全文获取类型
收费全文 | 414篇 |
免费 | 76篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 96篇 |
大气科学 | 8篇 |
地球物理 | 257篇 |
地质学 | 3篇 |
海洋学 | 7篇 |
天文学 | 14篇 |
综合类 | 69篇 |
自然地理 | 37篇 |
出版年
2022年 | 3篇 |
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 9篇 |
2018年 | 8篇 |
2017年 | 14篇 |
2016年 | 18篇 |
2015年 | 19篇 |
2014年 | 20篇 |
2013年 | 22篇 |
2012年 | 23篇 |
2011年 | 31篇 |
2010年 | 17篇 |
2009年 | 12篇 |
2008年 | 18篇 |
2007年 | 25篇 |
2006年 | 15篇 |
2005年 | 11篇 |
2004年 | 8篇 |
2003年 | 9篇 |
2002年 | 5篇 |
2001年 | 14篇 |
2000年 | 44篇 |
1999年 | 38篇 |
1998年 | 45篇 |
1997年 | 9篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1987年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有491条查询结果,搜索用时 484 毫秒
101.
层析模型在GNSS探测电离层中的研究进展 总被引:1,自引:0,他引:1
在利用全球导航卫星系统GNSS(GPS、GLONASS、Galileo等卫星定位系统)进行精密定位和导航时,电离层延迟误差是影响其精度和准确度的主要误差源之一,故对电离层模型研究至关重要。本文介绍了传统电离层模型的缺点,重点阐述了不同的电离层层析方法,同时分析了现在电离层层析方法所存在的主要问题,最后介绍了目前全世界电离层层析模型的研究展望。 相似文献
102.
103.
J. W. MacDougall D. A. Andre G. J. Sofko C.-S. Huang A. V. Koustov 《Annales Geophysicae》2001,18(12):1550-1559
Based on modeling of the perturbations in power and elevation angle produced by travelling ionospheric disturbances (TIDs), and observed by the Super Dual Auroral Radar Network, procedures for determining the TID properties are suggested. These procedures are shown to produce reasonable agreement with those properties of the TIDs that can be measured from simultaneous ionosonde measurements. The modeling shows that measurements of angle-of-elevation perturbations by SuperDARN allows for better determination of the TID properties than using only the perturbations of power as is commonly done.On sabbatical from Department Electrical Engineering, University of Western Ontario, London, Ontario, Canada 相似文献
104.
Ionospheric electron content (IEC) observed at Delhi (geographic co-ordinates: 28.63°N, 77.22°E; geomagnetic co-ordinates: 19.08°N, 148.91E; dip Latitude 24.8°N), India, for the period 1975/80 and 1986/89 belonging to an ascending phase of solar activity during first halves of solar cycles 21 and 22 respectively have been used to study the diurnal, seasonal, solar and magnetic activity variations. The diurnal variation of seasonal mean of IEC on quiet days shows a secondary peak comparable to the daytime peak in equinox and winter in high solar activity. IECmax (daytime maximum value of IEC, one per day) shows winter anomaly only during high solar activity at Delhi. Further, IECmax shows positive correlation with F10.7 up to about 200 flux units at equinox and 240 units both in winter and summer; for greater F10.7 values, IECmax is substantially constant in all the seasons. IECmax and magnetic activity (Ap) are found to be positively correlated in summer in high solar activity. Winter IECmax shows positive correlation with Ap in low solar activity and negative correlation in high solar activity in both the solar cycles. In equinox IECmax is independent of Ap in both solar cycles in low solar activity. A study of day-to-day variations in IECmax shows single day and alternate day abnormalities, semi-annual and annual variations controlled by the equatorial electrojet strength, and 27-day periodicity attributable to the solar rotation. 相似文献
105.
High time resolution data from the CUTLASS Finland radar during the interval 01:30–03:30 UT on 11 May, 1998, are employed to characterise the ionospheric electric field due to a series of omega bands extending 5° in latitude at a resolution of 45 km in the meridional direction and 50 km in the azimuthal direction. E-region observations from the STARE Norway VHF radar operating at a resolution of 15 km over a comparable region are also incorporated. These data are combined with ground magnetometer observations from several stations. This allows the study of the ionospheric equivalent current signatures and height integrated ionospheric conductances associated with omega bands as they propagate through the field-of-view of the CUTLASS and STARE radars. The high-time resolution and multi-point nature of the observations leads to a refinement of the previous models of omega band structure. The omega bands observed during this interval have scale sizes 500 km and an eastward propagation velocity 0.75 km s–1. They occur in the morning sector (05 MLT), simultaneously with the onset/intensification of a substorm to the west during the recovery phase of a previous substorm in the Scandinavian sector. A possible mechanism for omega band formation and their relationship to the substorm phase is discussed. 相似文献
106.
