A Search for Meteoroid Lunar Impact Generated Electromagnetic Pulses     

Saiveena Kesaraju  John D. Mathews  Juha Vierinen  Phil Perillat  David D. Meisel 《Earth, Moon, and Planets》2016,117(1):1-22

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基于NoSREx实验的积雪演化及微波辐射分析研究     

周静恬  邱玉宝  黄琳  LEMMETYINEN Juha  石利娟  李青寰  施建成 《冰川冻土》2022,44(5):1501-1512

在被动微波雪水当量反演中,积雪物理参数随时间的变化特征影响着反演精度,为理解积雪随时间演化的特征及其对微波辐射亮温的影响,本研究选用2009—2013年北欧积雪实验（Nordic Snow Radar Experiment, NoSREx）积雪地面观测和微波辐射测量数据,通过雪深和温度把雪期分为积累期（10月—次年2月）、稳定期（2—4月）和消融期（4—5月）,发现各个雪期的积雪演化特征为：雪颗粒形状在积累期前期以融态颗粒（Melt Forms, MF）为主,积累期后期和稳定期以圆形颗粒、片状颗粒、深霜为主,消融期以MF为主;整个雪季底层雪粒径从小变大再变小的过程,粒径最大值出现在稳定期的2至3月,约为2.5~4.0 mm,均出现在近地表雪层,而表层粒径较小且较为稳定。通过雪深和微波亮度差（18~37 GHz）的关系分析,表明亮温差在不同雪期对于雪深的依赖关系不同,在积累期和稳定期,雪深变化与亮温差变化具有明显的正相关;在消融期由于积雪融化的影响,其相关性较差;基于多层积雪微波辐射模型（MEMLS）构建了一维微波辐射模拟环境,模拟表明MEMLS模型在3个雪期的垂直极化10.65 GHz和18.7 GHz模拟结果较37 GHz和90 GHz更好;10.65 GHz V极化在入射角为50°且稳定期时,微波亮温模拟均方根误差（RMSE）结果最小,为2.49 K。3个雪期90 GHz模拟结果水平极化优于垂直极化,由于受表层积雪变化影响,90 GHz模拟结果较不稳定,尤其是消融期时,RMSE最小也达到了42.7 K。本研究有助于理解积雪随时间演化的特征及其对微波辐射模拟的影响,表明在被动微波雪水当量反演算法中,针对不同积雪期需要考虑积雪演化动态过程。  相似文献   

Simulation of the apparent anomalies of the galactic structure     

T. Jaakkola  N. Holsti  E. Laurikainen  P. Teerikorpi 《Astrophysics and Space Science》1984,107(1):85-107

In the maps of the galactic structure based on the kinematical method, several systematic heliocentric anomalies are found: in the northern galactic hemisphere the spiral arms are more tightly wound and the extent of neutral hydrogen is smaller than in the southern hemisphere; with separate rotation curves for the north and the south the arms become anomalously circular with a consequent discrepancy to the stellar distribution; there are straight portions in the arms pointing towards the Sun, as well as systematic strong curvatures and knee-like features; the inner arms affect the structure of the outer arms; with the northern rotation model, Hii-regions and Hi avoid the southern tangential circle; in the rear of the Galaxy, at symmetric longitudes, enhanced Hi-densities are found; the Perseus arm is displaced atl=180°. All of these anomalies can be explained with a simple model involving a non-velocity redshift field within the Galaxy, with an enhancement within the spiral arms. This is demonstrated by numerical simulations of the structural anomalies. Reducing the redshift effect from the kinematic data, the Galaxy's structure and kinematics appear symmetric. The significance of the result for the redshift problem is discussed.  相似文献   

Interpretation of the apparent north-south asymmetry and fluctuations of galactic rotation     

T. Jaakkola  M. Moles  J. P. Vigier 《Astrophysics and Space Science》1978,58(1):99-112

A model of the apparent north-south asymmetry of the rotation curve of the Galaxy and of the asymmetry appearing for the rotation of its outskirts has been presented, in terms of interaction redshifts. Fluctuations in the rotation curve for neutral hydrogen are discussed and interpreted as expanding motions in the arms. The expansion of the disk of the Galaxy which has been suggested previously as an explanation for the asymmetry has been shown to meet some serious difficulties. A remarkable north-south symmetry both in the spatial distribution and in the kinematics of neutral hydrogen in the subsolar region follows from the present model.  相似文献   

Temporal and spatial variations of global ionospheric total electron content under various solar conditions     

Jingbin Liu  Manuel Hernandez-Pajares  Xinlian Liang  Jiachun An  Zemin Wang  Ruizhi Chen  Wei Sun  Juha Hyyppä 《Journal of Geodesy》2017,91(5):485-502

