mid-latitudes. The annual component of log density, like the temperature variation, is largest at high latitudes up to 125 km. The semi-annual variation has a minimum at 110–115 km, above which amplitudes increase with height, reaching 5–12 per cent at 130 km according to latitude. The phases at and near 130 km for the annual and semi-annual density variations are very close to those found at greater heights from satellite orbits and amplitudes could be readily extrapolated to agree with those in the satellite region.     

湍流大气的像质衰减参数及测量

讨论了经过大气后天文图象的象质衰减,研究了对这种衰减进行描述的视宁度参数ｒ０及其它几个大气光学参数,介绍了几种测量象质衰减参数的方法,重点介绍了我们将差分像运动法运用于白日视宁度测量的尝试以及新设计的可兼顾昼夜的视宁度测量仪,仪器已用于云台白日的视宁度测量。最后对近场近似假设进行了定量分析,在此基础上给出了新确定的近场近似成立的范围,这一范围比以前所给的要更大些。     

像运动法测量视宁度参数中曝光时间的重要性及其测定

大气湍流会导致天文图像的像质衰减 ,而视宁度r0 则是描述这种衰减的特征参数。在本文中 ,我们给出了像运动法测量中r0 和曝光时间之间的关系 :曝光时间必须小于大气相干时间 ,否则测量的r0 值将偏大。基于这个原因以及对仪器检测的需要 ,我们必须对视宁度测量仪的曝光时间进行准确地测量。为此我们设计了一种实验方法来测定视频CCD的曝光时间 ,实验的结果表明这种方法是可靠的     

VLA observations of a radio plage at centimeter wavelengths

VLA observations of a solar plage region at 6 and 20 cm wavelengths are presented. The high frequency 6 cm emission correlates well with the associated sunspots, whereas 20 cm emission shows good correlation with the H plage. Large temperature variations over a period of one day are observed in the plage associated component without any significant changes in the sunspots. The dominant emission mechanisms at 6 and 20 cm are found to be gyroresonance radiation and bremstrahlung respectively. It is concluded that the coronal condensation above the chromospheric H plage has an electron density of 5 × 10⁹ cm^–3 and it extends to a height of 5 × 10⁴ km.     

Reconstruction of past decades sea level using thermosteric sea level, tide gauge, satellite altimetry and ocean reanalysis data

This study investigates past sea level reconstruction (over 1950–2003) based on tide gauge records and EOF spatial patterns from different 2-D fields. In a first step, we test the influence on the reconstructed signal of the 2-D fields temporal coverage. For that purpose we use global grids of thermosteric sea level data, available over 1950–2003. Different time spans (in the range 10–50 yr) for the EOF spatial patterns, and different geographical distributions for the 1-D thermosteric sea level time series (interpolated at specific locations from the 2-D grids), are successively used to reconstruct the 54-year long thermosteric sea level signal. In each case we compare the reconstructed trend map with the reference. The simulation indicates that the longer the time span covered by the spatial EOFs, the closer to the reference the reconstructed thermosteric sea level trends. In a second step, we apply the method to reconstructing 2-D sea level data over 1950–2003, combining sparse tide gauge records available since 1950, with EOF spatial patterns from different sources: (1) thermosteric sea level grids over 1955–2003, (2) sea level grids from Topex/Poseidon satellite altimetry over 1993–2003, and (3) dynamic height grids from the SODA reanalysis over 1958–2001. The reconstructed global mean sea level trend based on thermosteric EOFs (case 1) is significantly lower than the observed trend, while the interannual/decadal sea level fluctuations are well reproduced. Case 2 (Topex/Poseidon EOFs over 1993–2003) leads to a global mean sea level trend over the 54-year time interval very close to the observed trend. But the spatial trends of the reconstruction over 1950–2003 are significantly different from those obtained with thermosteric EOFs. Case 3 (SODA EOFs over 1958–2001) provides a reconstruction trend map over 1950–2003 that differs significantly from the previous two cases. We discuss possible causes for such differences. For the three cases, on the other hand, reconstructed spatial trends over 1993–2003 agree well with the regional sea level trends observed by Topex/Poseidon.     

Reconstruction of paleotemperatures in the Northwest Pacific over the past 3000 years from δO values of the micro-bivalvia Carditella iejimensis found in a submarine cave

We measured the δ¹⁸O values of the whole shells of the cavernicolous micro-bivalvia Carditella iejimensis obtained from sediments within a submarine cave (31 m water depth) at Ie Island (Okinawa Island, Japan) in the subtropical Northwest Pacific. Our results show no significant millennial-scale trend in the δ¹⁸O record, implying that both springtime temperature and the δ¹⁸O of sea water at 30 m depth around the Okinawa Islands have been stable for the past 3000 years at values similar to those of today. Moreover, we found one exceptionally light δ¹⁸O value from specimens spanning the past 250 years. The δ¹⁸O-derived temperature represents a departure of 2.1 °C from the average value for the past 250 years, being equal to the departure recorded during unusually high temperatures in the spring of 1998. This finding may imply that such high springtime sea surface temperature has been a rare event over the past 3000 years.     

Reef-island topography and the vulnerability of atolls to sea-level rise

Low-lying reef islands on the rim of atolls are perceived as particularly vulnerable to the impacts of sea-level rise. Three effects are inferred: erosion of the shoreline, inundation of low-lying areas, and saline intrusion into the freshwater lens. Regional reconstruction of sea-level trends, supplementing the short observational instrumental record, indicates that monthly mean sea level is rising in the eastern Indian and western Pacific Oceans. This paper reviews the morphology and substrate characteristics of reef islands on Indo-Pacific atolls, and summarises their topography. On most atolls across this region, there is an oceanward ridge built by waves to a height of around 3 m above MSL; in a few cases these are topped by wind-blown dunes. The prominence of these ridges, together with radiocarbon dating and multi-temporal studies of shoreline position, indicate net accretion rather than long-term erosion on most of these oceanward shores. Less prominent lagoonward ridges occur, but their morphology and continuity are atoll-specific, being a function of the processes operating in each lagoon. Low-lying central areas are a feature of many islands, often locally excavated for production of taro. These lower-lying areas are already subject to inundation, which seems certain to increase as the sea rises. Tropical storms play an important role in the geomorphology of reef islands in those regions where they are experienced. Topographical differences, as well as features such as emergence of the reef flat and the stability of the substrate, mean that islands differ in terms of their susceptibility to sea-level rise. Further assessment of variations in shoreline vulnerability based on topography and substrate could form the basis for enhancing the natural resilience of these islands.