Meridian-scanning photometer, coherent HF radar, and magnetometer observations of the cusp: a case study 总被引:2,自引:0,他引:2
S. E. Milan M. Lester S. W. H. Cowley J. Moen P. E. Sandholt C. J. Owen 《Annales Geophysicae》1999,17(2):159-172
The dynamics of the cusp region and post-noon sector for an interval of predominantly IMF By, Bz < 0 nT are studied with the CUTLASS Finland coherent HF radar, a meridian-scanning photometer located at Ny Ålesund, Svalbard, and a meridional network of magnetometers. The scanning mode of the radar is such that one beam is sampled every 14 s, and a 30° azimuthal sweep is completed every 2 minutes, all at 15 km range resolution. Both the radar backscatter and red line (630 nm) optical observations are closely co-located, especially at their equatorward boundary. The optical and radar aurora reveal three different behaviours which can interchange on the scale of minutes, and which are believed to be related to the dynamic nature of energy and momentum transfer from the solar wind to the magnetosphere through transient dayside reconnection. Two interpretations of the observations are presented, based upon the assumed location of the open/closed field line boundary (OCFLB). In the first, the OCFLB is co-located with equatorward boundary of the optical and radar aurora, placing most of the observations on open field lines. In the second, the observed aurora are interpreted as the ionospheric footprint of the region 1 current system, and the OCFLB is placed near the poleward edge of the radar backscatter and visible aurora; in this interpretation, most of the observations are placed on closed field lines, though transient brightenings of the optical aurora occur on open field lines. The observations reveal several transient features, including poleward and equatorward steps in the observed boundaries, braiding of the backscatter power, and 2 minute quasi-periodic enhancements of the plasma drift and optical intensity, predominantly on closed field lines. 相似文献
107.
R. G. Rastogi 《Annales Geophysicae》1999,17(2):210-219
The study describes the time and space morphologies of a rather new type of counter electrojet event on the basis of data from the excellent chain of magnetic and ionospheric observatories along the Indo-Russian longitude sector. Abnormally large westward currents are observed during almost the whole of the daytime hours on a series of days. These events do not form any vortices in the current system and do not apparently seem to be associated with tidal effects or any solar magnetosphere events or geomagnetic disturbances. The existence of a westward electric field over the equatorial ionosphere has been confirmed by the absence of an equatorial type of sporadic E in the ionograms at Thumba precisely during the periods when H at Trivandrum minus H at Alibag is negative. The equatorial F region anomaly was also absent on the counter electrojet day. Such counter electrojet events during the northern winter months of low solar activity years are suggested to be the result of the modified wind system in the ionosphere associated with stratospheric warming events. 相似文献
108.
利用南极长城站(62°13′S,58°58′W)频高图和磁照图研究了1989年3月13日磁暴的亚极光区电离层效应。在紧接着磁暴急始后F层的虚高h′F急剧上升,临频f0F2急剧下降,然后出现扩展F并持续几小时。在磁暴主相,电离层出现严重的吸收,但有时仍能观测到h′F的增加和伴随着的f0F2的降低。在磁暴急始后的第二和第三个晚上观测到极光型Es和夜间E层,其峰值电子浓度高达7.5×105el/cm3,文中讨论了造成这些现象的原因。在同一经度扇区的4个电离层站的h′F和f0F2的行为表明,观测站的纬度越高出现的吸收越严重,f0F2呈下降的时间越长。 相似文献
109.
Effects on the Ionosphere Due to Phenomena Occurring Below it 总被引:4,自引:0,他引:4
The terrestrial thermosphere and ionosphere form the most variable part of theEarth's atmosphere. Because our society depends on technological systems thatcan be affected by thermospheric and ionospheric phenomena, understanding,monitoring and ultimately forecasting the changes of the thermosphere–ionosphere system are of crucial importance to communications, navigation and the exploration of near-Earth space. The reason for the extreme variability of the thermosphere–ionosphere system isits rapid response to external forcing from various sources, i.e., thesolar ionizing flux, energetic charged particles and electric fields imposed via the interaction between the solar wind, magnetosphere and ionosphere, as well as coupling from below (meteorological influences) by the upward propagating, broad spectrum,internal atmospheric waves (planetary waves, tides, gravity waves) generated in thestratosphere and troposphere. Thunderstorms, typhoons, hurricanes, tornadoes andeven seismological events may also have observable consequences in the ionosphere.The release of trace gases due to human activity have the potential to cause changes inthe lower and the upper atmosphere.A brief overview is presented concerning the discoveries and experimentalresults that have confirmed that the ionosphere is subject to meteorologicalcontrol (especially for geomagnetic quiet conditions and for middle latitudes).D-region aeronomy, the winter anomaly of radiowave absorption, wave-liketravelling ionospheric disturbances, the non-zonality and regional peculiaritiesof lower thermospheric winds, sporadic-E occurrence and structure, spread-Fevents, the variability of ionospheric electron density profiles and Total ElectronContent, the variability of foF2, etc., should all be considered in connection withtropospheric and stratospheric processes. Ionospheric weather, as a part of spaceweather, (i.e., hour-to-hour and day-to-day variability of the ionospheric parameters)awaits explanation and prediction within the framework of the climatological, seasonal,and solar-cycle variations. 相似文献
110.