By utilizing the numerical technique of principal component analysis (PCA), this work analyses temporal and spatial variations of the ionosphere under various solar conditions during the period 1999–2013. Applying the PCA technique to the time series of the global ionospheric total electron content (TEC) maps provides an efficient method for analyzing the main ionospheric variability on a global scale that is able to decompose periodic variations (e.g., annual and semiannual oscillations) while retaining the asymmetry in the temporal and spatial domains (e.g., seasonal and equator anomalies). The TEC series of different local times are processed separately at two time scales: (1) the whole 15 years of the period of study and (2) the individual years. In contrast with previous studies, the analysis of the dataset of the 15 years shows that dawn (e.g., LT4–6) and late morning (LT10–12) are the more remarkable characteristic times for ionospheric variability. This study also reveals a cyclic trend of the variability with respect to local times. The first two modes, which contain 80–90% of the total variance, represent spatial distributions and temporal variations with respect to the different stages of the solar cycle and local times. Annual and semiannual variations are demodulated from the first two modes, and the results show that these variations evidently have distinct trends for daytime and nighttime. An exception is that, under active solar conditions, extremely strong solar irradiance during the daytime has a residual effect on the variability of the nighttime.  相似文献   

Sustainable environmental and resource management futures for Sub-Saharan Africa     

Juha I. Uitto  Walther Manshard 《GeoJournal》1993,29(1):105-106

Reports

MEDACTA '93 Modern Educational Technologies  相似文献   

Conflation of OpenStreetMap and Mobile Sports Tracking Data for Automatic Bicycle Routing          下载免费PDF全文

Cecilia Bergman  Juha Oksanen 《Transactions in GIS》2016,20(6):848-868

This article investigates how workout trajectories from a mobile sports tracking application can be used to provide automatic route suggestions for bicyclists. We apply a Hidden Markov Model (HMM)‐based method for matching cycling tracks to a “bicycle network” extracted from crowdsourced OpenStreetMap (OSM) data, and evaluate its effective differences in terms of optimal routing compared with a simple geometric point‐to‐curve method. OSM has quickly established itself as a popular resource for bicycle routing; however, its high‐level of detail presents challenges for its applicability to popularity‐based routing. We propose a solution where bikeways are prioritized in map‐matching, achieving good performance; the HMM‐based method matched correctly on average 94% of the route length. In addition, we show that the extremely biased nature of the trajectory dataset, which is typical of volunteered user‐generated data, can be of high importance in terms of popularity‐based routing. Most computed routes diverged depending on whether the number of users or number of tracks was used as an indicator of popularity, which may imply varying preferences among different types of cyclists. Revising the number of tracks by diversity of users to surmount local biases in the data had a more limited effect on routing.  相似文献   

Determination of Exterior Orientation Using Linear Features from Vector Maps     

Mika  Karjalainen  Juha  Hyyppä  Risto  Kuittinen 《The Photogrammetric Record》2006,21(116):329-341

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Integrating climate and local factors for geomorphological distribution models          下载免费PDF全文

Juha Aalto  Miska Luoto 《地球表面变化过程与地形》2014,39(13):1729-1740

Earth surface processes (ESPs) drive landscape development and ecosystem processes in high‐latitude regions by creating spatially heterogeneous abiotic and biotic conditions. Ongoing global change may potentially alter the activity of ESPs through feedback on ground conditions, vegetation and the carbon cycle. Consequently, accurate modeling of ESPs is important for improving understanding of the current and future distributions of these processes. The aims of this study were to: (1) integrate climate and multiple local predictors to develop realistic ensemble models for the four key ESPs occurring at high latitudes (slope processes, cryoturbation, nivation and palsa mires) based on the outputs of 10 modern statistical techniques; (2) test whether models of ESPs are improved by incorporating topography, soil and vegetation predictors to climate‐only models; (3) examine the relative importance of these variables in a multivariate setting. Overall, the models showed high transferability with the mean area under curve of a receiver operating characteristics (AUC) ranging from 0.83 to 0.96 and true skill statistics (TSS) from 0.52 to 0.87 for the most complex models. Even though the analyses highlighted the importance of the climate variables as the most influential predictors, three out of four models benefitted from the inclusion of local predictors. We conclude that disregarding local topography and soil conditions in spatial models of ESPs may cause a significant source of error in geomorphological distribution models. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Empirical Status in Cosmology and the Problem of the Nature of Redshifts     

T. Jaakkola  M. Moles  J.-P. Vigier 《Astronomische Nachrichten》1979,300(5):229-238

When comparing the results of the cosmological tests obtained by various authors using various methods, it is found that there has not been any trend of convergence in the values obtained for the deceleration parameter q₀, in spite of the accumulated and more accurate data. The empirical data – those on the global and the local cosmological tests and those pointing to the non-existence of cosmic evolutionary effects – can be interpreted in a plausible way in the framework of a static EUCLID ean model. This well reconciles the non-DOPPLER ian nature of redshifts implied by the studies of the redshift phenomenon itself.  相似文献